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6-2020

Integrating Detailed Petrography, Geochemistry, and Mineralogy to Integrating Detailed Petrography, Geochemistry, and Mineralogy to

Elucidate Extensive Early Diagenesis in the Eocene Carbonates of Elucidate Extensive Early Diagenesis in the Eocene Carbonates of

Qatar Qatar

Brooks H. Ryan Western Michigan University, brookshryan@gmail.com

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Recommended Citation Recommended Citation Ryan, Brooks H., "Integrating Detailed Petrography, Geochemistry, and Mineralogy to Elucidate Extensive Early Diagenesis in the Eocene Carbonates of Qatar" (2020). Dissertations. 3562. https://scholarworks.wmich.edu/dissertations/3562

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INTEGRATINGDETAILEDPETROGRAPHY,GEOCHEMISTRY,ANDMINERALOGYTOELUCIDATEEXTENSIVEEARLYDIAGENESISINTHEEOCENECARBONATESOFQATAR

by

BrooksH.Ryan

AdissertationsubmittedtotheGraduateCollegeinpartialfulfillmentoftherequirementsforthedegreeofDoctorofPhilosophyGeologicalandEnvironmentalSciences

WesternMichiganUniversityJune2020

DoctoralCommittee: StephenE.Kaczmarek,Ph.D.,Chair JohnM.Rivers,Ph.D. R.V.Krishnamurthy,Ph.D.

©2020BrooksH.Ryan

INTEGRATINGDETAILEDPETROGRAPHY,GEOCHEMISTRY,ANDMINERALOGYTOELUCIDATEEXTENSIVEEARLYDIAGENESISINTHEEOCENECARBONATESOFQATAR

BrooksH.Ryan,Ph.D.

WesternMichiganUniversity,2020

EocenecarbonatescomprisingtheUmmerRadhuma(UER),Rus,andDammamFormations,

cover80%ofthesurfaceandextend>300mintothesubsurfaceofQatar.Theserocksrecordmarine

sedimentation in shallow sub-tidal to restricted settings.Despite undergoing only shallowburial

(<1000m),extensivediageneticalterationhasoccurred.Giventhatlittleworkonthistopichasbeen

publishedpreviously,theobjectivesofthisworkaretodocumentdiagenesisintheEocene,andto

integratepetrography,mineralogy,andgeochemistrytodelineatethetimingandenvironmentsof

diagenesisoftheseEocenecarbonates.Detailedpetrographicdatashowthatdolomitizationoccurred

earlyintheUER,beforetheformationofdiageneticchert,palygorskite,pyrite,calcite,andgypsum

cements.Bulkdolomiteδ18Ovalues,coupledwithclumpedisotope-derived(∆47)temperaturesand

dolomitizing fluid δ18O values, suggest dolomitization took place in near-normal marine fluids,

perhapsduringshallowburial.Thesedatachallengethecurrentparadigmoflarge-scale,top-down

hypersalinerefluxdolomitizationintheUER.Associateddepthtrendsofincreasingcrystalsizeand

with increasing stoichiometry and cation ordering but variable δ18O further suggest that UER

dolomiteswereextensivelyrecrystallizedprior to the formationofallothermineralphases.This

finding challenges an analysis of the literature which suggests that extensive dolomite

recrystallizationonlyhappenslateinthediagenetichistoryand/orwithdeepburial.Similarly,the

overlyingRusFm.consistsofdolomitizedperitidalfaciesinterbeddedwithgypsum,aswellasmore

openmarinedepositshigherintheformation.However,petrographicdatashowsdolomiteformed

priortogypsumcementation,andbulkδ18Oandδ13Cdatasuggestanon-evaporativeorigin.The

intimateassociationbetweendolomitesandmeteoriccalciteselsewhereinthesectionindicatethat

Rusdolomiteshavelikelybeenrecrystallizedmultipletimes,possiblyinmeteoric-relatedfluids.The

key findings of this study are that early diagenesis can be extremely complex, and that current

paradigms related to reflux dolomitization and dolomite recrystallization may need to be

reevaluated.

ii

ACKNOWLEDGEMENTS

Iwouldliketothankallofmyfamilymembers,friends,andcolleagueswhohave

unconditionallysupportedmethroughoutthis4-yearjourney.SpecialthanksgotoSteve,whohas

beenmorethananacademicadvisor,butalsoalifeadvisor,duringmytimeinKalamazoo.His

effortsinshapingmylifeafterthePhDshouldnotgounrecognized.Iwouldalsoliketothankthe

criticismsandsupportofmycommitteemembersJohnandRV.Johnhasessentiallybeenaco-

advisorforthisdissertation,andIamgratefulforhisideasinhelpingshapemuchofmy

interpretationspresentedherein.IalsowanttothankJohnandhisfamilyfortheirhospitalitywhile

westayedwiththeminQatarforaweekin2018and2019.IalsowanttoacknowledgeDr.David

BuddfromtheUniversityofColorado,Boulderforhishighlyinsightfulandeducationalcritiquesof

twochaptersofthisdissertationthatweresubmittedforpublication.Hisinputhasbeeninvaluable,

andIamveryappreciativeforhisthoroughreviewsofmywork.Specialthanksalsogotomy

parentswhohavealwaystaughtmetoworkhardanddotherightthingatallstagesofmylife.I

wouldliketoacknowledgethefundingsupportofExxonMobilResearchQatar,whonotonly

providedfundingforallresearchaspectsofthisprojectbutalsofundedmefor1.5yearsasa

ResearchAssistant.ThankstoWesternMichiganUniversity’sDepartmentofGeologicaland

EnvironmentalSciences,whofundedmetheother2.5yearsthroughaTeachingAssistantship.The

AmericanAssociationofPetroleumGeologistsandtheSocietyforSedimentaryGeologyarealso

acknowledgedforfundingthroughoutthisdissertation.Lastly,Iwouldliketoacknowledgesupport

frommyGermanShepherd,Dolo,whomIraisedduringthelasthalfofthisdissertation.Inamed

himinhonorofthemineraldolomite,whichmakesupthebulkofthisdissertation.

BrooksH.Ryan

iii

TABLEOFCONTENTS

ACKNOWLEDGEMENTS..........................................................................................................................................................ii

LISTOFTABLES.........................................................................................................................................................................v

LISTOFFIGURES.......................................................................................................................................................................vi

CHAPTER

I. INTRODUCTION...........................................................................................................................................................1

References.............................................................................................................................................................6

II. EARLYANDPERVASIVEDOLOMITIZATIONBYNEAR-NORMALMARINEFLUIDS:NEWLESSONSFROMANEOCENEEVAPORITVESETTINGINQATAR........................................................10

Abstract...............................................................................................................................................................10

Introduction......................................................................................................................................................11

GeologicalBackground.................................................................................................................................13

MaterialsandMethods.................................................................................................................................17

Results.................................................................................................................................................................23

Discussion..........................................................................................................................................................36

Conclusions........................................................................................................................................................50

Acknowledgements........................................................................................................................................51

DataAvailabilityStatement........................................................................................................................51

References..........................................................................................................................................................51

III. EARLYANDEXTENSIVERECRYSTALLIZATIONOFCENOZOICDOLOMITESDURINGSHALLOWBURIAL...................................................................................................................................................62

Abstract...............................................................................................................................................................62

Introduction......................................................................................................................................................63

GeologicalSetting............................................................................................................................................64

iv

TableofContents—continued

Methods...............................................................................................................................................................65

Results.................................................................................................................................................................66

Discussion..........................................................................................................................................................68

Conclusions........................................................................................................................................................72

Acknowledgements........................................................................................................................................72

References..........................................................................................................................................................72

SupplementaryMaterial..............................................................................................................................75

IV. DOLOMITEDISSOLUTION:ANALTERNATIVEDIAGENETICPATHWAYFORTHEFORMATIONOFPALYGORSKITECLAY..........................................................................................................97

Abstract...............................................................................................................................................................97

Introduction......................................................................................................................................................98

GeologicBackground...................................................................................................................................100

MaterialsandMethods...............................................................................................................................101

Results...............................................................................................................................................................103

Discussion........................................................................................................................................................110

Conclusions......................................................................................................................................................127

Acknowledgements......................................................................................................................................128

References........................................................................................................................................................128

V. MULTI-EPISODICRECRYSTALLIZATIONANDISOTOPICRESETTINGOFDOLOMITESINNEAR-SURFACESETTINGS................................................................................................................................137

Abstract.............................................................................................................................................................137

Introduction....................................................................................................................................................139

GeologicBackground...................................................................................................................................142

MaterialsandMethods...............................................................................................................................144

Results...............................................................................................................................................................146

v

TableofContents—continued

Discussion........................................................................................................................................................157

Implications.....................................................................................................................................................171

Conclusions......................................................................................................................................................173

Acknowledgements......................................................................................................................................174

References........................................................................................................................................................175

vi

LISTOFTABLES

CHAPTERII

1. Stoichiometrystatisticsfor4UERdolomitesanalyzedbyXRD,EMPA,andEDS.......................32

2. UERDolomiteStableIsotopeAnalysis..........................................................................................................33

3. Qatargypsumδ18Oanalysis...............................................................................................................................35

CHAPTERIII

1. Core1clumpedisotopeanalysis......................................................................................................................67

SupplementaryMaterial

1. TotalVarianceExplained.....................................................................................................................77

2. ComponentMatrixandFactorLoadings......................................................................................77

3. AnalysisofNaturalRecrystallizedDolomitesinLiterature.................................................78

vii

LISTOFFIGURES

CHAPTERII

1. GeneralizedstratigraphiccolumnoftheEocenesedimentsofQatar..............................................14

2. ApalaeogeographicmapoftheeasternhalfoftheArabianPeninsula...........................................16

3. MineralogicalpercentagesfortheUmmerRadhuma(UER)recoveredincores1,2,and3....................................................................................................................................................................19

4. Planepolarizedlight(PPL)thinsectionimagesofobserveddolomitetextures........................24

5. Thinsectionimagesinplanepolarizedlight(PPL)ofvariousallochemfeatures.....................25

6. Non-carbonatediageneticfeatures.................................................................................................................26

7. Diageneticrelationshipsbetweendolomite,palygorskiteandgypsum.........................................28

8. Thinsectionimagesinplanepolarizedlight(PPL)displayingtherelationshipbetweencalcite,dolomite,palygorskiteandchalcedony.........................................................................................29

9. Thinsectionimages(planepolarizedlight–PPL)fromtheunconsolidatedsectionofcore2.......................................................................................................................................................................30

10. Coreprofileofcore1displayingmineralogyandfaciesassociations(A)withcorrespondingdolomitestoichiometry(mol%MgCO3)(B),cationordering(C),bulkdolomiteδ18Ovalues(D),bulkdolomiteδ13Cvalues(E),andSrandNaconcentrations(F)..................................................................................................................................................30

11. Themineralogyandfaciesassociationsofcore2(A)correlatedtodolomitestoichiometry(mol%MgCO3)(B),cationordering(C),bulkdolomiteδ18Ovalues(D),bulkdolomiteδ13Cvalues(E),andSrandNaconcentrations(F).....................................................31

12. Depthprofileofcore3exhibitingmineralogyandfaciesassociations(A)correspondingtodolomitestoichiometry(mol%MgCO3)(B),cationordering(C),bulkdolomiteδ18Ovalues(D),bulkdolomiteδ13Cvalues(E),andSrandNaconcentrations(F)..............................31

13. ParageneticsequencefortheUmmerRadhuma(UER)inQatarbasedonpetrographiccross-cuttingrelationshipsandgeochemicaldata...................................................................................37

CHAPTERIII

1. AstratigraphiccolumnofthePaleogenestrataofQatar(modifiedafterRyanetal.,2020),withthespecificstudyinterval(red).......................................................................................65

viii

ListofFigures—continued

2. Crossplotsofcationorderingandstoichiometry(A)andstableisotopiccomposition(B)asafunctionofdolomitetextureforallcores............................................................................................67

CHAPTERIV

1. MapofQatarandsurroundingregion.........................................................................................................100

2. StratigraphiccolumnforWellRR-01...........................................................................................................102

3. PhotographsshowingsomeofthecommonsedimentologicalfeaturesobservedintheUmmerRadhuma.................................................................................................................................................104

4. PowderX-raydiffractionpatternofarepresentativesample(110.45mdepth)containingca76%dolomiteandca20%palygorskitefrom5°to55°2q...................................106

5. High-resolution,planepolarizedlight(PPL)thinsectionimagesofdolomite(lightbrowntolightgreyrhombs)andpalygorskite(greenhaze)fromtheUmmerRadhuma.................................................................................................................................................108

6. Scanningelectronmicrophotograph(SEM)imagesofdolomiteandpalygorskite.................109

7. ElectrondispersiveanalysisofthesamedolomitecrystalasFig.6AandC...............................110

8. Scanningelectronmicrophotograph(SEM)ofdolomiteandpalygorskitefromwelldepthof89.55m...................................................................................................................................................111

9. BackscatterSEMphotomicrographshowingthelocationswhereenergy-dispersivespectroscopy(EDS)spotanalyseswerecollectedonthehighlypolishedthinsection(78.19m)..................................................................................................................................................................115

10. Evolutionarymodelofthedolomitizationreaction,growthofcloudy-centreclear-rimdolomitefabricsandpost-dolomitizationdissolutionofdolomitewithsubsequentpalygorskiteformation.......................................................................................................................................117

11. StabilitydiagramsofpalygorskiteasafunctionofpH,H4SiOH4,andMg.....................................120

12. StabilitydiagramsmodifiedfromBirsoy(2002)displayingthestabilityboundariesofvariousmineralswithinasevencomponentMgO-CaO-Al2O3-SiO2-H2O-CO2-HClsystem....122

CHAPTERV

1. AgeneralizedstratigraphicsectionofthePaleoceneandEocenesedimentsofQatar,aswellasalocationmapoftheArabianPeninsula,Qatar,andthethreeresearchcores....143

2. Percentmineralogyforcores1,2,and3fortherecoveredRusFm..............................................144

ix

ListofFigures—continued

3. PlanepolarizedlightthinsectionimagesofvariouspetrographicfeaturesofthedolomitesintheTrainaMbr............................................................................................................................149

4. AplanepolarizedlightthinsectionimageofadolomitizedmiliolidpackstonewiththefabricpreservedfromtheAlKhorMbr.......................................................................................................150

5. Featuresofthedolomite-calcitecontactobservedat~28mdepthincore1............................150

6. IntheTrainaMbr.ofcore2,large(>200µm)poikilotopiccalcitecrystals(pink)engulfplanar-edolomitecrystals................................................................................................................................151

7. PlanepolarizedlightimageoftheTrainaMbr.,exhibitingdolomitecrystals(tan)cementedbypalygorskite(darkgreen)......................................................................................................152

8. Planepolarizedlightimageofchalcedony(white)thathaspartiallyreplacedcalcitecrystals(pink)(A)andacrosspolarizedlightimageofeuhedralquartzcrystals(greyandblack)includedincalcitemosaics(pink)(B)incore2...................................................152

9. Plotsofstoichiometrywithdepthforcores1(A),2(B),and3(C)................................................153

10. Crossplotofbulkdolomiteδ18Oandδ13CvaluesforallRussamples.......................................155

11. Bulkrockδ18OfromtheRusplottedasafunctionofdepthforcores1(A),2(B),and3(C)....................................................................................................................................................................156

12. Bulkrockδ13CfromtheRusplottedasafunctionofdepthforcores1(A),2(B),and3(C)..................................................................................................................................................................157

13. ParageneticsequenceoftherelativetimingofdiageneticeventsimpactingtheRusinQatar,basedonpetrographiccross-cuttingrelationships..................................................................158

1

CHAPTERI

INTRODUCTION

TheGreaterGhawarUpliftandPersianGulfprovincesoftheArabianPeninsulaare

associatedwithsomeofthelargesthydrocarbonreservesonearth(Pollastro,2003).TheSouth

Pars-NorthField,offshoreQatar,isthelargestgasandcondensatefieldintheworld,withestimated

reservestotaling~500trillioncubicfeetofgas(TCFG)(Pollastro,2003;Perottietal.,2011).The

geologyofsuchfieldshasbeenwelldocumented,withmajorreservoirsexistingintheGreater

Paleozoic,Jurassic,andCretaceousPetroleumSystems(Pollastro,2003).InQatar,Cenozoicrocks

ranginginagefromLatePaleocenetoRecentoverlytheCretaceousSystem.TheEocenerocks,

althoughnothydrocarbonreservoirs,areofparticularimportance,astheyarevaluable

groundwateraquifersandprovidewaterresourcestothecountry(Ecclestonetal.,1981;

Baalousha,2016).

TheEocenestratigraphyofQatarisrepresentedbytheUmmerRadhuma,Rus,and

DammamFormations.MuchofwhatisknownaboutthesedimentologyoftheEoceneinQatar

comesfromearlystudiesoftheArabianPeninsula(Powersetal.,1966),thesurficialsediments

(Cavelier,1970),andthesubsurfacewaterresources(Ecclestonetal.,1981).Afewrecentstudies,

however,havefurthercharacterizedthesediments,rocks,andstratigraphyoftheQatarEocene

formations,reportingthattherocksaredominatedbylimestonesanddolomitesformedinshallow

marinetointermittentlyrestrictedenvironments(Abu-Zeid,1991;Al-HajariandSt.C.Kendall,

1992;Al-Saad,2003;Al-Saad,2005;Holailetal.,2005;Riversetal.,2019).Muchfocushasalsobeen

placedonrefiningthebiostratigraphyandformationboundaries(Smout,1954;Powersetal.1966;

Hasson,1985;Hewaidy,1994;Boukharyetal.2011).Mostofthisknowledgehasledtothe

understandingthatQatarexperiencedatransgressionduringtheLatePaleocene-EarlyEocene,a

2

regressionduringtheEarlyEocene,andasubsequenttransgressionintheMiddleEocene.These

eventscorrespondtothedepositionoftheUmmerRadhumaFm.,RusFm.,andDammamFm.,

respectively.

DespitetherecenteffortstoadvancetheunderstandingoftheEocenedepositionalsystem,

littlehasbeendonetocharacterizethediageneticalterationsthathavetakenplacesince

deposition.HolailandAl-Hajari(1997)andHolailetal.(2005)aretheearliestpublishedstudiesto

analyzethediageneticfeaturesinQatar,buttheyarelimitedtotheLowerDammamandUpper

Dammamsubformations,respectively.Abetterunderstandingofthediageneticimpactsonthe

UmmerRadhumaandRusformationshasrecentlycometolight(Riversetal.2019),butthe

generallackofdiageneticunderstandinginQatarispuzzlingconsideringthatdiageneticalterations

ofallthreeEoceneformationshavebeenrecordedacrosstheArabianPeninsula,includingKuwait,

Oman,SaudiArabia,andtheUnitedArabEmirates(Bou-RabeeandBurke,1987;Whittleand

Alsharhan,1994;Whittleetal.,1996;Al-Awadietal.,1998;El-SaiyandJordan,2007;Hersi,2011;

Khalaf,2011;Pollittetal.,2012;KhalafandAbdullah,2013;Khalafetal.,2018).

Aspartofanin-depthgeologicalstudyoftheEocenecarbonatesinQatarbeingspearheaded

byExxonMobilResearchQatarincollaborationwithWesternMichiganUniversity,thepurposeof

thepresentstudyistobridgethegapbetweenthediageneticalterationsimpactingtheEocene

successionsofQatarandthosedocumentedinthesurroundingareas.Adiageneticstudyofthe

EocenesuccessionsofQatarisofimportanceforenergyexploration,becauseitisinterpretedthat

thePaleocene-Eocenestratahavenotbeenburiedfurtherthanmoderndaydepths(50-400m)

basedonstudiesfocusingonthetectonicanddepositionalhistoryofQatar(VanBuchemetal.,

2014;RiversandLarson,2018).WhatthismeansisthatitisunlikelytheEocenerockshavebeen

subjectedtodeepburial(>1km)alterations,andthustheearlynear-surfacediageneticeffectsare

likelypreservedintheserocks.ThishassparkedtheinterestofExxonMobilResearchQatar,as

theserockslikelycontaininformationabouthowearlydiagenesisimpactstheporesystem,andcan

3

thusbeutilizedasananalogueforhydrocarbonexplorationandrecoveryinotherlocalities.This

studyseekstoaddressthefirstpartoftheproblem:characterizingtheearlydiageneticalterations.

From2016-2018,3wellsweredrilledandcoredinordertobetterunderstandthe

stratigraphy,diagenesis,andhydrologyoftheEoceneformations.Datawascollectedfromthese

coresandincorporatedintothisdissertationtospecificallyanswerthefollowingquestions:

(1) WhatdiageneticalterationsareobservedintheEocenerocksofQatar?

(2) Whatistherelativeorderofdiageneticevents,specificallywithregardtothecarbonate

minerals?

(3) Whatenvironmentsand/orprocessesareresponsibleforthediageneticalterationstothe

carbonateminerals?

Extensivepetrographicmethodswereutilizedtoanalyzecrosscuttingrelationshipsanddetermine

thetimingofalldiageneticmineralsandtexturesobserved.Subsequentmineralogicaland

geochemicaldatawasintegratedtoshedlightoninterpretationsrelatingtodiagenetic

environmentsandprocessesresponsibleforthealterations.Theultimateoutcomeisacomplete

anddetailedparageneticsequencecharacterizingtheEoceneformations,andinsightsintothe

diageneticenvironmentsthatwerepresent.Thisstudyhasresultedinthefirstextensivediagenetic

characterizationoftheEocenesedimentsofQatar,presentedinthefollowingchaptersandbriefly

summarizedbelow.

ChapterIIpresents“Earlyandpervasivedolomitizationbynear-normalmarinefluids:New

lessonsfromanEoceneevaporativesettingQatar,”whichreportsonthediageneticalterationsand

ultimateparageneticsequenceoftheUmmerRadhuma(UER)inQatar.Previousstudiesonthe

UmmerRadhumainSaudiArabiaandKuwaithadsuggestedthatdolomitizationoftheUmmer

Radhumaoccurredbyrefluxingofhypersalinebrines,implyingthatevaporitemineralssuchas

anhydriteandgypsumweredepositedandprecipitatedearly,whereasdolomiteformedlaterby

thedownwardflowofdensehypersalinebrines(Pollittetal.,2012;Salleretal.2014).However,

4

petrographicrelationshipspresentedinChapterIIshowthatthedolomitesintheUERofQatar

formedpriortogypsumcementation.Furthermore,downwardtrendsinstoichiometry(mol%

MgCO3)directlyunderneaththeoverlyingRusevaporitebedareinconsistentwiththatexpected

fromrefluxingfluidsthatshowadecreasedinMg/Caratioswithdepth.Lastly,thedolomite

geochemicaldata,includingdolomiteδ18O,Sr,andNa,aremorecompatiblewithanear-normal

marineorigin(normalseawaterwithpossiblyslightlyelevatedtemperaturesorsalinities),rather

thanahypersalineorigin.Theprinciplefindingfromthisstudywasthatdolomitesoverlainand

cementedbyevaporitesdonotneedtohaveformedfromevaporativefluids.Thisfindingbuilds

uponotherrecentstudieswhichcametosimilarconclusions,andchallengesthelong-held

paradigmthatdolomitesoverlainbyevaporitesformedviarefluxdolomitization.Thisarticlewas

firstpublishedbyRyanetal.(2020)inthepeer-reviewscientificjournalSedimentologyandhas

beenreprintedhereincompliancewiththecopyrightlicenseagreement.

ChapterIIIbuildsonChapterII,andutilizeschangesindolomitetextureswhichdisplay

increasingcrystalsizewithdepth,andcorrelatesthesetexturalchangeswithchangesindolomite

stoichiometry,cationordering,andδ18O.Thesedatasuggestthatmimeticdolomites(finestcrystal

size),whicharetheleaststoichiometric,leastordered,andcontainanarrowrangeofδ18Ohave

beennotbeensignificantlyrecrystallized,whereasthecoarserplanar-eandnonplanardolomites,

whicharemorestoichiometric,morewell-orderedandvariableinδ18Ohaveundergonesignificant

recrystallization.Aprincipalcomponentanalysisisconsistentwiththeinterpretationthatchanges

instoichiometry,cationordering,andδ18Oarecorrelatedtodolomiterecrystallization.

Furthermore,nonplanarintervalsinallcorescontainachertbandwithsilicifiedplanar-e

dolomites,andonecorecontainstwosuchbands—onewithplanar-edolomitesandonewith

mimeticdolomites.Itisinterpretedthatthesesilicifiedbandscapturetheearlyrecrystallizationof

thesedolomites.Basedonthe∆47-derivedtemperaturessuggestingburialdepths<400mand

δ18OwvaluesthatareindicativeofslightlyevaporatedEoceneseawater,themainfindinginthis

5

chapteristhatCenozoicdolomitescanbeextensivelyrecrystallizedearlyandinthenear-surface

realm.Thisisatoddswithalargeliteraturereviewofrecrystallizeddolomitesthatdemonstrates

that~90%arepost-Cenozoicinage,and~94%ofallstudiesinterpretanintermediate(500-1000

m)todeep(>1000m)burialorigin.ThisworkhasbeensubmittedtoGeologyandiscurrently

underreview.

ChapterIVshedslightontheparageneticsequenceafterdolomiterecrystallization

occurred,focusingontherelationshipbetweendolomiteandpalygorskiteintheUER.Thischapter

presentsresultsshowingthatplanar-edolomitecrystalsdisplaycloudycoresandclearrims,in

whichthecloudycoresarepartiallytocompletelydissolvedandtheclearrimsarepristineand

intact.Palygorskiteisobservedcoatingtheoutsideofdolomitecrystals,nucleatingonpartially

dissolvedcrystals,andhaspartiallytocompletelyfilleddissolveddolomitecores.Theseresults

suggestthattheformationofpalygorskitepostdatesdolomitizationandisconcurrentwith

dolomitedissolution.However,thepresenceofclaymineralswithininterpreteddepositionalcycles

suggeststhatpalygorskiteisanalterationproductofapre-existingclay.Thischapterdiscusseshow

dolomitedissolutionaltersporewaterchemistrybybufferingmeteoric-influencedfluidsthatare

slightlyacidicandcontainH4SiO4,andreleasingMg2+ionstodrivethetransformationofsmectiteto

palygorskite.Thefocusofthischapterisprovidingdirectevidencethatdolomitedissolutioncan

drivetheformationofpalygorskite,whichisahypothesisthathasbeenputforthpriorbutlacked

physicalevidence.Thisstudy(Ryanetal.,2019)iscurrentlypublishedinSedimentology.

ChapterVshiftsthefocusawayfromtheUERandintotheoverlyingRusFm.Thischapter

outlineshowdolomitesthatareinterbeddedwithevaporiteslikelydidnotforminevaporative

fluidsbasedondolomiteδ18Ovalues,δ13Cvalues,andtheobservationthatdolomiteisincludedin

gypsum,suggestingdolomitizationoccurredpriortodepositionofgypsum.Thisbuildsuponthe

principlefindingsinChapterII,andfurtherchallengesthecommonparadigmofhypersalinereflux

dolomitization.TheRusdolomitesalsocontainanegativeδ13Csignaturethattrendstowardsnear-

6

normalmarinevalueswithdepthawayfromanexposuresurfacecontainingmeteoriccalcites.This

isinspiteofthefactthatthedolomiteshavesignificantlyhigherδ18Ovalues,anddolomitecrystals

areincludedincalcitecrystals,suggestingdolomitesformedpriortocalcite.Theimportantfindings

ofthischapterarethatnear-surfacediagenesiscanbeextremelycomplex,dolomitesoverlainby

beddedevaporitesdonotneedtohaveformedfromrefluxdolomitization,andthatdolomitesare

relativelysusceptibletomultiphaserecrystallization,especiallyinmeteoric-influencedfluids.This

researchiscurrentlyunderreviewintheJournalofSedimentaryResearch.

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Bou-Rabee,F.andBurke,C.D.(1987)Lithostratigraphyandenvironmentofdepositionofthe

topoftheWaframember,Radhumaformation(Wafrafield,Kuwait):Palaeogeography,P

alaeoclimatology,Palaeoecology,v.59,p.269-277.

Boukhary,M.,Hewaidy,A.G.,Luterbacher,H.,Bassiouni,M.E.,andAl-Hitmi,H.,2011,

ForaminferaandostracodesofEarlyEoceneUmmerRadhumaFormation,DukhanOilField,

Qatar:Micropaleontology,v.57,p.37-60.

Cavelier,C.,1970,GeologicDescriptionoftheQatarPeninsula,DepartmentofPetroleum

Affairs,Paris,France,39pp.

Eccleston,B.L.,Pike,J.G.,andHarshash,I.,1981,ThewaterresourcesofQatarandtheir

development:Vol.1.FoodandAgriculturalOrganization(FAO)oftheUnitedNations,Doha,

TechnicalReportNo.5.,431pp.

El-Saiy,A.K.,andJordan,B.R.,2007,DiageneticaspectsoftertiarycarbonateswestoftheNorthern

OmanMountains,UnitedArabEmirates:JournalofAsianEarthSciences,v.31,p.35-43.

Hasson,P.F.,1985,NewobservationsonthebiostratigraphyoftheSaudiArabianUmmerRadhuma

Formation(Paleogene)anditscorrelationwithneighboringregions:Micropaleontology,v.

31,p.335-364.

Hersi,O.S.,2011,LithologicanddiageneticattributesoftheSharwayn(Maastrichtian)andUmmer

Radhuma(latePaleocene-Eocene)formationsandtheirsignificancetotheK-T

unconformity,JabalSamhanarea,Dhofar,SultanateofOman:ArabJGeosci,v.4,p.147-160.

Hewaidy,A.G.,1994,BiostratigraphyoftheUmmerRadhumaFormationinSouth-EastQatar,

ArabianGulf.N.Jb.Geol.Palaont.Abh.,v.193,p.145-164.

Holail,H.,andAl-Hajari,S.,1997,EvidenceofanAuthigenicOriginforthePalygorskiteina

MiddleEoceneCarbonateSequencefromNorthQatar.QatarUniv.Sci.J.,v.17,p.405-418.

Holail,H.M.,Shaaban,M.N.,Mansour,A.S.,andRifai,I.,2005,DiagenesisoftheMiddle

8

EoceneUpperDammamsubformation,Qatar:Petrographicandisotopicevidence:

CarbonatesandEvaporites,v.20,p.72-81.

Khalaf,F.I.,andAbdullah,F.A.,2013,Petrographyanddiagenesisofcavity-filldolocretes,Kuwait:

Geoderma,v.207-208,p.58-65.

Khalaf,F.I.,Abdullah,F.A.,andGharib,I.M.,2018,Petrography,diagenesisandisotope

geochemistryofdolostonesanddolocretesintheEoceneDammamFormation,Kuwait,

ArabianGulf:CarbonatesandEvaporites,v.33,p.87-105.

Pollastro,R.M.,2003,TotalPetroleumSystemsofthePaleozoicandJurassic,GreaterGhawarUplift

andAdjoiningProvincesofCentralSaudiArabiaandNorthernArabian-PersianGulf:U.S.

GeologicalSurveyBulletin,2202-H,107pp.

Powers,R.W.,Ramirez,L.F.,Redmond,C.D.,andElberg,E.L.,1966,Geologyofthe

ArabianPeninsula(SedimentaryGeologyofSaudiArabia).WashingtonDC:U.S.Geological

Survey.Survey,ProfessionalPaper560D,147pp.

Perotti,C.R.,Carruba,S.,Rinaldi,M.,Bertozzi,G.,Feltre,L.,andRahimi,M.,2011,TheQatar-

SouthFarsArchDevelopment(ArabianPlatform,PersianGulf):InsightsfromSeismic

InterpretationandAnalogueModelling.In:NewFrontiersinTectonicResearch–atthe

MidstofPlateConvergence(Ed.U.Schattner),p.325-352.

Pollitt,D.A.,Anthonissen,E.,Saller,A.H.,BouDagher-Fadel,M.K.,andDickson,J.A.D.,

2012,AbruptearlyEoceneglobalclimaticchangeasacontrolofcarbonatefaciesand

diagenesis:anewrecordofthePalaeocene-EoceneThermalMaximumintheUmmer

RadhumaFormation,SaudiArabiaandKuwait:TerraNova,v.24,p.487-498.

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faultingalongtheDukhan‘anticline’:MarineandPetroleumGeology,v.92,p.953-961.

Rivers,J.M.,Skeat,S.L.,Yousif,R.,Liu,C.,Stanmore,E.,Tai,P.andAl-Marri,S.M.,2019a,

9

Thedepositionalhistoryofnear-surfaceQataraquiferrocksanditsimpactonmatrixflow

andstorageproperties:ArabianJournalofGeosciences,v.12,

https://doi.org/10.1007/s12517-019-4498-6.

Ryan,B.H.,Kaczmarek,S.E.,andRivers,J.M.,2019,Dolomitedissolution:Analternative

diageneticpathwayfortheformationofpalygorskiteclay:Sedimentology,v.66,p.1803-

1824.

Ryan,B.H.,Kaczmarek,S.E.,andRivers,J.M.,2020,Earlyandpervasivedolomitizationby

near-normalmarinefluids:NewlessonsfromanEoceneevaporativesettinginQatar:

Sedimentology(inpress),https://doi.org/10.1111/sed.12726.

Saller,A.H.,Pollitt,D.A.,andDickson,J.A.D.,2014,Diagenesisandporositydevelopmentin

theFirstEocenereservoiratthegiantWafraField,PartitionedZone,SaudiArabiaand

Kuwait:AAPGBulletin,v.98,p.1185-1212.

Smout,A.H.(1954)LowerTertiaryForaminiferoftheQatarPeninsula.In:London:BritishMuseum

(NaturalHistory),90pp.

VanBuchem,F.,Svendsen,N.,Hoch,E.,Pedersen-Tatalovic,R.,andHabib,K.,2014,

DepositionalhistoryandpetroleumhabitatofQatar.inMarlow,L.,Kendall,C.C.G.,andYose,

L.A.,eds.,PetroleumsystemsoftheTethyanregion:AAPGMemoir,106,p.641-678.

Whittle,G.L.,andAlsharhan,A.S.,1994,DolomitizationandcertificationoftheEarlyEoceneRus

FormationinAbuDhabi,UnitedArabEmirates:SedimentaryGeology,v.92,p.273-285.

WhittleG.L.,Alsharhan,A.S.,andElDeeb,W.M.Z.,1996,Faciesanalysisandearlydiagenesisofthe

Middle-LateEoceneDammamFormation,AbuDhabi,UnitedArabEmirates:Carbonatesand

Evaporites,v.11,p.32-41.

10

CHAPTERII

EARLYANDPERVASIVEDOLOMITIZATIONBYNEAR-NORMALMARINEFLUIDS:NEW

LESSONSFROMANEOCENEEVAPORATIVESETTINGINQATAR

BrooksH.Ryan1,StephenE.Kaczmarek1,andJohnM.Rivers21DepartmentofGeologicalandEnvironmentalSciences,WesternMichiganUniversity,Kalamazoo,Michigan49008,U.S.A.2QatarCenterforCoastalResearch(QCCR),ExxonMobilResearchQatar,P.O.Box22500,QatarScienceandTechnologyPark-Tech,Doha,QatarRyan,B.H.,Kaczmarek,S.E.,andRivers,J.M.,2020,Sedimentology,https://doi.org/10.1111/sed.12726

Abstract

TheupperPalaeocene–lowerEoceneUmmerRadhumaFormationinthesubsurfaceofQatar

isdominatedbysubtidalcarbonatedepositionalpackagesoverlainbybeddedevaporites.InSaudi

Arabia and Kuwait, peritidal carbonate depositional sequences with intercalated evaporites and

carbonates inUmmer Radhuma have been previously interpreted to have been dolomitized via

downward reflux of hypersaline brines. Here, textural,mineralogical and geochemical data from

three research cores in Qatar are presented which, in contrast, are more consistent with

dolomitization by near-normal marine fluids. Petrographic relationships support a paragenetic

sequencewherebydolomitizationoccurredprior to the formationofallotherdiageneticmineral

phases, including chert,pyrite,palygorskite, gypsum, calciteandchalcedony,which suggests that

dolomitization occurred very early, possibly syndepositionally. The dolomites occur as finely

crystalline mimetic dolomites, relatively coarse planar-e dolomites, and coarser nonplanar

dolomites, all ofwhicharenear-stoichiometric (50.3mol%MgCO3) andwell-ordered (0.73).The

dolomitestableisotopevalues(range-2.5‰to+1‰;meanδ18O=-0.52‰)andtraceelement

11

concentrations(Sr=40–150ppmandNa=100–600ppm)arecompatiblewithdolomitizationby

near-normal seawater or mesohaline fluids. Comparisons between δ18O values from Umm er

RadhumadolomiteandtheoverlyingRusFormationgypsumfurthersuggeststhatdolomitization

did not occur in fluids related to Rus evaporites. This study provides an example of early

dolomitizationof evaporite-related carbonatesbynear-normal seawater rather thanby refluxing

hypersalinebrinesfromoverlyingbeddedevaporites.Further,itaddstorecentworksuggestingthat

dolomitization by near-normal marine fluids in evaporite-associated settings may be more

widespreadthanpreviouslyrecognized.

Introduction

Dolomite[CaMg(CO3)2]isacommondiageneticconstituentofancientsedimentarymarine

successions. Despite its abundance in the rock record, the scarcity of dolomite in modern

environmentshas ledtoacentury-longdebateabouthow,whenandwheredolomite forms(Van

Tuyl,1916;Land,1985;Budd,1997;Warren,2000;Machel,2004;Greggetal.,2015;Kaczmareket

al.,2017).Geochemicalproxydatacommonlypermitmultipleinterpretationsastothetemperature

and chemistry of the dolomitizing fluids and environmental conditions (Machel, 2004), but

frequentlypoint to low-temperature,near-surface settings (Machel, 2004;Greggetal., 2015).To

explainplatform-scalepervasivedolomitizationoflimestone,variousdiageneticmodelshavebeen

proposed.Someofthemostprominentmodelsincludedolomitizationbynormalseawater(Sass&

Katz,1982;Land,1985;Carballoetal.,1987;Land,1991;Manche&Kaczmarek,2019),hypersaline

reflux(Adams&Rhodes,1960;Deffeyesetal.,1965;Land,1967;Land,1985;Warren,2000;Machel,

2004;Dravis&Wanless,2018),mixingzonedolomitization(Badiozamani,1973;Land,1973;Folk&

Land,1975;Humphrey&Quinn,1989)anddolomitizationbyburialfluids(Mattes&Mountjoy,1980;

Barnaby&Read,1992).Althoughthesemodelsdifferintheirhydrologicalandgeochemicaldetails,

mostworkersagreethatwidespreaddolomitizationrequiresaprecursorlimestonetoprovidethe

12

necessary carbonate, a sufficient reservoir of aqueousMg, andan efficienthydrological pumping

mechanismtomovethefluidsthroughthelimestone(Morrow,1982;Land,1985).

Popular among these dolomitization models is the hypersaline reflux model, which is

commonly posited as the responsible mechanism for dolomitizing shallow-marine, peritidal

carbonatesworldwide(Sun,1995;Warren,2000;Machel,2004).Ingeneral,thismodeldescribesa

mechanism whereby hypersaline brines originating from an overlying evaporite pool seep

downwardanddolomitizeunderlyingcarbonatestrata(Adams&Rhodes,1960).Fromageochemical

perspective, the model is attractive because the precipitation of evaporites, such as gypsum or

anhydrite,inwarmaridenvironmentslowerstheconcentrationofCa2+ionsinsolution,thusraising

theMg/Caratioandcausingfluidstobecomemoresupersaturatedwithrespecttodolomite(Adams

&Rhodes,1960;Deffeyesetal.,1965;Land,1967;McKenzieetal.,1980;Patterson&Kinsman,1982;

Land,1985).Fromthehydrologicalperspective,evaporationalsoprovidesadriveforfluidflow.As

evaporativebrinesbecomedenserthanthesurroundingporefluids,theyhavethepotentialtoseep

downwardintotheunderlyinglimestone(Machel,2004).Takentogether,warmsalinefluidswith

elevatedMg/Caratioscoupledwithhydrologicalflowhavebecomethebasisforawidelyaccepted

explanationforlarge,platform-scaledolomitization,particularlyinrocksassociatedwithoverlying

evaporites(Adams&Rhodes,1960;Mooreetal.,1988;Ruppel&Cander,1988;Saller&Henderson,

1998;Cantrelletal.,2004;Fullmer&Lucia,2010;Dravis&Wanless,2018;andmanyothers).These

interpretationshavealsobeensupportedbyreactivetransportmodellingstudiesthatsuggestreflux

of hypersaline brines is a viablemechanism for platform-scale dolomitization (e.g. Jones& Xiao,

2005;Garcia-Fresca&Jones,2011;Al-Helaletal.,2012;Garcia-Frescaetal.,2012;Xiaoetal.,2018).

OneexampleofaperitidalmarinecarbonatethatispervasivelydolomitizedistheUmmErr

RadhumaFormation(UER).TheUERisawidespreadcarbonateunitthatliesinthesubsurfaceacross

muchoftheArabianPeninsula(Powersetal.,1966;Sharlandetal.,2001).Basedonmineralogical,

texturalandgeochemicaldata,ithasbeenhypothesizedthattheUERwasdolomitizedbydownward

13

flowingevaporativebrinesthatoriginatedinevaporativelagoonsduringdepositionoftheoverlying

Rusevaporitebeds(Pollittetal.,2012;Salleretal.,2014).TheUERconstitutesamajorhydrocarbon

reservoirintheWafraFieldlocatedbetweenKuwaitandSaudiArabia(Danielli,1988;Meddaughet

al.,2007;Bargeetal.,2009;Rubin,2011).TheUERhasalsoattractedattentionbecauseitcontains

the Palaeocene–Eocene Thermal Maximum (PETM) as evidenced by a distinct carbon isotope

excursionthatiscorrelativeacrosstheArabianPeninsula(Pollittetal.,2012).Effortstoassessthe

impact of diagenesis on reservoir properties at Wafra has produced a paragenetic sequence

suggesting that gypsum formation pre-dates dolomitization, supporting the contention that

dolomitizationoccurredbyrefluxinggypsum-saturatedfluids(Salleretal.,2014).Thepresentstudy

examines themechanismofdolomitization in theUER in thesubsurfaceofQatarusingasuiteof

textural,mineralogicalandgeochemicaldata.Specifically,thisstudyseekstoanswerthefollowing:

whatisthemechanismofdolomitization,andhowdoesitdifferfromthesurroundingregion?The

datapresentedhereinsuggestthatalthoughUERdolomitesareoverlainandcementedbyevaporites,

theywere formedearly in fluidswithanear-normal seawatersignature (definedhereasnormal

seawaterwithpossiblyelevatedtemperatures[≤40°C]orsalinities[35-50‰],butnottogypsum

saturation[140‰]).

GeologicalBackground

TheUmmerRadhumaFormationmarks theonsetofCenozoicdeposition (Fig. 1)during

transgressionandregressionalonganorth-east-facingramp(Sharlandetal.,2001).Thesouthern

margin of the Tethys region was covered by shallow carbonate ramps during much of the late

Palaeocene and early Eocene (Sharland et al., 2001), resulting in subtidal to peritidal carbonate

sequenceswithintercalatedclaysandminorevaporitesthatcovertheArabianPeninsula(Powerset

al.,1966;Sharlandetal.,2001).TheUERiscomposedprimarilyofmetre-scaleperitidalcarbonate

andevaporitesequencestowardtheeasternbordersofSaudiArabiaandKuwait(Salleretal.,2014),

14

withmoresubtidaltoopenmarinecarbonatedepositsinQatar(Riversetal.,2019a),whereitlies

exclusivelyinthesubsurfaceandisupto370mthick(Powersetal.,1966;Cavelier,1970;Eccleston

et al., 1981). The UER overlies the lithologically similar Cretaceous Simsima Formation with a

boundary that is biostratigraphically unconformable (Cavelier, 1970; Hewaidy, 1994). The UER

represents a large-scale regression, and is unconformably overlain by the lower Eocene Rus

Formation,which is characterized bymetre-scale cycles of dolomite, gypsum and clay (Cavelier,

1970;Ecclestonetal., 1981;Al-Saad,2003;Riversetal., 2019a).Althoughmicropalaeontological

studiesareinconclusive,thelowerUERisinterpretedtobeupperPalaeoceneinage,whereasthe

uppermostintervalsareinterpretedtobelowerEocene(Powersetal.,1966;Hasson,1985;Hewaidy,

Geologic Age Group Formation Lithology

PA

LE

OG

EN

E

Pale

ocen

eEo

cene

Early

Mid

dle

Has

a

Rus Fm.

Umm er

Radhuma Fm.

DammamFm.

Dolomite Limestone Gypsum

Figure1:GeneralizedstratigraphiccolumnoftheEocenesedimentsofQatar.AgesandlithologiesadaptedfromCavelier(1970),Al-Saad(2003)andRiversetal.(2019a).

15

1994;Boukharyetal.,2011;Pollittetal.,2012).Pollittetal.(2012)datedtheuppermostintervalsof

theUERtobeEocenebasedonacarbonisotopeshiftobservedacrosstheregion,whichtheseauthors

interpretedtobethePETMinterval,asdefinedgloballybyZachosetal.(2001).

Based on the abundance of open marine to semi-restricted inner ramp limestones with

associated shales and evaporites,much of theArabian Shelf has been inferred to be structurally

quiescent during Eocene time (Powers et al., 1966). A recent study by Rivers & Larson (2018),

however,mappedaseriesofhigh-anglenormalfaultsacrossQatarthatoccurredneartheendofUER

deposition.ThesefaultseffectivelyseparatedQatarintoasouth-westernproximalandnorth-eastern

distalbasin,relativetotheArabianPlate,attheendofthePalaeocene.Bifurcationofthebasincan

explainthehigherabundanceofsiliciclasticandevaporiticmaterial inthesouthcomparedtothe

relativelypurecarbonatepackagesinthenorth(Riversetal.,2019a).

Previousfaciesandenvironmentalinterpretations

RecentworkindicatesthattheUERinQatarisprimarilycomposedofsubtidaldeposits,with

ageneralshallowing-upwardtrendtowardstheUER/Rusboundary(Boukharyetal.,2011;Riverset

al.,2019a).InanassessmentofthefaunapresentintheUERinsouth-westernQatar,includingboth

planktonic and large benthic foraminifera, Boukhary et al. (2011) interpreted theUER to reflect

depositioninanopenmarinetoshelteredlagoonenvironment.Riversetal.(2019a)analyzedthe

sameresearchcoresutilizedinthepresentstudy(Fig.2)andcametosimilarconclusionswithregard

to depositional environments. The paragraphs that follow summarize the key observations and

interpretationsofRiversetal.(2019).

16

TheUERincore1isdefinedasmetre-scalefining-upwarddepositionalcyclesofdolomitic

packstones andwackestoneswith variable thickness. Cycle bases are characterized by dolomitic

packstonesrepresentingcoarselagdeposits,whereascyclecapsarecharacterizedbycentimeter-

scale thin-bedded or laminated dolomitic wackestones to mudstones commonly containing

palygorskite. These rocks are identified as facies type FA5, defined as bioturbated dolomitic

packstonesrichinlargebenthicforaminifera,echinoderms,dasycladgreenalgae,coralfragments,

andbothplanktonicandsmallbenthicforaminifera.Thisfaciesisinterpretedtoreflectdepositionin

normalmarinewatersbelownormalwavebaseonalow-energy,mid-rampsetting.Intheuppermost

8.5mofcore1,theUERconsistsofdolomiticpackstonesandwackestones,withbothstraightand

crinklylaminations.Theseupper8.5moftheUERincore1wasdesignatedfaciesFA3,definedas

bioturbateddolomiticmudstones,wackestonesandpackstonesrichinmiliolidsandrotalids,aswell

Figure2:ApalaeogeographicmapoftheeasternhalfoftheArabianPeninsula(modifiedfromZiegler,2001),alongwithaninsetmapofQatar(modifiedfromRyanetal.,2019)withlocationsofcores1,2and3.

17

asostracodsandsmallbivalves.Thisfaciesisinterpretedasaprotectedinteriorramporshallower

subtidallagoonenvironmentthanthatinterpretedfortheunderlyingFA5.

Similarobservationstocore1weremadeforcore3,thenorthern-mostcore,inwhichthe

uppermostca10marecomposedofFA3,underlainbyca20mofFA5,althoughtheseinterpretations

aremoreambiguousbecausetheyareobscuredbycoarsedolomitetextures.Incontrasttocore1,

thebottom-mostca20mofcore3arebioturbateddolomiticmud-leanpackstones,designatedFA4,

rich in large bivalves and gastropods, with a variety of small benthic foraminifera and minor

planktonicforaminiferaanddasycladgreenalgae.FaciesFA4wasinterpretedtoreflectcarbonate

sedimentationonanenergetic,openshallowsubtidalinnerramp.

DuetotheobliterativenatureofdolomitetexturesintheUERofcore2,faciesassociations

werenotassigned.However,ageneraldescription includesbothbioturbatedandmud-leandolo-

packstoneswithlargebenthicforaminifera,dasycladgreenalgae,echinodermsandmolluscs,which

aresimilartofaciesFA4andFA5.Theobliterativenatureofdolomitetexturesinallcoresalsoled

Riversetal.(2019a)toconcludethatahighlydetailedsequencestratigraphicinterpretationofthese

strataisnotpossible.ThetopoftheUERinallcoresdisplaysanerosivesurface,interpretedtobean

informal higher order sequenceboundarywithin thePg20 sequenceboundary of Sharlandet al.

(2001). One identifiable feature across all three cores is a chert band previously mapped as

continuousacrossQatarbyEcclestonetal.(1981).Twoothersequenceboundariesweresuggested

byRiversetal.(2019),althoughtheirprecisepositionsandhowtheycorrelateacrosswellsishighly

uncertain(seefig.20ofRiversetal.,2019a).

MaterialsandMethods

Allpetrographic,mineralogicalandgeochemicaldataacquiredforthisstudyweregathered

fromthreeresearchboreholesdrilledincentralandnorthernQatar(Fig.2).Theanalyticalmethods,

sedimentologyandgeneralstratigraphyofthesecoresaredescribedindetailinRiversetal.(2019a).

18

Researchcores

Although core recovery approached 100% in core 1, approximately 15 m of material

extractedfromcore2(90–105mdepth)andcore3(75–90mdepth)hadpoorcompetencydueto

karstification.Relevanttothecurrentstudyisthatthebottomca50–70mofeachcoreconsistsof

theuppermostpartoftheUmmerRadhumaFormation(Fig.3).Intotal,391cylindricalcoreplugs

(ca 2.5 cm diameter, ca 3.8 cm long)were taken at 0.02 – 2.0m vertical spacing fromUmm er

Radhuma.Sub-samplesofrockweretakenfromthosecoreplugsandgroundintopowdersusingan

electricrotarydrillandgroundbyhandwithamortarandpestleforca5minutesthatwereusedfor

X-ray diffraction, inductively coupled plasma – mass spectrometry (ICP-MS) and stable isotope

analysisasdescribedbelow.

Thinsectionpetrographyandscanningelectronmicroscopy

Thinsectionswerepreparedfromtheendsof391coreplugs,witheachimpregnatedwith

blueepoxyinordertoanalyzeporespace,andsubsequentlystainedwithAlizarinRedS(ARS)to

differentiatedolomitefromcalcite.Fourthinsectionsfromcore1,chosenbecausetheyrepresentan

example of each dolomite texture and also span the entirety of the UER, underwent additional

polishingtoasub-micronfinishforanalysisbyscanningelectronmicroscopy(SEM)onaJEOLJSM-

IT100 InTouchScope (JEOLLimited,Tokyo, Japan).Analytical parameters include an accelerating

voltageof15kV,workingdistanceof10mmandprobecurrentof85eV.EnergydispersiveX-ray

spectroscopy(EDS)wasusedinconjunctionwiththeSEMtodetermineelementalcompositionatthe

micrometre-scale.TheEDSreportselementalmass%,whichwasthenconvertedtomol%inorderto

calculatedolomitestoichiometry.TheEDSspectrathatcontainedelementsotherthanCa,Mg,CorO

wereexcluded,becausethiswouldindicatethatanythingotherthanpuredolomitewasanalyzed.At

least76SEM-EDSpointsmetthesecriteriaineachofthefoursamplesanalyzed.

19

70

80

90

120

Depth (m)50

100

110

60

70

80

90

120

Depth (m)

100

110

60

130

140

70

80

90

120

Depth (m)

100

110

60

Clastics

Dolomite

Calcite

Gypsum

Chert band

Mineralogy0% 100%

S NCore 1

Core 2

Core 3

(-20 mbsl)

(-17 mbsl)

(-41 mbsl)

Mineralogy0% 100%

Mineralogy0% 100%

FA3

FA5?

FA4?

FA5?

OrFA3?

FA5?

FA4?

Karst

Karst

Figure3:MineralogicalpercentagesfortheUmmerRadhuma(UER)recoveredincores1,2,and3,basedonX-raydiffraction(XRD)generatedbyCoreLaboratories,aswellasfaciesassociations(FA3,FA4andFA5)adaptedfromRiversetal.(2019a,b).Clasticcomponentsincludeclaysandchalcedony.Faciesdesignationswithquestionmarksrepresentuncertainfaciesinterpretationsduetotheobliterativenatureofdolomitetextures.Intervalsinredrepresentkarstsections.NotethattheUERtopsdonotlieatthesamedepthbelowthesurfaceinallcores,noraretheythesamedepthinmetresbelowsealevel(mbsl).

20

X-raydiffraction

Mineralogical characterization of core plugs was carried out in two separate ways, as

similarlydescribedinRyanetal.(2019).Bulkmineralogyforall391coreplugswasquantifiedby

CoreLaboratoriesusingtheirstandardX-raydiffraction(XRD)procedure.Thisinvolvesdispersing

each sample in a dilute sodium hexa-meta phosphate solution that is then centrifugally size

fractionedtoisolateclay-sizedparticles(<2–4µm).Thesuspensionsarevacuum-depositedonsilver

membranefilters,airdried,attachedtoaluminumstubsandanalyzedbothbeforeandafterexposure

toethyleneglycolforaminimumof2hoursat60°C.TheXRDdatawerecollectedwithaPanalytical

automatedpowderdiffractometer(MalvernPanalyticalLimited,Malvern,UK)equippedwitha40kV

Cusource,anX’celeratorlineardetectorusingRealTimeMultipleStripTechnology(RTMS),aNi-

Filterandagraphitemonochromator.A2θscanrangeof4°to70°wasusedatarateof4.2°/minute.

Forclaysamples,a rangeof2.5° to40°atarateof6.4°/minutewasused. Inorder todetermine

individualclaypercentageswithintheclayfraction,integratedpeakareasandempiricalreference

intensityratio(RIR)factorswereutilized.

In order to more fully characterize percent dolomite (relative to calcite), dolomite

stoichiometry and the degree of dolomite cation ordering, additional XRD data was collected at

WesternMichiganUniversityon125ofthe391coreplugs.StandardXRDtechniqueswereemployed

usingCuKαradiationwithaBrukerD2PhaserDiffractometer(Bruker,Billerica,MA,USA).Coreplug

powderswereextractedusinganelectricrotarydrillandfurthergroundbyhandwithamortarand

pestle to homogenize the sample. Samplesweremounted on a Boron-doped silicon P-type zero

backgrounddiffractionplate.AllXRDspectrawerecollectedinthe2θrangeof20to40°withastep

sizeof0.01°andacounttimeof1.0sperstep.Dolomitepercentageswerecalculatedfollowingthe

methodofRoyseetal.(1971),whichincludestakingtheratioofthedolomited(104)peaktothesum

ofthecalcited(104)anddolomited(104)peak.Dolomitestoichiometry(i.e.mol%MgCO3)wascalculated

usingtheequationderivedbyLumsden(1979)whichempiricallyrelatesd-spacingofthedolomite

21

d(104)peaktothemol%CaCO3.Severalinternalstandardswereusedtoshiftpeakpositions.Reeder

&Sheppard(1984)showedthattheequationderivedbyLumsden(1979)canleadtoinaccuraciesof

upto3mol%CaCO3,andthusthestoichiometrydatayieldedbyXRDwascheckedwithbothEDSand

electronmicroprobedata.Thedegreeofcationorderingwascalculatedbytakingtheratioof the

dolomited(015)peaktothedolomited(110)peak(Goldsmith&Graf,1958).

Electronmicroprobeanalysis

Electron microprobe analysis (EMPA) was carried out at the Eugene Cameron Electron

MicroprobeLab,UniversityofWisconsin-Madison.Thesamefourpolishedthinsectionsutilizedfor

SEM-EDSanalysiswerecarboncoatedwithca20nmcarbonandanalyzedwithaCamecaSXFiveFE

electronprobemicro-analyzer(Cameca,Gennevilliers,France).Analyticalparametersincludea15

kV electron beam, 10 nA current, and a 5–10 µm beam (spot) size, and elemental percentage

detection limits of 0.021%, 0.014%, 0.040%, 0.024% and 0.023% for Ca, Mg, Fe, Mn and Sr,

respectively.Fiveormorecrystalswereanalyzedoneachthinsectioncontainingfabric-destructive

dolomite,whileatleastfiveallochemswereanalyzedononemimeticallydolomitizedsample.More

than95pointsweretakenoneachsampleandanalyzedforelementalpercentofCa,Mg,Fe,Mnand

Srutilizinganinternalstandardforeachelement.Onlypointsinwhichthesumelementalmass%

was³98%wereutilized,removingpointsthatincludedsilicates.Thisresultedin³25usefulpoints

for each sample. Dolomite stoichiometry was determined by converting the average elemental

percentofMgandCaforeachsampletomol%MgandCa.

Inductivelycoupledplasma–massspectrometry

Bulkrocksamplesfrom25coreplugsacrossallthreecoreswereanalyzedfortraceelement

concentrations via inductively coupled plasma-mass spectrometry (ICP-MS) at SGS Canada Inc.,

MineralServices.Rocksampleswerepulverizedandpassedthrougha75µmsievebeforeundergoing

22

atwo-acidaquaregiadigestion.ThisdigestionincludesacombinationofHClandHNO3ata3:1ratio,

respectively.Thisparticularacidcompositionwasusedbecauseityieldsthemostaccuratedepiction

of the carbonate geochemistrybecause itwill dissolve carbonateminerals, butdoesnotdissolve

silicatesorgypsum.AnalyticalprecisionforSrandNais±12%and±43%,respectively.

Gypsumstableoxygenisotopes

OnegypsumsamplefromtheUERFormationandtwosamplesfromRuswereanalyzedfor

δ18O at Queen’s University, Kingston. Samples were dissolved in 2N HCl and subsequently re-

precipitatedasBaSO4withasaturatedsolutionofBaCl2.Precipitateswerethenmeasuredfortheir

oxygenisotopiccompositionusingaMAT253StableIsotopeRatioMassSpectrometercoupledtoa

Thermo Scientific TC/EA High Temperature Conversion Elemental Analyzer (Thermo Fisher

Scientific,Waltham,MA, USA). Precision is ±0.5‰. All gypsum δ18O data is reported relative to

VSMOW(Viennastandardmeanoceanwater).

Dolomitestableisotopeanalysis

BulkrockstableisotopedatausedherewerefirstreportedinRiversetal.(2019a).However,

someminormodificationsweremade.First,samplescontaininganyquantityofcalcite(>1%)were

removeddueto itspotentialeffectonbulkrockδ18Oandδ13C.Secondly,samplesrecoveredfrom

unconsolidated intervals due to karstification are not included, such that only lithified host-rock

dolomitesampleswereconsidered.Thisresultedinanalysisof191samples.Datareportedherewere

measuredattheCenterforStableIsotopeBiogeochemistry(CSIB),UniversityofCalifornia,Berkeley

using aGV IsoPrimemass spectrometerwithDual-Inlet andMultiCarb systems (GV Instruments

(Micromass)Limited,Manchester,UK).SampleswerereactedwithH3PO4at90°Cfor10minutes.

ReplicatesoftheinternationalstandardNBS19andtwolaboratorystandards(CaCO3 IandII)are

analyzedwitheachrun.Analyticalprecisionisapproximately±0.05‰forδ13Cand±0.07‰forδ18O.

23

Valuesforall isotopiccompositionsarereportedrelativetotheViennaPeedeeBelemnite(VPDB)

standard.

Results

Bulkrockmineralogy

ThebulkmineralogyoftheupperUERisdominatedbydolomite(62to100%)inallthree

cores, althoughmineralogical differences do occur laterally between and stratigraphicallywithin

cores(Fig.3).GypsumisonlypresentintheuppermostUERofcore1(above93m)inquantities

rangingfrom0–21.2%.Thin(<1m)intervalsofcalcitemixedwithdolomiteandpalygorskiteare

onlypresent towardsthetopof theUER incores2and3,with thecalciteandminoramountsof

chalcedonycomprising≤80%ofbulkrock.Palygorskiteandchertoccurthroughoutallthreecores,

withtheircombinedabundancetypicallybeing<30%.Adistinctivecentimetre-thickintervalrichin

chertisobservedinallthreecores(forexample,127mincore1,88.5mincore2and71mincore

3),withtwosuchbandsobservedincore1,andthesearediscussedinmoredetailbelow.

Thinsectionpetrography

Dolomite

Meandolomitecrystalsizeincreasesfrom<10µmatthetopoftheUERto>200µmatthe

lowestpartofcore1.Thedominantdolomitetextureintheuppermost5moftheUERincore1is

mimetic,wherebymicrocrystallinedolomitehaspreservedwackestonesandpackstonesdominated

bymiliolidsandothersmallbenthicforaminifera(Fig.4A).Incontrast,themiddle(ca65–110m)

sectionofthecoreconsistsofrelativelycoarse(>30µm)planar-edolomitecrystalswithacloudy-

coreclear-rim(CCCR)fabric(Fig.4B).Thebaseofthecoreisdominatedbylarge(>100µm)planar-

sandnonplanardolomitemosaics(Fig.4C).Althoughmostofcore1consistsoffabric-destructive

dolomite, some echinoderms, green algae and large benthic foraminifera (LBF) are preserved

24

throughdolomitization(Fig.5AandB).Additionally,manymouldsintheshapeofLBFarefrequently

observed(Fig.5C).

Cores2and3differinthatthebottomhalfoftheUERisdominatedbyplanar-stononplanar

dolomites,while the tophalf ismainly comprisedof planar-e sucrosicdolomites.Meandolomite

crystalsizerangesfrom30–200µminbothcores.Finercrystalsaretypicallyplanar-etononplanar,

whilecoarsercrystalsarenonplanarandplanar-s.Infrequently,bothtexturesandcrystalsizesoccur

ina single sample (Fig.4D).Similar tocore1, the fabricofechinodermsandLBFsarepreserved

throughdolomitization (Fig. 5D), and isolatedLBFmoulds are alsopresent.Dissimilar to core1,

100 µm 100 µm

200 µm100 µm

A B

C D

100 µm

Figure4:Planepolarized light (PPL) thin section imagesofobserveddolomite textures. (A)Core1,65.85m.Mimeticdolomitethathaspreservedmiliolidsandothersmallbenthicforaminifera.(B)Core3,86.0m.Planar-edolomitedisplayingcloudycoreclearrim(CCCR)fabrics,inwhichcoresarecommonlypartiallytocompletelydissolvedbutrimsremainintact.(C)Core1,132.76m.Nonplanardolomitewithlittleintercrystallineporespace.(D)Core2,97.75m.Abimodaldistributionof large(100to200µm)nonplanardolomitemosaicsandrelatively lesscoarse(<100µm)planar-eCCCRdolomites isexhibited.

25

however,dolomitetowardsthetopoftheUERincores2and3isnon-mimetic,andmiliolidsand

smallbenthicforaminiferaareabsent.

Pyriteandchertbands

AlthoughnotdetectedbyXRD,smallamountsofpyriteareobservedpetrographicallyinall

cores,whereitoccurswithintheintercrystallineporesofdolomitecrystals(Fig.6A).Withinthechert

bandspanningallwells,planar-edolomitecrystals(>30µm)arepartiallytocompletelyreplacedin

allthreecores(Fig.6B).Incore1,wheretwochertbandsareobserved,thetopbandischaracterized

byplanar-edolomitesreplacedbychert,similartothatobservedincores2and3.Ahalfmetrelower

250 µm

1 mm

A B

C D

200 µm

100 µm

Figure5:Thinsectionimagesinplanepolarizedlight(PPL)ofvariousallochemfeatures.(A)Core1,100.55m.Numerousechinodermfragmentshaveundergonefabricpreservingreplacementbydolomite(redarrows).(B)Core1,103.3m.Largebenthicforaminifera(>250µm)havebeenreplacedbydolomitethatpreservedthefabric(redarrows).(C)Core1,104.06m.Moulds <1mm are observed and take the shape of precursor allochems. (D) Red arrow points to a large benthicforaminifer(Nummulites).Thedolomitizedlargebenthicforaminiferillustratestexture-preservingnon-mimeticdolomite.

26

(ca127m);however,foraminiferatestspreservedinthechertbandcontainthinrimsthatappearto

be carbonate (Fig. 6C and D). Although SEM-EDS analysis of this thin section was unable to

characterize the elemental composition of the rims, bulk XRD shows stoichiometric (49.8mol%

MgCO3)andwell-ordered(0.78)dolomite.

Clay,gypsum,calciteandnon-bandedchalcedony

Theothermineralsobservedinthecores–palygorskite,gypsumandcalcite–primarilyoccur

as cements, although some gypsum and calcite is also replacive. The relationship between

palygorskite and dolomite is similar in all three wells. Palygorskite always occurs as an

A B

C D

200 µm 200 µm

200 µm200 µm

Figure6:Non-carbonatediageneticfeatures.Allimagestakeninplanepolarizedlight(PPL)unlessotherwisenoted.(A)Core3,81.0m.Dolomitecrystals(tan/grey)arecementedandpartiallyreplacedbypyrite(opaque,black).(B)Core1,126.83m.Dolomitecrystals(grey)encasedinchert.Partialreplacementindicatedbybothchertwithindolomitecrystals(redarrows)andthefactthatmostdolomitecrystalsincentreportionoftheimage‘float’withinthechert.(C)Core1,127.3m.SilicifiedNummulitesdisplayingteststhatstillcontaincarbonatematerial(redarrow).(D)Cross-polarizedimageof(C).

27

intercrystallineorintracrystallinepore-fillingcementwithinthedolomite,asevidencedbydolomite

crystals included within palygorskite and palygorskite observed within partially to completely

dissolved dolomite cores (Fig. 7A). Gypsum occurs as an intercrystalline cement that surrounds

dolomite and palygorskite (Fig. 7B), and within the intrafossil pores of mimetically replaced

foraminifera in the uppermost 5 m (Fig. 7C). Frequently, dolomite rhombs float within gypsum

crystals,indicatingthatsomegypsumisreplacive(Fig.7D).Thecalcite-bearingintervalsincores2

and3arecomprisedoflarge(>500µm)polyhedralcalcitecrystals,whichcommonlyincludepartially

dissolveddolomitecrystals(Fig.8A)thatarepartially tocompletelyreplacedbythesurrounding

calcite(Fig.8B).Theseintervalsalsocommonlyexhibitpalygorskiteasanintercrystallinecement

surroundingcalciteanddolomitecrystals(Fig.8C).Somecalcitecrystals(Fig.8C)exhibittextures

similartoMicrocodium(sensuKabanovetal.,2008)andMicrocodium(a)(sensuEsteban,1974).Some

ofthesecoarsecalcitecrystalsarealsopartiallyreplacedbychalcedony(Fig.8D).

Karst

Many of the features described above are also observed in the clasts comprising the

incompetentintervalsofcores2and3thatoccurbelowthekarstedintervals.Featuresincludeplanar

dolomite,nonplanardolomite,microcrystallinedolomiteandcalcitecrystalsthatincludedolomite

crystals(Fig.9).

Dolomitemineralogy

DolomitestoichiometrydataarepresentedinFigs10B,11Band12B,andshowanaverageof

50.3mol%MgCO3(±0.45%).Inallcores,dolomitestoichiometrycorrelateswithdolomitetexture.

Incore1,mimeticandnonplanardolomiteshavesimilarstoichiometryvaluesaveraging49.7%and

50.1%,respectively.Planar-eCCCRdolomites,incontrast,averageca1%higherMgat50.7%.At-

testshowsthatthisdifference isstatisticallysignificantat the99%confidence(𝛂=0.01)withp-

28

values7.5x10-9and2x10-10,respectively.NonplanardolomitesaresignificantlymoreMg-richthan

mimeticdolomitesat99%confidence(𝛂=0.01)withap-valueof6.5x10-5.Incores2and3,Mg-

content ishighestat the topof theUERanddecreaseswithdepth(Figs11Band12B). Incore2,

stoichiometryrangesfrom50.9%atthetopoftheUERto50%atthebottomofthecore(R2=0.86).

Similar tocore1,planar-edolomites incore2areslightlymoreMg-rich(50.6%)thannonplanar

dolomites(50%).At-testdeterminedthatthisdifferenceissignificant(𝛂=0.01;p-value=1x10-7).

Stoichiometryincore3rangesfrom50.7%atthetopto49.3%atthebottom(R2=0.49).Planar-e

200 µm100 µm

A B

C D

100 µm 100 µm

Figure7:Diageneticrelationshipsbetweendolomite,palygorskiteandgypsum.Allimagesinplanepolarizedlight(PPL).(A)Core3,74.3m.Palygorskite(lightgreenhaze)isobservedasacementarounddolomitecrystals(whitearrows),aswellasinsideofpartiallydissolveddolomitecores(redarrows).(B)Core1,73.44m.Palygorskite(redarrows)anddolomiteareencasedwithingypsumcement(white).(C)Core1,66.48m.Gypsum(white)hasoccludedmuchoftheporespacebetweenmimeticallydolomitizedforaminifera,aswellasminoramountsofintrafossilporespace.(D)Core1,74.95m.Planar-e,cloudycoreclearrim(CCCR)dolomitecrystals(grey)floatwithingypsum(white)indicatingthatthisgypsumis,inpart,replacive.

29

dolomitesaremoreMg-richincore3(50.2%)thannonplanardolomites(49.7%)(t-test;𝛂=0.01;p-

value=7x10-4).

ThestoichiometrydeterminedbyEMPA(Table1)was48.5%(±0.7%)and48.6%(±0.01%)

for themimeticandnonplanardolomite, respectively.For theplanar-esamples, theMg%ranged

from49.4to49.7%.Asawhole,170EMPAanalysesindicatethatplanar-edolomitesaresignificantly

moreMg-richthanmimeticornonplanardolomitesbasedonat-testat99%confidence(𝛂=0.01)

withp-valuesof3.4x10-8and3.2x10-8,respectively.AnalysisbySEM-EDSyieldedsimilarresults.

Themimeticandnonplanarsamplesaveraged48.5%and48.6%Mg,respectively.Thetwoplanar-e

500 µm

100 µm

A B

C D

500 µm

100 µm200 µm

500 µm

Figure 8: Thin section images in plane polarized light (PPL) displaying the relationship between calcite, dolomite,palygorskiteandchalcedony.(A)Core3,62.5m.Dolomitecrystals(grey)containingpartiallydissolvedcoresareincludedwithinlargecrystallinecalcitecrystals(pink).(B)Core2,61.05m.Remnantsofdolomite(darkgrey/black)inclusionsfloatwithina largecrystallinecalcitecrystal(pink).(C)Core2,61.05m.Alarge(>500µm)cal-citicMicrocodiumstructureincludedwithinpalygorskite(lightgreenhaze).Notethatdarkinclusionsaredolo-mitecrystalsthathavebeenpartiallytocompletelyreplacedbythecentralcrystalofcalcite.(D)Core2,65.1m.Calcitecrystals(pink)arepartiallyreplacedbychalcedony(white).Darkmatrixsurroundingcrystalsconsistsofpalygorskite.

30

samplesaveragedMg%between49.5%and49.7%.At-testat99%confidence(𝛂=0.01)determined

thatplanar-edolomitesweresignificantlymoreMg-rich thanmimetic (p-value=1.1x10-10)and

nonplanar(p-value=3.5x10-18)dolomites.

500 µm100 µm

A B

100 µm100 µm

Figure9:Thinsectionimages(planepolarizedlight–PPL)fromtheunconsolidatedsectionofcore2.(A)From118.1m.Clastsofdark,microcrystallinedolomite(left),calcite(pink)withinclusionsofreplaceddolomitecrystals(centre),andnonplanardolomitemosaics(right).(B)Largeclastsofnonplanartoplanar-sdolomite‘floating’withinamatrixconsistingof10to20µmsizedplanar-edolomitecrystalswithmuchintercrystallineporespace.

Figure 10: Core profile of core 1 displaying mineralogy and facies associations (A) with corresponding dolomitestoichiometry(mol%MgCO3)(B),cationordering(C),bulkdolomiteδ18Ovalues(D),bulkdolomiteδ13Cvalues(E),andSrand Na concentrations (F). Dolomite textures are represented as different stratigraphic shades. Red arrows point tonegativecarbonisotopeexcursionsthatpotentiallyrepresentthePalaeocene–EoceneThermalMaximum.

31

Figure11:Themineralogyandfaciesassociationsofcore2(A)correlatedtodolomitestoichiometry(mol%MgCO3)(B),cationordering(C),bulkdolomiteδ18Ovalues(D),bulkdolomiteδ13Cvalues(E),andSrandNaconcentrations(F).Dolomitetexturesarerepresentedasdifferentstratigraphicshades,andtheredarrowpointstoanegativecarbonisotopeexcursionthatpotentiallyrepresentsthePalaeocene–EoceneThermalMaximum.

Figure 12: Depth profile of core 3 exhibiting mineralogy and facies associations (A) corresponding to dolomitestoichiometry(mol%MgCO3)(B),cationordering(C),bulkdolomiteδ18Ovalues(D),bulkdolomiteδ13Cvalues(E),andSrandNa concentrations (F).Dolomite textures are represented as different stratigraphic shades.Red arrow indicates anegativecarbonisotopeexcursionthatpotentiallyrepresentsthePalaeocene–EoceneThermalMaximum.

32

Table1.Stoichiometrystatisticsfor4UERdolomitesanalyzedbyXRD,EMPA,andEDSDepth(m)

Dolomitetexture

XRDMg% EMPAaverageMg%

±1𝞼(%) n

EDSaverageMg%

±1𝞼(%) n

65.85 Mimetic 49.6 48.5 0.7 25 48.5 0.01 76

78.19 Planar-e 50.7 49.4 0.6 67 49.5 0.01 82

97.26 Planar-e 50.9 49.7 0.8 42 49.7 0.01 82

126.38 Nonplanar 50 48.6 0.01 40 48.6 0.01 90

Dolomite cation ordering also correlateswith dolomite texture (Figs 10C, 11C and 12C).

Mimeticdolomiteshavethelowestaveragedegreeofcationordering(0.57±0.06),whilethecation

orderingofplanar-e(0.77±0.07)andnonplanar(0.71±0.08)dolomitesaresignificantlyhigher(t-

test,𝛂=0.01;p-values=6.5x10-9and7.5x10-7,respectively).

Dolomitestableisotopeanalysis

Dolomiteδ18Oandδ13CvaluesarereportedrelativetodepthinFigs10to12andinTable2.

Ingeneral,dolomiteδ18Ovaluesarebetween-2.5‰and+1‰,whereasδ13Cvariesbetween-3.5‰

and+2‰.Averagedolomiteδ18Oandδ13Cvaluesfrom191samplesare-0.5‰(s=0.9)and-0.1‰

(s=1.2),respectively.δ18Odecreaseswithdepthincore1(R2=0.50),whereas it increaseswith

depthincores2(R2=0.57)and3(R2=0.70).Thenonplanardolomitesthatliebelowthekarstified

intervals in core2 (>105mdepth) and core3 (>91mdepth) areca 1‰morepositive than all

dolomitesasawhole(average=0.39‰and+0.45‰,respectively).δ13Cismorevariableinallcores

andalthoughnoapparenttrendwasobservedwithdepth,multipleisotopeexcursionsareidentified.

Incore1,a-2‰excursionisobservedat122m,a+3.5‰excursionat104m,anda-3‰excursion

at96m.Incores2and3,a-4.5‰and-3.8‰excursionisobservedat105mand108m,respectively.

33

Table2.UERDolomiteStableIsotopeAnalysisCore1 Core2 Core3

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

60.3 -0.88 0.32 63.00 -0.81 -0.46 51.85 -1.72 -1.67

60.3 -1.22 -1.13 63.90 -0.86 -0.82 52.64 -1.17 -1.86

61.29 -1.77 -0.31 65.95 1.37 -0.47 53.30 0.27 -1.28

61.30 -1.69 -0.45 66.50 1.33 -0.28 54.45 0.01 -1.73

61.8 -2.12 0.29 67.50 -0.39 -0.86 54.78 -1.81 -2.28

62.25 -0.35 0.18 69.05 1.44 -0.52 55.53 0.34 -1.46

62.70 -0.41 -0.31 69.50 1.42 -0.48 56.56 -2.03 -2.00

64.55 -1.06 0.19 70.00 1.01 -0.58 57.40 -0.79 -1.88

65.2 -0.52 0.38 70.50 1.40 -0.88 58.15 -2.87 -1.16

66.5 -0.24 0.16 70.50 1.35 -1.02 58.42 -3.47 -1.36

66.77 -0.30 0.13 70.90 0.99 -0.66 59.04 -0.74 -1.33

68.00 -0.69 -0.18 71.50 0.66 -0.64 63.30 -0.85 -1.45

69.00 -0.79 -0.09 72.00 0.36 -0.80 63.85 -0.56 -1.20

69.74 -1.00 -0.41 73.85 -0.24 -0.89 65.69 -1.79 -1.01

69.97 -0.83 -0.34 74.30 -0.54 -1.03 66.40 -1.82 -1.05

70.65 -0.60 -0.04 75.00 -0.98 -1.17 70.54 -0.88 -0.66

71.00 -0.67 -0.76 76.50 -0.47 -1.14 71.54 -0.91 -0.29

72.00 -1.06 -0.78 77.00 -0.53 -1.19 71.94 -0.74 -1.52

73.00 -0.53 -0.82 77.35 -1.17 -1.85 72.35 -0.91 0.68

74.00 0.17 -0.52 78.50 -0.73 -1.10 74.05 -0.35 -0.24

75.00 -0.09 -0.59 82.00 -0.76 -0.63 91.10 0.16 1.17

75.35 0.13 -0.93 83.00 -0.97 -1.39 92.28 0.05 1.00

75.82 0.06 -1.09 83.80 -0.68 -1.07 94.00 -0.88 0.81

76.15 0.02 -0.73 84.45 -1.05 -0.59 97.75 -0.92 0.31

77.00 -0.36 -0.97 86.00 -0.87 -0.50 98.60 -0.19 0.70

78.20 -0.67 -0.85 87.00 -0.93 -0.03 99.70 1.19 0.20

79.00 -0.63 -1.04 105.50 -2.31 0.15 101.40 -1.82 -0.46

80.00 -0.97 -1.24 105.90 -1.61 -0.11 101.85 -0.55 -0.07

81.00 -0.70 -1.05 106.50 -1.38 -0.01 102.25 0.04 0.15

82.00 -1.17 -1.47 107.05 -1.21 0.20 102.82 0.60 0.46

83.00 -0.72 -1.23 107.55 -0.83 -0.06 103.58 -2.45 -0.89

84.00 -0.87 -2.08 108.00 -2.12 -0.76 104.15 -2.05 -0.83

84.25 -0.41 -1.66 108.30 -1.01 0.24 104.48 0.05 0.54

85.00 -0.46 -1.54 109.00 -0.80 0.40 106.38 -1.36 -0.39

86.00 -0.30 -1.11 109.50 -0.10 0.60 106.90 0.02 0.39

87.00 -0.05 -0.87 110.00 0.08 0.50 107.89 -2.40 -0.67

34

Table2.(continued)

Core1 Core2 Core3

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

88.00 0.58 0.05 110.45 0.57 0.89 108.35 -2.57 -0.72

88.92 0.33 -0.16 111.00 0.94 -0.85 109.10 -0.72 0.44

90.00 1.30 -0.69 111.50 1.45 0.82 110.05 -1.59 -0.02

91.00 0.29 -0.98 112.00 1.52 0.87 110.45 0.54 0.29

91.20 0.08 -1.49 112.50 1.41 0.29 111.45 1.20 0.99

92.00 0.34 -0.89 113.20 1.72 0.52 112.00 1.27 1.16

93.00 0.30 -1.18 113.50 1.77 0.73 113.50 1.04 0.64

94.00 0.22 -1.52 113.70 1.25 0.47 114.73 1.59 1.25

95.00 0.33 -1.22 113.70 1.67 0.99 115.22 1.05 0.91

96.00 -1.63 -1.62 114.00 1.65 0.69 115.96 1.39 0.81

97.00 -1.20 -1.20 114.20 1.82 0.74 118.10 1.45 0.87

98.00 -0.52 -0.89 114.90 1.82 0.66 118.95 0.80 0.86

99.00 -0.90 -1.02 115.60 1.78 -1.43 120.65 0.33 1.06

100.1 -0.10 -1.29 115.95 1.79 0.78 121.17 1.30 1.39

100.50 0.40 -1.24 116.25 2.06 0.99 121.30 0.97 1.02

102 0.94 -1.41 117.00 1.80 0.48 122.13 1.23 0.97

103.1 1.23 -1.20 117.50 1.56 1.09

104.35 1.55 -0.92 117.80 1.78 0.89

105 1.16 -0.93 118.36 1.99 0.96

106.00 0.54 -1.30 118.85 2.05 0.40

107.00 0.34 -1.29 119.50 2.10 0.51

108.00 0.01 -1.10 120.00 2.22 -0.18

109.00 -0.71 -1.74 120.00 2.04 0.33

109.20 -0.63 -1.74

110.00 -1.38 -1.63

112.00 -1.94 -2.43

114.00 -0.92 -1.57

116.00 -1.07 -1.24

118.00 -0.55 -1.41

120.00 -1.07 -1.56

120.75 -1.61 -1.52

122.00 -1.58 -1.69

125.00 -0.69 -1.32

125.10 -0.52 -1.09

126.90 0.25 -2.01

127.85 0.41 -1.65

35

Table2.(continued)

Core1 Core2 Core3

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

Depth(m)

δ13C(‰VPDB)

δ18O(‰VPDB)

128.00 0.60 -1.49

130.5 0.62 -1.37

131.1 0.20 -2.29

132.7 0.14 -1.33

134 0.26 -1.24

134.4 0.15 -1.33 Overalltotal:

134.7 -0.18 -1.57 Average -0.14 -0.52

134.70 -0.32 -1.68 Stddev. 1.17 0.91

Dolomitetraceelementgeochemistryandgypsumstableoxygenisotopes

ResultsofICP-MSanalysisforSrandNaforcores1,2,and3arepresentedinFigs10F,11F

and12F,respectively.Ingeneral,Srlevelsforalldolomitesarelow,rangingfromca40to150ppm,

averaging101ppm.Sodiumconcentrationsaremoderateandrangefrom<100to600,averaging342

ppm.

Theresultsofgypsumδ18OanalysisarepresentedinTable3.Theδ18Oofagypsumsample

from the UER was measured to be +15.4‰ VSMOW. Gypsum samples from the overlying Rus

Formationhaveasimilar isotopiccomposition,rangingfrom+15.7to+15.9‰VSMOW.Theδ18O

valuesareslightlymorepositivethanexpectedbasedonthesecularevaporiteδ18OcurveofSealet

al.(2000),whichshowsthatCenozoicmarineevaporitesrangefrom+12to+14‰VSMOW.

Table3.Qatargypsumδ18OanalysisFormation Depth(m) δ18O(‰VSMOW)

Rus 50.45 +15.7

Rus 53.24 +15.9

UER 90.4 +15.4

36

Discussion

Paragenesisinterpretation

TheinterpretedparageneticsequencepresentedinFig.13indicatesthattheUERofQatar

has a multi-component diagenetic history. Given the petrographic relationships observed,

dolomitization appears to be the earliest diagenetic process affecting the UER as evidenced by

dolomitecrystalseitherbeingincludedwithinorreplacedbyallotherdiageneticmineralphases.

Dolomite is includedwithin and replaced by the chert band (Fig. 6B to D),which indicates that

dolomitizationoccurredpriortotheregionalsilicificationevent(Ecclestonetal.,1981).Moreover,

whendolomitecrystalsareobserved inassociationwithpalygorskite,calciteand/orgypsum, the

dolomitecrystalsareeithercementedtogetherbythesemineralphasesorobservedasinclusions

withinauthigeniccalciteorgypsumcrystals.Thepresenceofpyriteintheintercrystallineporesalso

suggeststhatdolomitizationalsooccurredpriortopyriteformation.Dolomitedissolutionobviously

followeddolomitization,andthepresenceofauthigenicpalygorskite(Fig.7A;seealsoRyanetal.

2019)andcalcite(Fig.8)inthedissolvedcoresofmanydolomitecrystalsindicatesthatdolomite

dissolutionisconcurrentwithorpre-datesthesephases.

The silicificationevent resulting in the chertband thathasbeenmappedasa continuous

horizonacrosseasternandsouthernQatar(Ecclestonetal.,1981;Riversetal.,2019)isinterpreted

to post-date dissolution of dolomite. It is, however, difficult to discern whether chertification

occurredexclusivelypriorto,orconcurrentwith,theotherdiageneticmineralsduetoanabsenceof

cross-cuttingrelationships.Butts&Briggs(2011)havearguedthatretentionofcrystalstructures

duringchertificationisanindicatorofearlyreplacement.Thelackofcross-cuttingrelationscould

alsomeantheotherphaseswerenotyetpresentwhenthatbandformed.Giventhatthechertband

37

is stratigraphically lower than intervals containing other diagenetic minerals, it is unlikely that

chertificationpost-datesmineralssuchaspalygorskite,calciteorgypsum.Ifthechertbandformed

early,itlikelyformedbeforetheupper75mwasdepositedandthuspre-datesauthigenicphasesin

theupper75m.

Diagenetic EventDolomitization

Dolomite dissolution

Chertification

De-dolomitization via Microcodium

Smectite replaced by palygorskite

Replacement ofcalcite by chalcedonyGypsum cementation

Pyrite cementation

Karstification

Relative Order1

8?

8

7

6

5

4

3

2

9?

Figure 13: Paragenetic sequence for the Umm er Radhuma (UER) in Qatar based on petrographic cross-cuttingrelationshipsandgeochemicaldata.Thetimingofdolomitizationintheuppermost50to60mofthecoresisesti-matedtobeapproximately56MabasedoncarbonisotopeexcursionsinterpretedasthePalaeocene–EoceneThermalMaximum(seetextfordetails).LatestagegypsumcementsinterpretedasbeingassociatedwithbrinesfromtheoverlyingRusFormation(ca52Ma)suggestthatdiageneticevents1to7wereconcludedby52Ma.Karstfeaturesincores2and3cross-cutmostofthediageneticfeaturessuggestingthatkarstificationoccurredverylateintheparageneticsequence.Karstisnotdistinctincore1;however,wherethelate-stagegypsumcementispresent,sothetimingofkarstrelativetogypsumcementationisuncertain.

38

The fifth diagenetic event was the formation of palygorskite, which was previously

interpreted by Ryan et al. (2019) to be a replacement of smectite that in turn formed fromMg

liberatedbythedissolutionofdolomite.Thisclayoccursbothasanintercrystallinecementandas

an intracrystalline cement inside of partially to completely dissolved dolomite cores. Such

relationships imply that dolomitization occurred before the palygorskite, and that dolomite

dissolutionwasconcurrentwithpalygorskiteprecipitation.

Giventhepetrographicrelationshipbetweenthecoarsecalcitecrystalsandthesurrounding

palygorskiteincores2and3,palygorskiteformationlikelyfollowedtheformationofMicrocodium

andcrystallinecalcite(Fig.8C).However,dolomitecrystalsarealsoincludedwithinMicrocodium,

and arepartially to completely replacedby calcite, i.e. dedolomitized (Fig. 8). This calcite is also

partially replaced by chalcedony sporadically throughout the UER (Fig. 8D). As a result, the

parageneticsequenceproposedinFig.13isdolomitization,de-dolomitizationviaMicrocodiumconcurrent

withalterationofsmectitetopalygorskite,andfinallyreplacementofcalcitebychalcedony.

The observation that gypsum contains dolomite and palygorskite inclusions leads to the

interpretationthatgypsum(asacementandareplacementofdolomite)isfollowedbyformationof

palygorskite,and thusallMicrocodium calcitealso.Gypsum is the lastdiageneticmineral tohave

formedintheUERincore1.Althoughmetre-scalegypsumbedsarecommonintheoverlyingRus

Formation,thereisnoevidenceofbeddedevaporitesintheUERinQatar(Riversetal.,2019a).This

observation, coupledwith the petrographic relationships documented above, suggests it ismost

likelythatauthigenicgypsumintheUERoriginatedduringdepositionoftheRus.

Finally,theUERisoverprintedbylatestagekarst,whichisobservedonlyincores2and3.

Rockpowderharvestedfrompoorrecoveryzonesfromthesecorescontainsrelictsofthediagenetic

features described above. It is therefore likely that karstification occurred after deposition and

diageneticalterationoftheUER.Thetimingofkarstificationrelativetotheauthigenicgypsumincore

1isunknown,however.Theonlysignsoferosionincore1occuratthetopoftheUERatthecontact

39

withtheoverlyingRus(Riversetal.,2019a).Itmaybepossiblethatthiserosionalcontactistime

equivalenttothekarstificationthatoccurredincores2and3,whichwouldsuggestthatkarstification

happenedaftertheformationofauthigenicgypsum.Itisunlikelythatkarstificationincores2and3

originatedfromthedissolutionofevaporitesbecause,aspreviouslynoted,nobeddedorauthigenic

gypsumispresentintheUERorRusincores2and3.However,giventhatevidenceofkarstification

isabsentincore1,thetimingofkarstificationrelativetoauthigenicgypsumcannotbeconclusively

determined.

Regionaldifferencesindepositionalenvironmentanddolomitizationmechanism

Salleretal.(2014)andPollittetal.(2012)investigatedtheparageneticsequenceoftheUER

andinterpretedthemechanismresponsiblefordolomitizationinSaudiArabiaandKuwait.Salleret

al. (2014) and Pollitt et al. (2012) described the UER as a burrowed peloidal wackestones and

packstones interbedded with laminated mudstone, wackestone and packstone beds with minor

molluscfragmentsandbenthicforaminifera.Theauthorsalsonotedthattheburrowedfaciespass

gradationally upward to laminated intervals, the tops ofwhich are sharpwith distinct erosional

truncations.Salleretal.(2014)interpretedtheburrowedintervalstoreflectdepositioninasubtidal

to lower intertidal setting, and the laminated intervals as upper intertidal and supratidal

environments.

BasedontheobservationofsupratidalfaciesandbeddedevaporitesnearthetopoftheUER,

Salleretal.(2014)interpretedUERdolomitestoformasaresultofhypersalinereflux.Giventhatthe

UERwas deposited in an upper intertidal to supratidalmarine setting situated stratigraphically

belowtheoverlyingRusevaporites,suchan interpretation isgeologicallyreasonable.Salleretal.

(2014)suggested thatbeddedanhydrite formedprior toUERdolomitization inSaudiArabiaand

Kuwaitbasedonverticalcrystalgrowthstructuresthatwereobservedinassociationwithlaminated

dolomitized stromatolitic cyanobacterial mats towards the top of the UER (Saller et al., 2014).

40

However,Salleretal.(2014)alsoreportedthatanhydriteandgypsumcommonlyincludedolomite

crystals.

ThepresenceofinterbeddedevaporiteswasusedbySalleretal.(2014)asoneoftheprinciple

piecesofevidenceinsupportofthehypothesisthatdownwardrefluxingbrineswereresponsiblefor

mostofthedolomiteintheUER.BuildingonearlierworkbyPollittetal.(2012),Salleretal.(2014)

hypothesizedthatthewaterstowardsthetopoftheUERweresaturatedwithrespecttogypsum,

resultingingypsumprecipitation,andthusadrawdownofCa2+fromthefluids.Thoseauthorsargued

that this process drove dolomitization by the downward flow of dense evaporative brines with

elevatedMg/Ca(seefig.16ofSalleretal.,2014).Thismodelisconsistentwiththeobservationthat

microcrystalline(mimetic)dolomitesoccurnearthetopoftheUER,butcoarsendownward(Pollitt

et al., 2012; Saller et al., 2014). Previous work suggests that fluids with a high degree of

supersaturation with respect to dolomite tend to precipitate microcrystalline, fabric-preserving

dolomitecomparedwithlesssupersaturatedfluids(Sibley&Gregg,1987).Basedonthisreasoning,

Pollittetal.(2012)andSalleretal.(2014)bothreasonedthatasdolomiteprecipitatedintheupper

UER,thesaturationstateofthedownwardrefluxingfluidwasreduced,thuscausingfewerdolomite

nuclei to form and slower crystal growth rates, which produced the coarser fabric-destructive

dolomitesobservedlowerintheUERofSaudiArabiaandKuwait.

IncontrasttotheobservationsofSalleretal.(2014),whichshowpredominantlyintertidal

andsupratidalfaciestowardsthetopoftheUER,theobservationsinRiversetal.(2019a)suggest

thatUERinQatarisdominatedbyopen-marine,subtidalfacieswithsomepartiallyrestrictedfacies

atthetopoftheUERincore1.ThesedifferencessuggestthatthedepositionalenvironmentinQatar

differed from those experienced in Saudi Arabia and Kuwait, possibly due to different tectonic

histories.Salleretal.(2014)notethattheUERthinsovertheWafraanticlineandinterpretedthisto

indicatethestructurehadalreadyformedatopographichighduringdepositioninthePalaeocene

andEocene.Salleretal.(2014,fig.16)interpretmultiplestructuralhighs,whichresultedinsemi-

41

restricted environments that would have promoted the formation of hypersaline brines. The

structuralhistoryoftheUERinQatardiffersinthatthemajorstructuralhighinthecentreofQatar,

whichseparatescore1fromcores2and3,wasformingsimultaneouslywithUERdeposition,and

hadnotfullydisconnectedthetwobasinsuntilthestartofdepositionoftheoverlyingRus(Riverset

al., 2019a).This scenarioalsoexplainswhymostof the facies in the three researchcores reflect

depositioninsubtidalenvironmentsloweronthecarbonateramp,withonlytheuppermost7mof

core1reflectingsemi-restrictedconditions.Itfollowsthatdifferencesindepositionalenvironments

mayhaveresultedindifferentmodesofdolomitizationinQatarcomparedwithSaudiArabiaand

Kuwait.

Petrographicrelationshipbetweendolomiteandgypsum

Petrographic relationships between dolomite and gypsum described above, indicate that

dolomite formed before precipitation of gypsum. In particular, gypsum is observed both as a

poikolitopiccementaswellasareplacementofdolomitecrystals(Fig.7),indicatingthatauthigenic

gypsum post-dates dolomitization. This in turn suggests that dolomitization was unrelated to

gypsum-saturatedfluids,becausegypsumclearlyformedafterdolomite.However,thisobservation

alonedoesnotprecludethepossibilitythatgypsumbedsweredeposited,andtheresultingfluids

with elevated Mg/Ca ratios subsequently dolomitized the underlying strata. Reactive transport

modelling,forinstance,predictsevaporitecementsassociatedwithrefluxdolomitizationbasedon

the premise that dolomitization releases excess Ca2+ into the pore-fluids and drives subsequent

gypsum precipitation (Jones & Xiao, 2005; Al-Halal et al., 2012). This implies that although

dolomitizationprecedestheauthigenicevaporitecementation,anoverlyingbrinepoolassociated

withthedepositionofRusgypsumbedsmayhavebeenresponsibleforrefluxdolomitizationinthe

UERofQatar,similartoSaudiArabiaandKuwait.However,duetothelackofbeddedevaporitesin

theUERofincore1,andthecompletelackofevaporitesintheUERandtheoverlyingRusincores2

42

and3,clearpetrographicevidenceofrefluxinghypersalinebrinesismissing,althoughthislackof

evidencealonedoesnotprecludeahypersalinerefluxorigin.

Dolomitetextures,stoichiometryandcationordering

SimilartoobservationsreportedinPollittetal.(2012)andSalleretal.(2014),thedolomites

nearthetopofcore1inthepresentstudyarefinelycrystallineandmimetic,whereascoarserplanar-

edolomitesareunderlyingmimeticdolomitesandthecoarsestnonplanardolomitesareobservedat

thebottom.Althoughmimeticdolomitesarenotobservedincores2and3,dolomitecrystalscoarsen

fromplanar-edolomiteatthetoptononplanardolomitesatbottom.Asdescribedabove,thisincrease

in crystal size could reflect a decrease in the saturation state of the fluid becauseMg/Ca ratios

decreaseddownwardasdolomitizationoccurred(Sibley&Gregg,1987;Jones&Xiao,2005).

However, an alternative hypothesis is that dolomite textures in the UER may be facies

controlled.Mimeticdolomites,forinstance,areassociatedwithfaciesFA3,whichischaracterizedby

muddiersedimentsoccasionallyrichinsmallbenthicforaminifera(Riversetal.,2019a).Incontrast,

theplanar-eandnonplanardolomitesarepredominantlyobservedinfaciesFA4andFA5,whichare

characterizedbymud-leanpackstonesrichinadiversefaunalassemblage(Riversetal.,2019a).Itis

therefore likely possible that mud content of the precursor facies was the primary control on

subsequent dolomite textures. Specifically, themuddier FA3 facieswould have a higher reactive

surfaceareafordolomitenucleation,andnucleationmayhaveoutpacedgrowthratesresultinginthe

mimeticnatureofthedolomite(Sibley&Gregg,1987).Inthecoarser,grain-richFA4andFA5facies,

thereislessreactivesurfaceareaandthuscrystalgrowthratesmayhavebeenfasterthannucleation

rates,thusresultingincoarseplanarandnonplanartextures(Sibley&Gregg,1987).Followingthis

logic,dolomite texturesareexplainedbyvariations in thecarbonateprecursor faciesrather than

decreasingMg/Caratiosasdolomitizingfluidsrefluxeddownward.

43

Theinterpretationthatdolomitetextureswerefaciescontrolledisfurthersupportedbythe

relationshipbetweendolomitetexturesandstoichiometry.Themimeticdolomitesincore1arenear-

stoichiometricandrelativelywellordered(Fig.10BandC;Table1).Belowthemimeticintervalin

core1,however,theplanar-eandnonplanardolomitesaremorestoichiometricthantheoverlying

mimeticdolomites(Fig.10B;Table1).Thiscompositionaltrendarguesagainstdownwardrefluxing

fluids related toMg depletion, but rather that themoreMg-rich dolomites lower in the section

perhaps formed from fluidswith either higherMg/Ca ratios (Kaczmarek & Sibley, 2007; 2011),

highertemperatures(Kaczmarek&Thornton,2017),highersalinity(Glover&Sippel,1967;Cohen

&Kaczmarek,2017)and/orwererecrystallizedtoagreaterextent(Sibley,1990;Mazzullo,1992;

Maloneetal.,1996;Machel,1997;Kaczmarek&Sibley,2014).

In contrastwith core 1, an overall decrease in dolomite stoichiometry is observed down

section in cores 2 and 3. Dolomites near the top of the UER in cores 2 and 3 are the most

stoichiometric, and stoichiometry decreases with depth. Evaporites are absent in the UER and

overlyingRusinthesecores,however,andbasedonthepresenceofdominantlyFA4andFA5faces

in these cores, there is no evidence that a highly evaporative environmentwaspresent to cause

refluxingofevaporativefluids.

Traceelementgeochemistry

Strontium and sodium concentrations of the dolomites are also inconsistent with

dolomitizationinhighlyevaporativefluids.Rather,thesedatasuggestthatdolomitizationoftheUER

inQatartookplaceinnear-normalmarinefluids.Near-normalmarinefluids,asdefinedhere,include

normalmarinetoslightlyevaporatedmesohalinefluidsthathavenotreachedgypsumsaturation.

TheSrcontentintheUERdolomitesrangesbetween40and150ppm(averaging101ppm)whichis

similartoorslightlylowerthanotherCenozoicdolomitesinterpretedtohaveformedinnear-normal

marineseawater(Vahrenkamp&Swart,1990;Budd,1997).BasedonthestudyofVahrenkamp&

44

Swart(1990),stoichiometricdolomiteshouldcontainca100ppmofSrifprecipitatedfromfluids

withtypicalseawaterratiosofSr/Ca.ThissuggeststhattheUERdolomitesinQatarthusprecipitated

fromnear-normalmarinefluids.

TheNaconcentrationsofUERdolomites,whichrangefromca100to600ppm(averaging

342ppm),arealsoconsistentwiththosereportedforothermarinedolomites(Staudtetal.,1993).

AlthoughNaconcentrationsindolomiteareequivocal(Budd,1997),somefieldstudieshaveshown

thattheycanbeusedtocomparegeneticallyrelateddolomitepopulations(Sass&Bein,1988;Staudt

etal.,1993).Sass&Bein(1988),forinstance,showedthatdolomitesassociatedwithgypsumhave

Nacontentashighas2700ppm,whereasnon-evaporiticmarinedolomitesrangebetween150–350

ppm.Staudtetal.(1993)measuredtheNaconcentrationsofdolomitesinwhichtheiroriginswere

constrainedbypetrographic,geochemicalandstratigraphicdata.Staudtetal.(1993)showedthat

dolomites formed from near-normal seawater containing Na values between 0 and 500 ppm,

whereasdolomitesformedinevaporiticenvironmentsspanarangebetween500and2000ppm.The

values reported here are therefore more consistent with near-normal marine seawater such as

mesohalinefluidsandargueagainstgypsum-saturatedbrines.

Dolomiteandgypsumstableisotopes

Whenexaminedcollectively,thebulkδ18OvaluesofUERdolomitesinallthreeresearchcores

(Figs10D,11Dand12D) furthersupport the interpretation thatdolomitizationdidnotoccurvia

refluxingofevaporativebrines.Assumingatemperaturerangeof25–35°C,whichiscompatiblewith

seasurfacetemperature(SST)estimatesduringthePalaeocene–Eocene(Pearsonetal.2001;Zachos

et al., 2006), and integrating an ice-free normal seawater value of -0.98‰(Zachoset al., 1994),

dolomiteformedinnear-normalmarinefluidsshouldhaveδ18Ovaluesbetween-1.5‰and+0.75‰

usingtheequationofHoritaetal.(2014).TherangeofUERdolomiteδ18Ovaluespresentedhere(-

2.5to+1.0‰,averaging-0.52‰)areslightlymorenegativethanthecalculateddolomiteδ18Ovalues

45

utilizingnear-normal seawaterδ18Oand temperatures.This suggests thatUERdolomites formed

fromnormalmarineseawatersundershallowburialconditionsattemperaturesslightlygreaterthan

35°C (as high as 40°C based on dolomite δ18O of -2.5‰ and seawater δ18O of -0.98‰), were

recrystallized,oracombinationofboth(Land,1980).Interestingly,theexpecteddolomiteδ18Orange

calculatedhere for anear-normalmarineorigin is similar to the rangeof δ18Ovalues (-1.0‰to

+2.5‰)presentedbySalleretal.(2014),suggestingthattheUERinSaudiArabiaandKuwaitmay

havealsoformedfromnear-normalmarinefluids.

Whether or not dolomitization occurred in fluids saturated with respect to gypsum can

further be constrainedby comparing the gypsumanddolomite δ18Odata. Given that the δ18O in

dissolvedSO4 from typical seawater isca +8.6‰,and that the fractionation factor forδ18O from

dissolvedSO4togypsumisca+3.5‰(Markovic,2016;Warren,2016),gypsumprecipitatingfrom

seawaterwithδ18Oof0‰shouldtheoreticallyhaveaδ18Ogypsumof+12‰.Bulkgypsumδ18Ovalues

inthisstudyrangebetween+15.4‰and+15.9‰,whichimpliesthatgypsumintheRusFormation

precipitatedfromseawaterwithδ18Obetween+3.7‰and+3.9‰basedonaback-calculation.This

argumentiscomplicated,however,becauseotherfactorsalsocontroltheultimateδ18Oofoceanic

sulphate,andthustheδ18Ogypsum.Therateofoxygenexchangebetweensulphateionsandseawater

atoceanictemperaturesisonthescaleof106to109years(Lloyd,1968;Chiba&Sakai,1985),and

theresidencetimeofmarinesulphate-boundoxygen(ca500kyr)farexceedstheoceanmixingtime

(1600yrs),suggestingthatchangesinδ18Ogypsumislikelynotanindicatorofchangesinseawaterδ18O

due to evaporation (Jorgensen & Kasten, 2006; Markovic, 2016). Rather, deviations from the

theoretical+12‰ indicate fluxesofpyriteweathering, volcanicdegassing, evaporiteweathering,

evaporite burial and sulphate reduction, among others (Markovic, 2016). Moreover, seawater

evaporatedtothepointofgypsumsaturationhasδ18Oashighas+7to+10‰(Knauth&Beeunas,

1986;Riversetal.,2019b),butmoderngypsumhasameanδ18Oofca+13‰(Sealetal.,2000).If

dolomites formed ingypsum-saturated fluidswith seawaterδ18Ovalues similar tomodern fluids

46

precipitating gypsum (+7 to +10‰) between temperatures of 25°C and 35°C, they should have

δ18Odolomitebetween+6.5‰and+11.8‰(Horita,2014).Thisrangeissignificantlymorepositivethan

thosemeasuredintheUERdolomitesinQatarandthosefromSaudiArabiaandKuwaitdescribedby

Salleretal.(2014),suggestingthatthedolomitesdidnotformfromfluidsassociatedwithgypsumin

theRusFormation.Ofcourse,recrystallizationorshallowburialunderelevatedtemperaturescould

alsocausetheδ18Ovaluestodecreasefromtheseinitialvalues(Land,1980).

It isalsonotable that thedolomitesclosest tooverlyingevaporitebedsboth in thisstudy

(core1;Fig.10D)andSalleretal.(2014,fig.13)commonlyhaveδ18Olessthan+1‰,andfrequently

lessthan+0.5‰,withheavierdolomiteδ18Ovaluesobserveddeeperinthesection.Thisiscontrary

towhatwouldbeexpected,inthatdolomitesclosesttotheoverlyingevaporitebedsshouldrecord

theheaviestδ18Ovaluesduetotheirproximitytotheevaporativefluidsource(Warren,2000).Itis

alsonotablethattheanticipatedtrendisobservedincore1inQatar(Fig.10D),butthedolomiteδ18O

onlyreachesamaximumvalueof+0.5‰,whichagainisbroadlyindicativeofnear-normalmarine

fluids.

Dolomitizationmodelandimplications

GiventhattheUERwasdolomitizedpriortoRusdeposition,asarguedfromthepetrographic

andgeochemicaldatapresentedabove,itispossibletoconstrainthetimingofdolomitizationandto

estimatethetimeandfluidamountrequiredtocompletelydolomitizetheunit.DepositionoftheRus

FormationcommencedduringthemiddleYpresian(Al-Saad,2005;Riversetal.,2019a),thusca52

MaprovidestheyoungestageofwhenUERdolomitizationended.ItisdifficulttoknowwhenUER

dolomitizationbegan,however.TheUERdolomitehasbeenreportedtobeasthickas370minthe

subsurface(Eccelestonetal.,1981),butonlytheuppermostca75mwascoredintheresearchwells

here.Thus,althoughitnotpossibletoextrapolatethefindingspresentedfortheuppermost75mto

theentireformation,anattemptismadetoconstrainthetimingforthe75mpresentedinthisstudy.

47

A2.0–4.5‰negativecarbonisotopeexcursionthatiscorrelativeacrossthestudyarea(Figs

10E,11Eand12E)providessomeconstraintonthetimingofdolomitizationintheupper75m.A

similarexcursionwastrackedregionally throughoutSaudiArabiaandKuwaitand interpretedby

Pollittetal.(2012)asthePETM,andthusitisinterpretedthattheexcursionspresentedheremay

representthePETMinQatar.Assumingthattobethecase,thepositionofthePETMprovidessome

constraintonthetimingofdolomitizationoftheuppermost75moftheUER.GiventhatthePETM

represents a ca 100 kyr peak in mean global temperatures approximately 55.53 to 56.33 Ma

(Westerholdetal.,2009),ca56Marepresentsaconservativeestimatefortheonsetofdolomitization

oftheuppermost50–60m(i.e.depthtothetopoftheexcursionineachcore).Thicknessofthemetre-

scalefining-upwardcyclesintheUERvarybetween1mand3m(Riversetal.,2019),meaning20to

60cyclesin4Ma,oranaveragedurationofca60to220kyrfordolomitizationofeachcycle.Using

theareaofQatar(ca11570km2),athicknessof60m,100%dolomitization,40%initialporosity

(Enos&Sowatsky,1981),andaPalaeocene–EoceneseawaterMgconcentrationofca0.033mol/L

(Tyrell&Zeebe,2004),itwouldrequire5.0x1010L/yearofseawatertodolomitizetheupper60m

of theUER(ca416km3)over4Myr.Thisannualestimateofseawatervolume isconsistentwith

estimatesbyMontanez&Read(1992),whopositedthat5x1012L/yearcouldbefluxedthrough1–

4mperitidalcyclesintheDevonianKnoxGroup.

TwopossiblemechanismsoffluidflowareconsideredfordolomitizationoftheUERbynear-

normalmarinefluids.Becausedolomitizationisinterpretedtohavehappenedrelativelyearlyafter

deposition,itispossiblethatsedimentsweredolomitizedwhiletheywereindirectcontactwiththe

overlyingmarinefluids,i.e.syndepositionally.ThisstyleofdolomitizationwaspositedinManche&

Kaczmarek (2019) to explain variable dolomitization of metre-scale peritidal deposits, in which

differences in cyclical vertical patterns were interpreted to reflect temporal changes in

environmentalconditions.Manche&Kaczmarek(2019)interpretedfinelycrystalline,stoichiometric

dolomitesinsupratidalandperitidaldepositstoreflectshallowconditionswithhigherfluidMg/Ca,

48

salinityandtemperature.Incontrast,deeperperitidalandsubtidaldepositswerecharacterizedby

coarselycrystallineandless-stoichiometricdolomites,inferredtoreflectdeeperwaterdepthsand

lower fluid Mg/Ca, salinity and temperature (Manche & Kaczmarek, 2019). A similar process,

whereby sediments are syndepositionally dolomitized, could have been responsible for early

dolomitization of themetre-scale cycles observed in theUER, although nometre-scale trends in

stoichiometry,stableisotopesormeancrystalsizeareobserved.

Another possible driver could be small-scale reflux of mesohaline fluids through the

underlying sediments. Based on the work of Simms (1984) and Jones & Xiao (2005), reflux of

mesohalinefluidsnottogypsumsaturation(salinity37–48‰)isaviablemechanismforplatform-

scale dolomitization, because slight increases in salinity can drive fluids through underlying

sediment.Basedonreactivetransportmodelling,pervasivedolomitizationbymesohalinefluidscan

occuroverdistancesofupto7kmand500mdepthover1Myr(Jones&Xiao,2005).However,given

anestimateof4MyrfordolomitizationoftheUER,itwouldbeexpectedthatdolomitizationcould

extendfurtherthan7km.Additionally,thegeneralshallowingtrendoftheUERwouldhavecaused

fluidstomovebasinward,whichwouldincreasethelateralextentofdolomitization.Furthermore,

the fining upward sequences observed in the UER are consistent with modelling showing that

grainier sediments,when overlain bymuddy sediments, provide conduits tomove fluids further

laterally(Al-Helaletal.,2012).Thus,itisalsopossiblethatsmall-scalerefluxofmesohalinefluids

couldhavepervasivelydolomitizedtheUER.

The observed geochemical and textural signatures also are compatible with dolomite

recrystallization,whichcouldexplainthenear-stoichiometricandrelativelywell-orderednatureof

thedolomites(e.g.Maloneetal.,1996;Kaczmarek&Sibley,2014), theslightlydepleteddolomite

δ18OrelativetocalculatednormalmarineEoceneseawatervalues(Land,1980;Mazzullo,1992)and

thedepletedSrconcentrations(e.g.Maloneetal.,1996)intheUERdolomites.IftheUERdolomites

were recrystallized, however, petrographic observations still support amodelwherebydolomite

49

crystalsformedpriortoallotherdiageneticmineralphases.Recrystallizationwouldlikelyobliterate

anytrendsintheinitialdolomitestoichiometrythatmightreflectchangesinfluidfloworchemistry.

However,theδ18O,SrandNavaluesofthedolomites,ifrecrystallized,arestillsuggestiveofnear-

normalmarinefluidsalbeitwithslightlyelevatedtemperatures.

Implications

This study provides an example of early, platform-scale dolomitization by near-normal,

mesohaline fluids in a setting thatwould otherwise suggest refluxing of hypersaline evaporative

brineswasresponsible.Thisisconsistentwithotherstudiesthathaveanalyzedevaporite-associated

dolomitesandprovideddatathatsuggestnear-normalseawaterwasresponsiblefordolomitization

(e.g.Newportetal.2017;Manche&Kaczmarek,2019).Forexample,Newportetal.(2017)analyzed

theUpperAlbian–LowerTuronianZebbagFormation insouthernTunisia.TheZebbagFormation

consistsofmetre-scaleshallowing-upwardperitidalandsubtidalcycles,witha1mthickgypsum

horizoninthemiddleKerkerMember.AlthoughnotpresentinthestudybyNewportetal.(2017),

theZebbagisalsooverlainbybeddedevaporitesnorthoftheauthors’studyarea(Newportetal.,

2017).Basedonfaciesdistributions,dolomitetexturevariations,slightlypositiveisotopesignatures,

slightly elevated Sr concentrations and a near-absence of evaporites, Newport et al. (2017)

hypothesized that the Zebbag Formation was pervasively dolomitized by reflux of mesohaline

seawater.Asbrieflydescribedabove,Manche&Kaczmarek(2019)analyzedtheCretaceousUpper

GlennRoseFormation,composedofmetre-scaleevaporite-associatedperitidaldolomitespreviously

interpreted to have formed by refluxing of hypersaline brines. Cyclical variations in dolomite

abundance, crystal size, stoichiometry and δ18O were inconsistent with such an interpretation,

however, and instead suggested that the data were more consistent with early syndepositional

dolomitization by near-normal marine fluids. Combined with these studies, the present study

provides yet another example of dolomitization by near-normal fluids in evaporite-capped

50

carbonates.Thisinturnsuggeststhatevaporativecapsdonotneedtoimplyrefluxdolomitizationby

hypersalinebrines.

Conclusions

Petrographic,mineralogicalandgeochemicalevidencefromtheUmmerRadhuma(UER),asubtidal

carbonateunitcappedbyevaporitesinQatar,suggeststhatdolomitizationhappenedearlyinnear-

normalmarine seawater. In all occurrences, dolomite is either included in or replaced by chert,

pyrite,palygorskite,gypsum,calciteandchalcedony,indicatingthatdolomitizationwastheearliest

diageneticphaseintheUER.PreviousstudiesconcludedthatUERdolomitesinnearbySaudiArabia

and Kuwait formed by large-scale, top-down refluxing of hypersaline brines driven by gypsum

precipitationintheoverlyingunit.Thetextural,mineralogical,andgeochemicaldatapresentedfrom

theUERinthisstudy,however,argueagainsta large-scale,top-downhypersalinerefluxmodelin

central Qatar. Mimetic UER dolomites closest to the overlying Rus evaporite beds are less

stoichiometric than dolomites deeper in the section, when the opposite trend is expected for

dolomitizationbydownwardrefluxingbrines.Dolomiteδ18Ovaluesfallwithinthoseexpectedfrom

dolomitization by near-normal marine seawater, or are slightly more negative possibly due to

recrystallizationordolomitizationinaslightlywarmer(≤40°C),shallowburialenvironment,orboth.

Strontiumandsodiumconcentrationsarealsoconsistentwithdolomitizationinmarine-likefluids.

Theaccumulationof evidence,which includes subtidalprecursor facies, diagenetically veryearly

dolomite, and dolomite geochemistry andmineralogy, is consistentwith early dolomitization by

near-normal marine fluids. This study adds to a growing understanding of platform-scale

dolomitizationofevaporite-associatedsequencesbynear-normalseawater.Further,thisstudyadds

toagrowingbodyofresearchonshallowwatercarbonatesthatsuggestsearlydolomitizationby

marinefluidsmaybemorecommonthanpreviouslyaccepted,especiallyinevaporativesettings.

51

Acknowledgements

ThisprojectwassponsoredbyExxonMobilResearchQatar.WethankExxonMobilmanagementfor

permission to publish this study. Katie Sauer and Zander Sorenson are acknowledged for their

assistance with XRD sample preparation and analysis. John Fournelle assisted in EMPA data

collection at the University of Wisconsin, Madison. Sabrina Skeat provided important logistical

support.WethankCameronMancheforcommentsonanearlierversionofthismanuscript.Wealso

wishtothankChiefEditorGiovannaDellaPorta,AssociateEditorCathyHollisandreviewerBeatriz

Garcia-Frescafordetailedreviewsandconstructivecomments,aswellasreviewerDavidBuddfor

hissignificanttechnicalandeditorialcontributions,allofwhichimprovedthefinalmanuscript.

DataAvailabilityStatement

Thedata that support the findingsof this studyareavailableon request from thecorresponding

author.Thedataarenotpubliclyavailableduetoprivacyorethicalrestrictions.

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62

CHAPTERIII

EARLYANDEXTENSIVERECRYSTALLIZATIONOFCENOZOICDOLOMITESDURINGSHALLOW

BURIAL

BrooksH.Ryan1,StephenE.Kaczmarek1,andJohnM.Rivers21DepartmentofGeologicalandEnvironmentalSciences,WesternMichiganUniversity,Kalamazoo,MI,U.S.A.2ExxonMobilResearchQatar,QatarScienceandTechnologyPark,Doha,QatarRyan,B.H.,Kaczmarek,S.E.,andRivers,J.M.,inreview,Geology.

Abstract

Recrystallizationcanalterthetextural,mineralogical,andgeochemicalsignaturesrecorded

duringdolomitization.However,mostexamplesofextensiverecrystallizationinancientdolomites

are attributed to deep burial. Textural,mineralogical, and geochemical data from the Paleocene-

Eocene Umm er Radhuma Formation (UER) in Qatar show that, in contrast, extensive dolomite

recrystallizationcanalsooccurearly inanear-surfacesetting.Dolomitestoichiometryandcation

ordering correlatewith textures that trendwith depth from finely crystallinemimetic to coarse

planar-etocoarsernonplanarwithdepth.Shallowmimeticdolomitesaretheleaststoichiometric

andleastordered,whereascoarserplanar-eandthecoarsestnonplanardolomitesobservedwith

deptharemorestoichiometricandbetter-ordered,suggestingrecrystallizationwithshallowburial.

Furthermore,aprincipalcomponentanalysisofdolomitestoichiometry,cationordering,δ18O,and

δ13C indicates that variations in the data can be explained by two factors. The first factor is

interpreted to be dolomite recrystallization,which shows positive loadings of stoichiometry and

cationorderingbutanegativeloadingofδ18O.Thesecondfactorisinterpretedtobethetemporal

evolutionofdissolvedinorganiccarbon,asδ13Cistheonlyheavilyloadedcomponent.Progressive,

63

earlyrecrystallizationduringshallowburialisconsistentwithpetrographicobservationsoftwodm-

thickchertbandswithinanonplanardolomiteintervalthatpreservelessalteredmimeticandplanar-

edolomites. ∆47-derivedtemperatures(35°Cto46°C)andδ18Owvalues(+0.8to+2.6‰)arealso

indicativeofshallowburialrecrystallization(<370m)byslightlyevaporatedseawater.Collectively,

these findings indicate that early diagenesis may reset most, but perhaps not all textural and

geochemicalattributes,whichstandsincontrasttothestronglyheldsuppositioninthe literature

thatextendedgeologictimeanddeepburialareneededforsignificantdolomiterecrystallization.

Introduction

Dolomiteisacommonreplacementoflimestoneandisasignificanthostofhydrocarbonsand

mineraldeposits(Warren,2000),yetdebatecontinuesoverthe“dolomiteproblem”(Land,1985;

Greggetal.2015).Atissueishowmassive,platform-scaledolomitizationhappens(Morrow,1982).

Conceptualmodels,whichdescribetheenvironments,fluids,andhydrologicalflowmechanismsof

dolomitization, are inferred from textural, mineralogical, and geochemical data. It is commonly

assumed,however,thatthesepetrologicalattributesreflecttheinitialdolomitizationevent,despite

thepotentialforrecrystallization(Mazzullo,1992).

Modern and geologically young dolomites are generally Ca-rich and poorly ordered

comparedtotheirancientcounterparts(LumsdenandChimahusky,1980).Laboratoryexperiments

alsoshowtheinitialdolomitephasetoformduringdolomitizationispoorlyorderedandoftenCa-

rich (Kaczmarek and Sibley, 2014), which is thermodynamically less stable (Navrotsky and

Capobianco,1987).Thus,thereisthepotentialfordolomiterecrystallization.

Awiderangeofdatahasbeencitedasevidenceofdolomiterecrystallization,includinglarge

crystals, a near stoichiometric composition, a high degree of cation ordering, low Sr and Na

concentrations,lightOisotoperatios,andmottledcathodoluminescencesignatures,amongothers

(Mazullo,1992). Ithasbeenarguedthatthesedatamayreflecttheconditionsofrecrystallization

64

ratherthantheoriginaldolomitizingconditions.This isespecially likely inancientdolomitesthat

haveundergonesignificantburialdiagenesis (Kupeczetal.,2003;Machel,2004),butwhatabout

youngdolomitesthathavenotsuffereddeepburial?

To address this question, we investigated the Umm er Radhuma (UER), a pervasively

dolomitizedunitinQatarthathaslikelynotbeenburieddeeperthanpresentdaydepthsof50-400

m(VanBuchemetal.,2014).Basedonpetrographicdata,recentworkontheUERsuggeststhatthe

replacement of limestone by dolomite (i.e. dolomitization) occurred very early in the diagenetic

history(52-56Ma),priortotheformationofchert,pyrite,palygorskite,calcite,andgypsum(Ryanet

al.,2020).Here,wepresentevidencethatsuggestsdespiteearlydolomitizationandlimitedburial

depths, early and extensive dolomite recrystallization is responsible for changes in the textural,

mineralogical,andgeochemicalsignatures.Inaddition,suchrecrystallizationofCenozoicdolomites

undershallowburialisuniqueasshownbyananalysisofseveralstudiesofrecrystallizeddolomite,

whichsuggeststhatrecrystallizationismostlyanancientanddeepphenomenon.

GeologicalSetting

DuringmuchofthelatePaleoceneandearlyEocene,shallowcarbonaterampscoveredthe

southernmarginoftheTethysregion(Sharlandetal.,2001).Transgressionandregressionalonga

northeast-facingrampontheArabianPeninsulamarkedtheonsetofCenozoicdeposition,resulting

insequencesofsubtidaltoperitidalcarbonatescomprisingtheUmmerRadhumaFormation(UER).

TheUERisasthickas370m,observedonlyinthesubsurface,andiscontinuousacrossQatarand

most of the eastern Arabian Peninsula (Eccleston et al., 198; Sharland et al., 2001). The unit is

characterized bym-thick, fining upward subtidal sequences dominated by dolomite, intercalated

siliciclastics,andsporadicreplaciveevaporitenodules(Riversetal.,2019).

65

Methods

Petrographic, mineralogical, and geochemical data were collected from three shallow

research cores from Qatar (Figure 1). Detailed methods for thin section petrography, x-ray

diffractometry,stableisotopes,andclumpedisotopesarepresentedintheGSADataRepository1.A

principalcomponentanalysis(PCA)wascarriedoutusingStatisticalProductandServiceSolutions

statisticalsoftwareinordertodeterminehowmanyfactorscontrolthevariabilityin71sampleswith

correspondingdataforstoichiometry,cationordering,δ18O,andδ13Cacrossallthreecores.Variables

thatcorrelate tooneanotherarecombined into factors,withdifferent loadings thatrelate to the

strength of the association between the factors and variables (Joliffe, 2002). Lastly, a literature

review of naturally recrystallized dolomites was conducted which includes only journal articles

postdating1992.Geologicageandrecrystallizationburialdepth(shallow≤0.5km≤intermediate≤

1km≤deep)wererecordedtotesttheassumptionthatdolomiterecrystallizationisassociatedwith

significantburial.

1GSADataRepositoryitemDR1,CompleteMethodologyandStatisticalAnalyses

Core 1

Qatar

SaudiArabia

PersianGulf

Kilometres

Core 2

Core 3

Geologic Age Stratigraphic Unit Lithology

PA

LE

OG

EN

E

Pale

ocen

eEo

cene

Early

Mid

dle

RusFormation

Umm er

Radhuma Formation

DammamFormation

70

80

90

120

Depth (m)

100

110

60

130

140 Mineralogy0% 100%

Core 1 UER

ClasticsDolomite

Gypsum

Mimetic

Planar-e

Nonplanar

A

B

C

Arabian Peninsula

D

E

12

This study

Calcite

Chert band

KEY

N

Figure1:AstratigraphiccolumnofthePaleogenestrataofQatar(modifiedafterRyanetal.,2020),withthespecificstudyintervaloutlinedinred.Locationsofcores1(+25.23773°,+51.22826°),2(+25.38417°,+51.14507°),and3(+25.86184°,+51.26197°)shownonmapofQatar(insetmapshowslocationofQatarontheArabianPeninsula).MineralogicprofileoftheUmmerRadhumaFm.incore1(depthinmetersbelowthelandsurface),withcorrespondingthinsectionimagesofmimetic(A),planar-e(B),nonplanar(C)dolomitetextures,planar-edolomitecrystalsinchertcement(D)fromtheupperchertband(#2)andasilicifiedmimeticallydolomitized(redarrow)foraminifera(E;modifiedfromRyanetal.2020)fromthelowerchertband.ScalebarsinA-Erepresent200µm.

66

Results

Theupper60-75moftheUERwereanalyzedinallcores,andaredominatedbydolomite,

comprising62to100%ofbulkmineralogy(Fig.1).Dolomitetexturesvarywithdepthinallcores

(Figs.1and2),withageneral increase incrystalsizewithdepth.Theuppermost~7mofcore1

containsmicrocrystallinedolomite(<10µm)thathasmimeticallyreplacedtheprecursorlimestone

(Fig.1).Themiddle50mofcore1,andtop20-30mofcores2and3aredominatedbyrelatively

coarse(30-100µm)planar-edolomiteexhibitingcloudycoresandclearrims(CCCR).Thebottom20-

30mofallthreecoresconsistsofcoarser(100-200µm)nonplanardolomite.Acorrelativedm-thick

chertbandwithplanar-edolomitecrystalinclusionsisobservedinallthreecores.Core1hastwo

chert bands separated by < 1 meter (Fig. 1). The lower chert band in core 1 includes mimetic

dolomite,whereastheupperoneincludesonlyplanar-edolomite(Fig.1).

Dolomite stoichiometry (ave. 50.3 mol%Mg) and cation ordering (ave. 0.73) values are

generally high, with statistically different averages as a function of dolomite texture1. Mimetic

dolomitesaretheleaststoichiometric(49.7mol%Mg)andleastordered(0.57),whereasnonplanar

dolomitesaremorestoichiometric(50.1%mol%Mg)andordered(0.71),andplanar-edolomitesare

themost stoichiometric (50.7%mol%Mg) andordered (0.77) (Fig. 2A).Dolomite δ18Oandδ13C

values,incontrast,arehighlyvariablewithrespecttodolomitetexture(Fig.2B).Mimeticdolomites

average+0.03‰(range-0.45to+0.38‰)δ18Oand-0.94‰(range-2.12to-0.24‰)δ13C(VPDB).

Nonplanar dolomites average +0.03‰ (-2.29 to +1.39‰) and +0.18‰ (-2.57 to +2.22‰),

respectively.Planar-edolomiteshavethelowestδ18Oaveraging-1.06‰(-2.43to+0.68‰)withδ13C

similartotheothertextures(ave-0.50‰;-3.47to+1.55).Inallthreecores,a~2.5-4‰negative

carbon isotope excursion is observed in nonplanar dolomites (Ryan et al., 2020). Temperatures

derivedfrom∆47analysisrangefrom35.1-46.2°Candcalculatedfluidδ18Owranges+0.84to+2.64‰

(VSMOW)(Table1).

67

PCAresultsyield2statisticallyusefulfactorswitheigenvalues>0.7andthatexplain~80%

ofthetotalvariance1(Joliffe,2002).Factor1accountsfor58%ofthetotalvarianceandisdominated

byhighpositiveloadingsofstoichiometry(0.774)andcationordering(0.823)andahighnegative

loadingofδ18O(-0.846)(Fig.2C).Factor2(23%oftotalvariance)isdominatedbyahighpositive

loadingofδ13C(0.790).

Atotalof71paperswereanalyzedfromtheliteraturefornatural,recrystallizeddolomites1.

Theresultsshowthat90%ofthesestudiesinvolvepre-Cenozoicdolomites,andthat81%ofpre-

Cenozoic dolomites interpreted to be recrystallized were recrystallized under self-reported

intermediate to deepburial conditions, althoughmany studies reporteddepth ranges aswell. In

contrast, only 6%of these 71 studies includeCenozoic dolomites that underwent shallowburial

recrystallization.Of all studies that interpreted shallowburial recrystallization,71% invoked the

possibleinfluenceofmeteoricfluids.

Table1.Core1clumpedisotopeanalysisDepth(m)

Dolomitetexture

Average∆47(‰)

± 1 SE(‰)

δ18Odol (‰VPDB)

±1SD(‰)

AverageTemperature(°C)

±1SE(°C)

δ18Ow (‰VSMOW)

±1SE(‰)

66.48 Mimetic 0.661 0.006 1.08 0.16 35.1 2.10 1.61 0.52

120.67 Nonplanar 0.634 0.007 -0.15 0.10 46.2 2.90 2.64 0.55

128.51 Nonplanar 0.654 0.025 -0.31 0.17 38.8 9.60 0.84 2.10

-4

-3

-2

-1

0

1

2

3

-4 -3 -2 -1 0 1 2

13C

(‰ V

PDB)

18O (‰ VPDB)

NonplanarMimeticPlanar-e

Sr (p

pm)

0

20

40

60

80

100

120

140

160

Mimetic Planar-e Nonplanar

A B C

0.4

0.5

0.6

0.7

0.8

0.9

1

49 49.5 50 50.5 51 51.5

Catio

n O

rder

ing

(015

:110

)

Stoichiometry (mol% Mg)

Planar-eNonplanarMimetic

Stoichiometry

Cation OrderingDolomite

δ18O

Dolomite δ13C

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Fact

or 2

Factor 1

CExpected dolomite δ18O values for latePaleocene to early Eocene seawater

Figure2:Crossplotsofcationorderingandstoichiometry(A)andstableisotopiccomposition(B)asafunctionofdolomitetextureforallcores.Loadingvaluesforstoichiometry,ordering,δ18O,andδ13Casafunctionoffactorareshownin(C).

68

Discussion

Thesubtlebutsignificantdifferencesinstoichiometry,cationordering,andtoalesserextent

δ18Ovalues,asafunctionoftexture,reflecteitherdifferencesindolomitizationprocessesassociated

witheachtexture,orvaryingdegreesofrecrystallization.Ryanetal.(2020)suggestedthatdolomite

texturesintheUERarerelatedtoprecursorlimestonefacies,inwhichmimeticdolomitesreplaced

muddier sediments whereas planar-e and nonplanar textures resulted from grainier sediments.

Furthermore,Ryanetal.(2020)arguedthattheUERdolomitesinitiallyformedfromLatePaleocene

toearlyEoceneseawater(δ18Owof-0.98‰andSST25-35°C,Zachosetal.,1994;2006)basedonthe

similaritybetweendolomite δ18Ovalues theoretically formedunder such conditions (-1.5‰and

+0.75‰)andthoseoftheUER(-2.5‰to+1‰).However,Ryanetal.(2020)notedthatthemore

negativerangeofUERdolomiteδ18Ovalues,coupledwiththenear-stoichiometricandwell-ordered

natureofthedolomites,permitsthepossibilityofrecrystallization.Therelationshipbetweentexture,

stoichiometry, ordering, δ18Opresentedhere further supports a recrystallizationhypothesis.The

mimeticdolomitesaretheleaststoichiometric,leastordered,andhaveatightrangeofδ18Ovalues

nearesttheupperlimitofthetheoreticalrange.Incontrast,theplanar-eandnonplanardolomites

aremorestoichiometric,moreordered,andoccupyawiderrangewithδ18Oasnegativeas-2.5‰,

withnonplanardolomitesofcores2and3commonly≥0‰(Fig.2A).Thegreaterδ18Orangeinthe

latter two textures implies that those values recordmore than the conditions during the initial

dolomitizationandhavelikelybeenrecrystallizedtosomedegreeeitherindifferenttemperatures

or fluids. Following this, the fine crystals comprising mimetic dolomites, along with lower

stoichiometry and ordering values, aswell as δ18O values closest to expected, reflect the lowest

degreeofrecrystallization(Mazzullo,1992).Theplanar-eandnonplanaranddolomites,incontrast,

representmorerecrystallizeddolomitebasedonlargercrystalsizes,beingmorestoichiometricand

well-ordered,andcontainingawiderδ18Orange.

69

ResultsofthePCAarealsoconsistentwiththeinterpretationthatchangesinstoichiometry,

cationordering,andδ18Oarerelatedtodolomiterecrystallization.Factor1,whichshowspositive

loadingsofstoichiometryandcationorderingbutanegativeloadingofδ18O,isinterpretedhereto

reflect recrystallization. Recrystallization of dolomite in laboratory experiments demonstrates

similar trends, in that stoichiometry and cation ordering concurrently increase (Kaczmarek and

Sibley, 2014) whereas dolomite δ18O generally decreases (Malone et al. 1996). Changes in UER

dolomite δ18O possibly also reflect changing temperature, fluid salinity, fluid δ18O, or precursor

calciteδ18O(Swart,2015).Ofthese,onlytemperatureandsalinityalsoaffectdolomitestoichiometry,

inwhichapositivecorrelationisobservedwithbothparameters(KaczmarekandThornton,2017;

Cohen,2019).However,noneoftheseconditionsaffectstherateatwhichcationorderingincreases,

whichhasbeenexperimentallyshowntoonlyincreasesimultaneouslywithstoichiometrythrough

dolomite recrystallization (Malone et al. 1996; Kaczmarek and Sibley, 2014). Thus, the most

reasonableinterpretationforsimultaneouspositiveloadingofstoichiometryandcationorderingand

negative loadingofδ18OonFactor1 isdolomite recrystallization.This suggests that theplanar-e

dolomites,whicharethemoststoichiometric,mostordered,andcontainthelowestδ18Ovalues,are

the most recrystallized of all dolomite types. The nonplanar dolomites have likely also been

significantlyrecrystallizedbasedonevenlargercrystalsizesandhighstoichiometryandordering

values, but their relatively higher δ18O values in cores 2 and 3 suggest either somewhat less

alteration, initial dolomitization of precursor a calcite with higher δ18O than in core 1, or

recrystallizingfluidswithhigherδ18O.

Theobservedincreaseincrystalsizeandchangeindolomitetexturewithdepthinthethree

coressuggeststhatrecrystallizationlikelyoccurredduringshallowburial.Thisissupportedbythe

observationthatthetwochertbandswithinanonplanardolomiteinterval incore1preservethe

mimeticandplanar-edolomitetexturesthatprevailhigherinthesection(Fig.1E,F).Theobservation

thatdolomitetexturesareretainedthroughsilicification,andthatthechertbandiscorrelativeacross

70

Qatar(Ecclestonetal.,1981),suggeststhatsilicificationwasassociatedwithwidespreadbutpossibly

short-livedexposureeventsthatoccurredafterbothmimeticandplanar-edolomites.Thetwochert

bands in core1,whichcapture themimeticandplanar-edolomites, suggest that therewere two

episodesofexposure.Riversetal.(2019)reportedaδ18Oforthelowerchertbandincore1of-6.5‰

andsuggestedthatmeteoricfluidswereresponsible.Thedifferencebetweentheδ18Oofthechert

andthatofthedolomitesbothwithinandoutsideofthechertbandareconsistentwithanexposure

eventthatintroducedsilica-saturatedmeteoricfluidspriortofurtherdolomiterecrystallization.The

replacementofdolomitebychertalsoclearlyshowsthatdolomiterecrystallizationoccurredearlyin

thediagenetichistory.Furtherevidenceofearlyrecrystallizationincludesplanar-edolomiteswith

partiallydissolvedCa-richcoresandpristinestoichiometricrims(Ryanetal.,2019)thatareboth

cross-cut by latermineral phases including chert, palygorskite, gypsum, and calcite (Ryan et al.,

2020).

It is possible that the meteoric fluids associated with silicification were involved with

subsequentdolomiterecrystallization,assuggestedbylessrecrystallizeddolomitesbeingreplaced

bychert.The∆47-derivedtemperaturesandδ18Owvalues(Table1),however,aremoreconsistent

withslightlyevaporativeandslightlywarmerEoceneseawater(Zachosetal.,1994;2006).The∆47-

derived temperatures can provide further constraints on how and where shallow the

recrystallizationoccurred.ConsideringanSSTwithin25-35°C,andtheinterpretationthatmimetic

dolomitesarenotsignificantlyrecrystallized,thenthe∆47-derived35°Cforthemimeticsamplelikely

represents surface conditions. Assuming a geothermal gradient of 30°C/km, then the nonplanar

samples,whichyieldcrystallizationtemperaturesof39-46°C,correlatetodeepestrecrystallization

between130-370m.Thus,recrystallizationoftheUERisinterpretedtohaveoccurredbymodified

seawaterduringshallow-burial,implyingtemperatures>50°Cmaynotberequiredfortheformation

ofnonplanardolomite(SibleyandGregg,1987).

71

Extensive,earlydolomiterecrystallizationofCenozoicdolomitesinashallowburialrealmis

significantfortworeasons.First,theoverwhelmingmajorityofinterpretedrecrystallizeddolomites

are post-Cenozoic in age. Secondly, inmost of these post-Cenozoic dolomites, recrystallization is

interpretedtohaveoccurredunderintermediate(>0.5km)todeep(>1km)burial,with72%being

exclusivelyofdeepburialorigin.TheseobservationsaloneindicatethatthePaleocene-EoceneUER

dolomites are a unique example of geologically young dolomites that have recrystallized under

shallowburialconditions.Furthermore,theinterpretationthatrecrystallizationhappenedinonly

modifiedEoceneseawaterissignificant,as71%ofrecrystallizeddolomitesofshallowaburialorigin

were interpreted to be recrystallized in the possible presence of meteoric fluids. Thus, the

interpretationspresentedhere,involvingshallowburialandslightlymodifiedseawater,impliesthat

deep burial, exceptional diagenetic fluids, and/or extended time are not necessary for extensive

dolomiterecrystallization.Thissuggeststhatextremecautionmustbeexercisedwheninterpreting

theconditionsorfluidsresponsiblefordolomitization,assubsequentrecrystallizationcanhappen

early and innear-surface fluids and conditions. This opens thequestion for future studies:what

exactlycausesdolomitestorecrystallize?

Equallyimportanttothetopicofearlydolomiterecrystallizationistheobservationthatδ13C

loadsheavilyonaseparatefactorasalonevariable.Ryanetal.(2020)proposedthatthe~2.5-4‰

δ13CexcursionscorrelatedacrossthethreewellsinthenonplanarintervalwaslikelythePaleocene-

EoceneThermalMaximum(REFS).This suggests thatwhileothergeochemicalandmineralogical

propertiesmayberesetduringdolomiterecrystallization,theδ13Cmayremainunalteredincertain

cases, indicating that information pertaining to past climatic conditionsmay be preserved even

throughrecrystallization.

72

Conclusions

Collectively,thetextural,mineralogical,andgeochemicaldatasupporttheinterpretationthat

theUmmerRadhumaFm.dolomiteshaveundergone extensive recrystallizationwithdepth in a

shallow burial environment. Results from the principal component analysis support this

interpretation,asmultiplevariablesaffectedbydolomiterecrystallization loadonasingle factor.

Petrographicdatafromtwochertlayersfurtherconstrainthetimingofrecrystallizationtobevery

early. These findings suggest that dolomite recrystallization is not only an ancient and deep

phenomenon,butalsothatexceptionalconditionsotherthanmodifiedseawaterandshallowburial

arenotrequiredforrecrystallization.Lastly,despitesignificantalterationofδ18O,stoichiometry,and

cation ordering, the depositional δ13Ccarb signature of the precursor limestone may have been

retainedintheUERdolomites,implyingthatsomerecrystallizeddolomitesmaybeviablecandidates

fromwhichtoextractδ13Crecords.

Acknowledgements

This work was supported by ExxonMobil Research Qatar. We are grateful to Sabrina Skeat for

logisticalsupport,andDr.SierraPetersen,UniversityofMichigan,forclumpedisotopeanalysis.We

appreciatethethoroughreviewfromDavidBudd,andthecommentsofChiefEditorGeraldDickens

andananonymousreviewer.

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SupplementaryMaterial

DataRepositoryFileDR1:CompleteMethodologyandStatisticalAnalyses

ThinsectionpetrographyandX-raydiffractometry

Thinsectionswerepreparedfromtheendsof391blue-epoxyimpregnatedcoreplugs.Thin

sectionsweresubsequentlystainedwithAlizarinRedS(ARS)todifferentiatedolomitefromcalcite.

X-raydiffractometry(XRD)datawascollectedatWesternMichiganUniversityon125coreplugs

across all three cores. An electric rotary drill was used to extract powders which were then

homogenizedbyhandwithamortarandpestle.PowderswereanalyzedunderCuKαradiationwith

aBrukerD2PhaserDiffractometerunderthe2θrangeof20to40°withastepsizeof0.01°anda

counttimeof1.0sperstep.Dolomitemol%MgCO3wascalculatedusingtheequationderivedby

Lumsden(1979)andcationorderingwascalculatedutilizingthemethodofGoldsmith&Graf,1958).

Severalinternalstandardswereusedtoshiftpeakpositions.

DolomiteStableIsotopeAnalysis

Bulkrockstableisotopedatausedhere(n=191)werefirstreportedinRiversetal.(2019a)

andthenmodifiedbyRyanetal.(2020)toexcludecalcitecontaminatedsamplesorsamplesthatmay

not represent thehost rock. Stable isotopedatawere yielded from theCenter for Stable Isotope

76

Biogeochemistry(CSIB),UniversityofCalifornia,Berkeley.SampleswerereactedwithH3PO4at90°C

for 10 minutes before being analyzed in a GV IsoPrimemass spectrometer with Dual-Inlet and

MultiCarb systems. Analytical precision is ±0.05‰ for δ13C and ±0.07‰ for δ18O. Values for all

isotopiccompositionsarereportedrelativetotheViennaPeedeeBelemnite(VPDB)standard.

ClumpedIsotopeAnalysis

Fourdolomitesamplesfromcore1,spanningtheentiretyoftherecoveredUERFormation

and representing all three dolomite textures (1 mimetic, 1 planar-e, and 2 nonplanar), were

measuredfortheirclumped(∆47)isotopiccompositionattheUniversityofMichiganStableIsotope

Laboratory.Threereplicateanalysesfromthreeoutofthefoursampleswereutilized,withthefourth

sample(planar-edolomite;depth89.55m)onlybeingreplicatedtwiceduetocontaminationofboth

analysesasindicatedbyhigh∆48values.Theplanar-esamplewasthusdiscardedfromtheclumped

isotope results. Measured dolomite ∆47 values are correctedwith an acid fractionation factor of

+0.072‰at75°CfollowingPetersenetal.(2019),whichshowsthatthisacidfractionationfactorcan

be applied to all carbonate mineralogies. Temperatures were calculated using the calibration

equationalsodevelopedbyPetersenetal.(2019),whichcombinessyntheticsamplesof6carbonate

mineralogies. Inordertocalculatetheδ18Oofthefluid(δ18Ow), the∆47-derivedtemperaturesand

dolomiteδ18OvalueswereintegratedintotheequationofHorita(2014).Allδ18Owdataarereported

relativetoVSMOW.

Dolomitestoichiometryt-tests

DatafromRyanetal.(2020)arebrieflyreviewedhere.Acrossallcores,mimeticdolomites

havetheloweststoichiometry,averaging49.7%(±0.24%).Thisisfollowedbynonplanardolomites,

whichaverage50.0%(0.29%),andplanar-edolomiteswhichaverage50.6%(0.29%).At-testat95%

confidence(𝛂=0.05)showsthatplanar-edolomitesaresignificantlymorestoichiometricthanboth

nonplanarandmimeticdolomites,withp-valuesof7.9x10-9and1.1x10-8,respectively.Nonplanar

77

dolomitesaresignificantlymorestoichiometricthanmimeticdolomitesat95%confidence(𝛂=0.05)

withap-valueof0.005.T-testsalsoshowthatmimeticdolomiteshavethelowestaveragedegreeof

cationordering(0.57±0.06),comparedtothatofplanar-e(0.77±0.07)andnonplanar(0.71±0.08)

dolomites(t-test,𝛂=0.01;p-values=6.5x10-9and7.5x10-7,respectively).

PrincipalComponentAnalysis

DRTable1.TotalVarianceExplained

Component InitialEigenvalues

Total %ofVariance Cumulative%

1 2.305 57.6 57.6

2 0.916 22.9 80.5

3 0.454 11.3 91.9

4 0.326 8.1 100.0

DRTable2.ComponentMatrixandFactorLoadings

Component

1 2 3 4

Stoichiometry 0.774 0.492 -0.128 0.379

CationOrdering 0.823 0.191 0.493 -0.206

d13C -0.560 0.790 -0.101 -0.228

d18O -0.846 0.113 0.429 0.296

78

ReviewofNaturalRecrystallizedDolomites

DRTable3.AnalysisofNaturalRecrystallizedDolomitesinLiterature

Authors Title Year Journal GeologicAge Burial(Shallow

<0.5<

Intermediate<1

km<Deep)

Dept

h

(km)

Temp

(°C)

If

shallow:

possible

meteoric

influence

?

Newman

andMitra

Fluid-influenced

deformationand

recrystallizationof

dolomiteatlow

temperaturesalonga

naturalfaultzone,

MountainCity

window,Tennessee

1994 GSABulletin Cambrian Deep 9 <300

Al-Aasm Chemicaland

IsotopicConstraints

forRecrystallization

ofSedimentary

Dolomitesfromthe

WesternCanada

SedimentaryBasin

2000 Aquatic

Geochemistry

Mississippian Shallow

Yes

Al-Aasm Chemicaland

IsotopicConstraints

forRecrystallization

ofSedimentary

Dolomitesfromthe

WesternCanada

SedimentaryBasin

2000 Aquatic

Geochemistry

Mississippian Deep 4

79

Al-Aasm

and

Packard

Stabilizationofearly-

formeddolomite:a

taleofdivergence

fromtwo

Mississippian

dolomites

2000 Sedimentary

Geology

Mississippian Deep

Reinhold Multipleepisodesof

dolomitizationand

dolomite

recrystallization

duringshallowburial

inUpperJurassic

shelfcarbonates:

easternSwabianAlb,

southernGermany

1998 Sedimentary

Geology

Jurassic Shallow

66-70 Yes

Reinhold Multipleepisodesof

dolomitizationand

dolomite

recrystallization

duringshallowburial

inUpperJurassic

shelfcarbonates:

easternSwabianAlb,

southernGermany

1998 Sedimentary

Geology

Jurassic Shallow

47 Yes

Maloneet

al.

Recrystallizationof

dolomite:evidence

fromtheMonterey

Formation

(Miocene),California

1994 Sedimentology Miocene Deep

>80

Maloneet

al.

Hydrothermal

dolomitizationand

recrystallizationof

dolomitebreccias

fromtheMiocene

1996 Journalof

Sedimentary

Research

Miocene Deep

>80-

100

80

MontereyFormation,

TepusquetArea,

California

Nielsenet

al.

Multiple-step

recrystallization

withinmassive

ancientdolomite

units:anexample

fromtheDinantianof

Belgium

1994 Sedimentology Mississippian Deep ~3

Lonneeand

Machel

Pervasive

dolomitizationwith

subsequent

hydrothermal

alterationinthe

ClarkeLakegasfield,

MiddleDevonian

SlavePoint

Formation,British

Columbia,Canada

2006 AAPGBulletin Devonianto

Mississippian

Deep >1 230-

267

Machelet

al.

Isotopicevidencefor

carbonate

cementationand

recrystallization,and

fortectonic

expulsionoffluids

intotheWestern

CanadaSedimentary

Basin

1996 GSABulletin Devonian Deep 5.5-

6.5

Touiretal. Polyphased

dolomitizationofa

shoal-rimmed

carbonateplatform:

examplefromthe

2009 Cretaceous

Research

Cretaceous Shallow

N/A

81

middleTuronian

Birenodolomitesof

centralTunisia

Lewchuck

etal.

LateLaramide

dolomite

recrystallizationof

theHuskyRainbow

“A”hydrocarbon

Devonianreservoir,

northwestern

Alberta,Canada:

paleomagneticand

geochemical

evidence

2000 Canadian

JournalofEarth

Sciences

Devonian Deep

100

Yooand

Lee

Originand

modificationofearly

dolomitesincyclic

shallowplatform

carbonates,

Yeongheung

Formation(middle

Ordovician),Korea

1998 Sedimentary

Geology

Ordovician shallow

Yes

Kirmaci Dolomitizationofthe

lateCretaceous–

Paleoceneplatform

carbonates,Golkoy

(Ordu),eastern

Pontides,NETurkey

2008 Sedimentary

Geology

Cretaceous intermediate 0.5

Yeand

Mazzullo

Dolomitizationof

LowerPermian

platformfacies,

WichitaFormation,

NorthPlatform,

MidlandBasin,Texas

1993 Carbonatesand

Evaporites

Permian shallow

Yes

82

Kirmaciet

al.

Multistage

dolomitizationin

LateJurassic–Early

Cretaceousplatform

carbonates(Berdiga

Formation),Başoba

Yayla(Trabzon),NE

Turkey:Implications

ofthegenerationof

magmaticarcon

dolomitization

2018 Marineand

Petroleum

Geology

Jurassic-

Cretaceous

Deep >2 170-

210

Adamand

Al-Aasm

Petrologicand

geochemical

attributesofcalcite

cementation,

dolomitizationand

dolomite

recrystallization:an

examplefromthe

Mississippian

PekiskoFormation,

west-centralAlberta

2017 Bulletinof

Canadian

Petroleum

Geology

Mississippian Deep

Lukoczkiet

al.

Earlydolomitization

andpartialburial

recrystallization:a

casestudyofMiddle

Triassicperitidal

dolomitesinthe

VillanyHills(SW

Hungary)using

petrography,carbon,

oxygen,strontium

andclumpedisotope

data

2020 International

JournalofEarth

Sciences

Triassic Deep 1.3 65

83

Durocher

andAl-

Aasm

Dolomitizationand

Neomorphismof

Mississippian

(Visean)Upper

DeboltFormation,

BlueberryField,

NortheasternBritish

Columbia:Geologic,

Petrologic,and

ChemicalEvidence

1997 AAPGBulletin Mississippian Deep 1

Veillardet

al.

Rock-buffered

recrystallizationof

MarionPlateau

dolomitesatlow

temperature

evidencedby

clumpedisotope

thermometryandX-

raydiffraction

analysis

2019 Geochmicaet

Cosmochimica

Acta

Miocene Shallow <0.7 <40 No

Wierzbicki

etal.

Burialdolomitization

anddissolutionof

UpperJurassic

Abenakiplatform

carbonates,Deep

Panukereservoir,

NovaScotia,Canada

2006 AAPGBulletin Jurassic Deep ~4

Zempolich

andHardie

Geometryof

DolomiteBodies

WithinDeep-Water

ResedimentedOolite

oftheMiddleJurassic

VajontLimestone,

VenetianAlps,Italy:

1997 AAPGMemoir

69

Jurassic Deep

84

Analogsfor

Hydrocarbon

ReservoirsCreated

ThroughFault-

RelatedBurial

Dolomitization

Chungand

Land

Dolomitizationofthe

periplatform

carbonateslope

deposit,theMachari

Formation(Middleto

LateCambrian),

Korea

1997 Carbonatesand

Evaporites

Cambrian Deep 1.1

Sallerand

Dickson

Partial

dolomitizationofa

Pennsylvanian

limestonebuildupby

hydrothermalfluids

anditseffecton

reservoirqualityand

performance

2011 AAPGBulletin Pennsylvania

ntoPermian

Deep

Adabi Multistage

dolomitizationof

UpperJurrassic

Mozduran

Formation,Kopet-

DaghBasin,N.E.Iran

2009 Carbonatesand

Evaporites

Jurassic deep 1-3.5

Srinivasan

etal.

Determiningfluid

sourceandpossible

pathwaysduring

burialdolomitization

ofMaryville

Limestone

(Cambrian),

1994 Sedimentology Cambrian Deep 2-3

km

85

Southern

Appalachians,USA

Swennenet

al.

Multiple

dolomitization

eventsalongthe

PozalaguaFault

(PozalaguaQuarry,

Basque–Cantabrian

Basin,Northern

Spain)

2012 Sedimentology Cretaceous Deep

Haeri-

Ardakaniet

al.

Petrologicand

geochemical

attributesof

fracture-related

dolomitizationin

Ordovician

carbonatesandtheir

spatialdistribution

insouthwestern

Ontario,Canada

2013 Marineand

Petroleum

Geology

Ordovician Shallow

N/A

Haeri-

Ardakaniet

al.

Petrologicand

geochemical

attributesof

fracture-related

dolomitizationin

Ordovician

carbonatesandtheir

spatialdistribution

insouthwestern

Ontario,Canada

2013 Marineand

Petroleum

Geology

Ordovician Deep

70-

100

Muelleret

al.

Testingthe

preservation

potentialofearly

diageneticdolomites

2020 Sedimentology Jurassic Intermediate 0.4-

0.8

86

asgeochemical

archives

Rottand

Qing

Earlydolomitization

andrecrystallization

inshallowmarine

carbonates,

MississipianAlida

Beds,Willistoin

Basin(Canada):

Evidencefrom

petrographyand

isotopegeochemistry

2013 Journalof

Sedimentary

Research

Mississippian intermediateto

deep

0.75-

1.4

Salleretal. Faciescontrolon

dolomitizationand

porosityinthe

DevonianSwanHills

Formationinthe

Roseveararea,west-

centralAlberta

2001 Bulletinof

Canadian

Petroleum

Geology

Devonian Deep

Lonneeand

Al-Aasm

Dolomitizationand

fluidevolutioninthe

MiddleDevonian

SulphurPoint

Formation,Rainbow

SouthField,Alberta:

Petrographicand

geochemical

evidence

2000 Bulletinof

Canadian

Petroleum

Geology

Devonian Intermediateto

Deep

Cioppaet

al.

Correlating

paleomagnetic,

geochemicaland

petrographic

evidencetodate

diageneticandfluid

2000 Sedimentary

Geology

Mississippian Shallowto

Intermediate

Yes

87

floweventsinthe

MississippianTurner

ValleyFormation,

MooseField,Alberta,

Canada

Cairdetal. Ediacaran

stromatolitesand

intertidal

phosphoriteofthe

SalitreFormation,

Brazil:

Phosphogenesis

duringthe

Neoproterozoic

OxygenationEvent

2017 Sedimentary

Geology

Precambrian Deep

Zhengetal. Stratigraphicand

StructuralControlon

Hydrothermal

Dolomitizationinthe

MiddlePermian

Carbonates,

Southwestern

SichuanBasin

(China)

2019 Minerals Permian Deep <1.5 <85

Ambers

andPetzold

Geochemicaland

petrologicevidence

oftheoriginand

diagenesisofaLate

Mississippian,

supratidialdolostone

1996 Carbonatesand

Evaporites

Mississippian Deep

Videtich Dolomitizationand

H2Sgenerationin

thePermianKhuff

1994 Carbonatesand

Evaporites

Permian Deep 3-5

km

88

Formation,offshore

Dubai,U.A.E.

Adamset

al.

Dolomitizationby

hypersalinereflux

intodense

groundwatersas

revealedbyvertical

trendsinstrontium

andoxygenisotopes:

UpperMuschelkalk,

Switzerland

2019 Sedimentology Triassic Shallow

Yes

Lukoczkiet

al.

Multi-phase

dolomitizationand

recrystallizationof

MiddleTriassic

shallowTmarine–

peritidalcarbonates

fromtheMecsekMts.

(SWHungary),as

inferredfrom

petrography,carbon,

oxygen,strontium

andclumpedisotope

data

2019 Marineand

Petroleum

Geology

Triassic Deep 1-5

km

Swartetal. Evidenceforhigh

temperatureand

18O-enrichedfluids

intheArab-Dofthe

GhawarField,Saudi

Arabia

2016 Sedimentology Jurassic Deep 2.5

Kupeczand

Land

Progressive

recrystallizationand

stabilizationofearly-

stagedolomite:

1994 IASSpecialPub Ordovician Shallow

Yes

89

LowerOrdovician

EllenburgerGroup,

westTexas

Kupeczand

Land

Progressive

recrystallizationand

stabilizationofearly-

stagedolomite:

LowerOrdovician

EllenburgerGroup,

westTexas

1994 IASSpecialPub Ordovician Deep

Greggetal. Earlydiagenetic

recrystallizationof

Holocene(<3000

yearsold)peritidal

dolomites,Ambergis

Cay,Belize

1992 Sedimentology Holocene shallow

Hartigetal. DolomiteinPermian

paleosolsofthe

BravoDomeCO2

Field,U.S.A.:Permian

refluxfollowedby

laterecrystallization

atelevated

temperature

2011 Journalof

Sedimentary

Research

Permian Intermediateto

Deep

Montanez

andRead

Fluid-rock

interactionhistory

duringstabilization

ofearlydolomites,

UpperKnoxGroup

(LowerOrdovician),

U.S.Appilachians

1992 Journalof

Sedimentary

Research

Ordovician Deep

100-

190

Nicolaides Originand

modificationof

Cambriandolomites

1995 Sedimentology Cambrian Deep

90

(RedHeartDolomite

andArthurCreek

Formation),Georgina

Basin,central

Australia

Vandeginst

eandJohn

Influenceofclimate

anddolomite

compositionon

dedolomitization:

Insightsfroma

multi-proxystudyin

thecentralOman

mountains

2012 Journalof

Sedimentary

Research

Jurassic shallow

Yes

Zhangetal. Formationofsaddle

dolomitesinUpper

Cambrian

carbonates,western

TarimBasin

(northwestChina):

Implicationsfor

fault-relatedfluid

flow

2009 Marineand

Petroleum

Geology

Cambrian deep

120-

200

Coniglioet

al.

Dolomitizationand

recrystallizationof

middleSilurianreefs

andplatformal

carbonatesofthe

GuelphFormation,

MichiganBasin,

southwestern

Ontario

2003 Bulletinof

Canadian

Petroleum

Geology

Silurian deep <2

km

Greggetal. Porosityevolutionof

theCambrian

Bonneterre

1993 Sedimentology Cambrian deep

91

Dolomite,south-

easternMissouri,USA

Soreghanet

al.

Glacioeustatic

transgressivereflux:

Stratiformdolomite

inPennsylvanian

biohermsofthe

westernOrogrande

Basin,NewMexico

2000 Journalof

Sedimentary

Research

Pennsylvania

n

deep

Wooand

Moore

Burialdolomitization

anddedolomitization

oftheLateCambrain

WagokFormation,

Yeongweol,Korea

1996 Carbonatesand

Evaporites

Cambrian deep

Gaswirthet

al.

Theroleandimpact

offreshwater–

seawatermixing

zonesinthe

maturationof

regionaldolomite

bodieswithinthe

protoFloridan

Aquifer,USA

2007 Sedimentology Eocene shallow

Yes

Rahimpour

-Bonabet

al.

Dolomitizationand

anhydrite

precipitationin

Permo-Triassic

carbonatesatthe

SouthParsGasfield,

offshoreIran:

Controlsonreservoir

quality

2010 Journalof

Petroleum

Geology

Permo-

Triassic

Deep 1.4

Dongetal. Hydrothermal

alterationof

2013 Sedimentary

Geology

Ordovician Deep

92

dolostonesinthe

LowerOrdovician,

TarimBasin,NW

China:Multiple

constraintsfrom

petrology,isotope

geochemistryand

fluidinclusion

microthermometry

Mriheel

and

Anketell

Dolomitizationofthe

EarlyEoceneJirani

DolomiteFormation,

Gabes-TripoliBasin,

westernoffshor,

Libya

2000 Journalof

Petroleum

Geology

Eocene Shallow <0.3 <60 Yes

Guoetal. Earlydolomitization

andrecrystallization

oftheLower-Middle

Ordovician

carbonatesin

westernTarimBasin

(NWChina)

2020 Marineand

Petroleum

Geology

Ordovician Deep >1

Özyurtet

al.

Geochemical

characteristicsof

UpperJurassic–

LowerCretaceous

platformcarbonates

inHazineMagara,

Gumushane

(northeastTurkey):

implicationsfor

dolomitizationand

recrystallization

2018 Canadian

JournalofEarth

Sciences

Jurassic-

Cretaceous

Deep 2.2-

4.5

95

93

Geskeetal. Impactofdiagenesis

andlowgrade

metamorphosison

isotope(δ26Mg,

δ13C,δ18Oand

87Sr/86Sr)and

elemental(Ca,Mg,

Mn,FeandSr)

signaturesofTriassic

sabkhadolomites

2012 Chemical

Geology

Triassic Shallow

(interpreted)

thenverydeep

100-

350

Moradetal. Diagenesisofa

mixed

siliciclastic/evaporiti

csequenceofthe

MiddleMuschelkalk

(MiddleTriassic),the

CatalanCoastal

Range,NESpain

1995 Sedimentology Triassic Deep

<65

Meisteret

al.

Dolomiteformation

intheshallowseasof

theAlpineTriassic

2013 Sedimentology Triassic Deep ~1-2 40-70

Martin-

Martinet

al.

Fault-controlledand

stratabound

dolostonesinthe

LateAptianeearliest

AlbianBenassal

Formation(Maestrat

Basin,ESpain):

Petrologyand

geochemistry

constrains

2015 Marineand

Petroleum

Geology

Jurassic-

Cretaceous

intermediate 0.5-

0.75

60-80

Naderetal. Refluxstratabound

dolostoneand

hydrothermal

2004 Sedimentology Jurassic-

Cretaceous

deep ~2 <90

94

volcanism-associated

dolostone:atwo-

stagedolomitization

model(Jurassic,

Lebanon)

Jameson Modelsofporosity

formationandtheir

impactonreservoir

description,Lisburne

Field,PrudhoeBay,

Alask

1994 AAPGBulletin Pennsylvania

n

Deep 1.3

Mazzullo Dolomitizationof

periplatform

carbonates(Lower

Permian,

Leonardian),

MidlandBasin,Texas

1994 Carbonatesand

Evaporites

Permian Deep 1.2-

1.8

Al-Aasmet

al.

Dolomitizationand

relatedfluid

evolutioninthe

Oligocene-Miocene

AsmariFormation,

GachsaranArea,SW

Iran:Petrographic

evidence

2009 Journalof

Petroleum

Geology

Oligocene-

Miocene

Shallow

Yes

Duetal. GenesisofUpper

Cambrian-Lower

Ordoviciandolomites

intheTaheOilfield,

TarimBasin,NW

China:Several

limitationsfrom

petrology,

2018 Marineand

Petroleum

Geology

Cambrian-

Ordovician

Deep

95

geochemistry,and

fluidinclusions

Guoetal. Multiple

dolomitizationand

laterhydrothermal

alterationonthe

UpperCambrian-

LowerOrdovician

carbonatesinthe

northernTarim

Basin,China

2016 Marineand

Petroleum

Geology

Cambrian-

Ordovician

Deep

Montanez LateDiagenetic

Dolomitizationof

LowerOrdovician,

UpperKnox

Carbonates:A

Recordofthe

Hydrodynamic

Evolutionofthe

Southern

AppalachianBasin

1994 AAPGBulletin Ordovician Deep 2-5

km

80-

160

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Lohmann,K.C.,Passey,B.H.,Peral,M.Y.,Petrizzo,D.A.,Rosenheim,B.E.,Tripati,A.,Venturelli,

R.,Young,E.D.,andWinklestern,I.Z.,2019,Effectsofimproved17Ocorrectionon

interlaboratoryagreementinclumpedisotopecalibrations,estimatesofmineral-specific

offsets,andtemperaturedependenceofaciddigestionfractionation:Geochemistry,

Geophysics,Geosystems,v.20,p.3495-3519.

Rivers,J.M.,Skeat,S.,Yousif,R.,Liu,C.,Stanmore,E.,Tai,P.,andAl-Marri,S.M.,2019,The

depositionalhistoryofnear-surfaceQataraquiferrocks,anditsimpactonmatrixflowand

storageproperties:ArabianJournalofGeosciences,v.12,p.1-33.

Ryan,B.H.,Kaczmarek,S.E.,andRivers,J.M.,2020,Earlyandpervasivedolomitizationby

near-normalmarinefluids:NewlessonsfromanEoceneevaporativesettinginQatar:

Sedimentology(inpress).https://doi.org/10.1111/sed.12726

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CHAPTERIV

DOLOMITEDISSOLUTION:ANALTERNATIVEDIAGENETICPATHWAYFORTHEFORMATIONOFPALYGORSKITECLAY

BrooksH.Ryan1,StephenE.Kaczmarek1,JohnM.Rivers21DepartmentofGeologicalandEnvironmentalSciences,WesternMichiganUniversity,Kalamazoo,Michigan49006,U.S.A.2QatarCenterforCoastalResearch(QCCR),ExxonMobilResearchQatar,P.O.Box22500,QatarScienceandTechnologyPark-Tech,Doha,QatarRyan,BH.,Kaczmarek,S.E.,andRivers,J.M.,2019,Sedimentology,v.66,p.1803-1824.https://doi.org/10.1111/sed.12559

Abstract

Palygorskite isa fibrous,magnesium-bearingclaymineralcommonlyassociatedwithLate

MesozoicandEarlyCenozoicdolomites.Thepresenceofpalygorskiteisthoughttobeindicativeof

warm,alkaline fluidsrich inSi,Al,andMg.Palygorskitehasbeen interpretedto forminperitidal

diagenetic environments, either as a replacement of detrital smectite clay during a dissolution-

precipitationreactionorsolid-statetransformation,orasadirectprecipitatefromsolution.Despite

a lack of evidence,most diagenetic studies involving these twominerals posit that dolomite and

palygorskiteformconcurrently.Here,petrologicalevidenceispresentedfromtheUmmerRadhuma

Formation (Paleocene-Eocene) in the subsurface of central Qatar for an alternative pathway for

palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal

carbonate cycles that are commonly capped by cm-scale beds rich in palygorskite. Thin section,

scanningelectronmicroscopy,andelementalanalysesdemonstratethatpalygorskitefibersformed

on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to

completely dissolved dolomite crystal cores. These observations suggest that dolomite and

palygorskite formedsequentially,andsupportamodelbywhichthereleaseofMg2+ ionsandthe

bufferingofsolutionpHduringdolomitedissolutionpromotetheformationofpalygorskite.Thisnew

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diageneticmodelexplainstheco-occurrenceofpalygorskiteanddolomite intherockrecord,and

providesvaluable insight into thespecificdiagenetic conditionsunderwhich thesemineralsmay

form.

Introduction

Palygorskite,withanidealcompositionof(Mg,Al)2Si4O10(OH)•4(H2O),isamagnesium-rich

monoclinicandorthorhombicclaymineralwithacharacteristicfibroushabit(Weaver&Beck1977;

Singer, 1979, 1984, 2002; Galan, 1996; Galan& Carretero, 1999; Guggenheim&Krekeler, 2011;

Murrayetal.,2011).Palygorskiteisobservedinawidevarietyofsedimentarydepositsworldwide

(Isphording,1973;Callen,1977;Callen,1984;Weaver&Beck,1977;Torres-Ruizetal.,1994;Akbulut

&Kadir, 2003),with the largest deposits locatedwithin theMiddleTertiary sediments of China,

Senegal,Spain,Turkey,Ukraine,andthesoutheasternUnitedStates(Krekeler,2004;Krekeleretal.,

2004;Garcia-Romeroetal.,2007;Murrayetal.,2011;Yeniyol,2012;Yeniyol,2014;Kadiretal.,2016;

Kadiretal.,2017).Themineralalsoservesasanimportantconstituentinsoilsinaridtosemi-arid

environments(Singer&Norrish,1974;Singer,1984;Singer,2002).

Palygorskitehasbeenreportedinenvironmentsofmarine,lacustrine,andcontinentalorigin

(e.g.,Weaver,1984;Botha&Hughes,1992;Singer,2002),but there is littleagreementabout the

controls,timing,andmechanisminvolvedwithitsformation.Multiplegeneticinterpretationshave

beenofferedtoexplainthepresenceofpalygorskiteintherockrecord.Theseincludepalygorskite

as a primary detritalmineral (Singer & Amiel, 1974; Singer, 1979), a diagenetic replacement of

detritalsmectite(Weaver&Beck,1977;Chenetal.,2004;Xieetal.,2013),andadirectprecipitate

fromaqueoussolutions(Weaver,1975;Shadfanetal.,1985a;Çağatay,1990;Botha&Hughes,1992;

Holail and Al-Hajari, 1997; Xie et al., 2013). Efforts to understand the fundamental controls on

palygorskiteformationhave,however,beenlimitedbyaninabilitytosynthesizeitinthelaboratory

atearthsurfaceconditions(Singer,1979;2002).Thermodynamicconsiderationscoupledwith its

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non-uniformdistributionintherockrecordsuggestthatspecificphysicochemicalfactorspromote

palygorskite formation, includingwarmschizohaline fluidswithpHbetween7.7 and9, andhigh

Mg/Alratios(Singer&Norrish,1974;Singer,1979;Birsoy,2002;Singer,2002).

Numerouscasestudieshavedocumentedtheco-occurrenceofdolomiteandpalygorskitein

therockrecord(Weaver,1975;Weaver&Beck,1977;Shadfanetal.,1985a;Shadfanetal.,1985b;

Çağatay,1988;Çağatay,1990;Ingles&Anadon,1991;Verrecchia&Le-Coustumer,1996;Holail&Al-

Hajari,1997;Krekeleretal.,2007;Xieetal.,2013;Draidiaetal.,2016;Kadiretal.,2017).Despitea

lackofcompellingpetrologicalevidence,anumberofmodelstoexplainthegenesisofpalygorskite

inassociationwithdolomitehavebeenproposed.Thepresenceofdolomite,forexample,hasbeen

citedasevidenceforelevatedMgconcentrationsinsolution(Singer,1984;Çağatay,1990;Xieetal.,

2013), the inference being that both dolomite and palygorskite form concurrently under such

conditions (Weaver, 1975;Weaver&Beck, 1977).However, large scale co-precipitation of these

minerals while in competition for Mg2+ ions is geochemically unfavorable because the

thermodynamicstabilityofbothmineralslargelydependsonmagnesiumconcentrationsinsolution.

Alternatively,studieshavehypothesizedthatdolomitedissolutioncouldprovidetherequisiteMgfor

palygorskiteformation(Ingles&Anadon,1991;Verrecchia&Le-Coustumer,1996;Holail&Al-Hajari,

1997).Thermodynamicstabilitydiagrams,however, indicate thatbothmineralsareconcurrently

stable(orunstable)insimilarenvironments(Birsoy,2002).

As neither mineral has been successfully synthesized in the laboratory at earth-surface

temperaturesandpressures(seereviewsbySinger,1979;Kaczmareketal.,2017),understanding

theco-occurrenceofpalygorskiteanddolomiteinnatureisintriguing.Thediageneticrelationship

betweenthesetwomineralsmayalsohaveimportantindustrialapplicationsbecausedolomitization

generally improves the quality of hydrocarbon reservoirs (Lucia, 2004)whereas precipitation of

claysgenerallyhasanegativeimpact(Aksuetal.,2015).Inaddition,understandingthegeochemical

constraintsonpalygorskiteformationmaybeusefulwhenreconstructingpaleoenvironments.Thus,

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the present study investigates the co-occurrence of palygorskite and dolomite in the Umm er

RadhumaFormation(UER),Qatar.Thepetrologicaldatapresentedprovideevidenceofpalygorskite

formationasadirectresultof,andconcurrentwith,dolomitedissolution.Thedataprovidedhere

supportspreviousstudiesthathavespeculatedthatpalygorskitecanformasaresultofdolomite

dissolution(Ingles&Anadon,1991;Verrecchia&Le-Coustumer,1996;Holail&Al-Hajari,1997).

GeologicBackground

Qatarcanbedescribedasanellipticalstructuraldomethatformedinassociationwiththe

North-SouthstrikingQatarArch(Fig.1;Cavelier,1970).DuringtheLatePaleocene-EarlyEocene,the

area was structurally quiescent (Cavelier, 1970). Semi-restricted, inner ramp limestones were

depositedacrossmuchoftheArabianShelf(Powersetal.,1966).Theselimestonesandtheassociated

shalesandevaporitesareknownregionallyastheUmmerRadhumaFormation,andareasmuchas

370mthickinthesubsurfaceofQatar(Powersetal.,1966;Cavelier,1970,Ecclestonetal.,1981).Al-

Saad(2003)interpretedtheLatePaleocene-EarlyEoceneasbeingpartofahighstandsystemstract

Well RR-01

Qatar

SaudiArabia

Persian Gulf

Kilometres

Figure1:MapofQatarandsurroundingregion.LocationofWellRR-01outlinedbyredcircle.

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acrossalowreliefbasin.LikemanyofthelimestonesacrossmuchoftheArabianPeninsula,theUmm

erRadhumahasundergoneextensivedolomitization(Powersetal.,1966).Otherthantheseearly

studies, little work has been done on the sedimentologic and diagenetic history of the Umm er

Radhuma inQatar.Thus, thepetrographicobservationsdescribedhereprovidea firstattempt to

unravelthediagenetichistoryoftheUmmerRadhumainQatar.

MaterialsandMethods

AlldataacquiredforthisstudywereextractedfromtheRR-01well,locatedincentralQatar

(Fig.1).TheRR-01well,thefirstofthreeresearchboreholesdrilledbyExxonMobilResearchQatar

toinvestigatetheshallowsubsurfacegeologyofthecountry,wasdrilledin2016.Approximately135

mofcorewasrecovered,includingmaterialfromtheupperUmmerRadhuma,Rus,andDammam

Formations (Fig.2).A totalof146cylindrical coreplugs (~2.5cmdiameter,~3.8cm long)were

extractedat0.02-2mverticalspacingthroughouttheUmmerRadhuma,theintervalofinterestfor

thisstudy.Standard,blueepoxy-impregnatedpetrographicthinsectionsweremadefromthecore

plugs.High-resolutionthinsectionscanswereacquiredbyCoreLaboratories(U.A.E.).

PowderstakenfromeachcoreplugwereanalyzedusingstandardpowderX-raydiffraction

(XRD)techniques.MineralpercentagesforeachsamplewerequantifiedbyCoreLaboratoriesusing

the following procedure. Each sample was dispersed in a dilute sodium hexa-meta phosphate

solutionandcentrifugallysizefractionedtoisolateclay-sized(<2-4µm)particles.Air-driedmounts

weremeasuredbeforeandafterexposuretoethyleneglycolat60°Cforaminimumof2hours.The

sampleswereanalyzedwithaPanalyticalautomatedpowderdiffractometerequippedwitha40kV

coppersource,anX’celeratorlineardetectorusingRealTimeMultipleStripTechnology(RTMS),a

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Ni-Filter,andaGraphitemonochromator.Bulksampleswerescannedovera2θrangeof4°to70°at

a rate of 4.2°/minute. Clay samples were scanned over a 2θ range of 2.5° to 40° at a rate of

6.4°/minute. Determinations of individual clay percentages were done utilizing integrated peak

areasandempirical reference intensity ratio (RIR) factors.Mineralogy fora select subsetof core

samples was confirmed with standard powder XRD techniques employed at Western Michigan

Dammam

Formation

Period Epoch

Rus

Ummer

Radhuma

Mid

dle

Early

Late

Pal.

Eoce

ne

Pala

eoge

ne

140

120

100

20

0

40

60

80

Depth (m)

Calcite Dolomite Gypsum Clays and Quartz

Mineralogy

Figure2:StratigraphiccolumnforWellRR-01.PercentmineralogyisbasedontheX-raydiffraction(XRD)proceduresofCore Laboratories (United Arab Emi- rates). Palaeocene–Eocene boundary for Umm er Rad- huma Formation wasinterpretedbycomparisonofbiostratigraphyandd13CvalueswiththoseofPollittetal.(2012).AgeboundariesfortheRusandDam-mamformationsaremodifiedfromWhittleetal.(1996),Holailetal.(2005)andAl-Saad(2005).

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UniversityusingaBrukerD2PhaserDiffractometerwithaCuKαanode.Powderswereextracted

usinganelectricrotarydrill,crushedbyhandusinganagatemortarandpestle,andmountedona

Boron-dopedsiliconP-typezerobackgrounddiffractionplates.XRDscanconditions includea2θ

rangeof5°to50°,astepsizeof0.004°,andacounttimeof2.0sperstep.

Scanning electron microscope (SEM) analysis was conducted on samples with both

palygorskite and dolomite as determined by XRD and thin section petrography. SEM

photomicrographswerecollectedonaJEOLJSM-IT100InTouchScope.Rocksampleswereprepared

by breaking small pieces from core plugs with a rock hammer. Samples were cleaned with

compressed air to remove particulate matter, mounted on aluminum stubs with electrically

conductivecarbon tape,andcoatedwitha thin (~20nm) layerof carbon.All rocksampleswere

analyzedatanacceleratingvoltageof20kV.Workingdistance(8-12mm)andprobecurrent(50-70

eV)werevariedasneededtoachieveoptimalimagingresults.SEMenergy-dispersivespectroscopy

(SEM-EDS)wasusedinconjunctionwithXRDtodetermineelementalcompositionattheµm-scale

forbothcoatedrocksamplesanduncoatedthinsections.Onehighlypolishedthinsectionandtwo

standard(non-polished)thinsectionswereanalyzedusingEDSinbackscattermodeat15kV-20kV.

Morethan300pointswereanalyzedondolomitecoresandrimsfrom18crystalsinahighlypolished

thinsectionfrom78.19m,whilemorethan250pointsweretakenfrom10crystalsinstandardthin

sectionsat86.08and89.55mdepth.

Results

Sedimentologicaldescription

TheUmmerRadhumaisdominatedbysedimentarysequencescomprisedofcarbonateand

clay-rich intervals. These intervals are defined based on the mineralogy data (Fig. 2) and core

observations. Carbonate intervals range from 5 to 25 m thick, and are characterized by vuggy

dolomitethatdisplayspreferentialdissolutionofrelativelylarge(0.5–2.0mm)benthicforaminifera

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(Fig.3A),andpartialmimeticreplacementofallochems,suchascrinoids(Fig.3B)andforaminifera

(Fig.3C).Thesefeaturessuggestthattheprecursorlimestonewasdepositedinasubtidaltoperitidal

Burrows

Dolomite

Higherclaycontent

Crinoids

Foram moulds

Dolomite

A

B

C

D

E

5 cm

Figure3:PhotographsshowingsomeofthecommonsedimentologicalfeaturesobservedintheUmmerRadhuma.(A)to(D)High-resolution,planepolarizedlight(PPL)thinsectionimages.(A)Relativelylarge(ca1mm)vugsformedasaresultofpreferentialdissolutionofforaminifera.Depthof122.25m;scalebar=1mm.(B)Partiallyandmimeticallydolomitizedechinoderms.Interparticlepalygorskitecement(green)fillsthespacebetweenechino-dermsanddolomitematrix.Depthof100.55m;scalebar=100µm.(C)Mimeticallydolomitizedlargebenthicforaminiferasurroundedbyadolomitematrixandpalygorskitecement.From96.85mdepth;scalebar=200µm.(D)Mimeticallydolomitizedmiliolidforaminifera(grey)withinpore-occludinggypsumcement(white).Depthof66.48m;scalebar=100µm.(E)Corephotographofthetransitionfromasubtidalvuggydolomitetoaburrow-dominatedclay-richcapandbacktoasubtidalvuggydolomite.Acoreplugwasacquiredat122.94mwiththethinsectionimagepresentedinFig.5A.

105

setting.Theabundanceofmimeticallyreplacedmiliolidforaminiferaincreasestowardthetopofthe

Ummer Radhuma (Fig. 3D),which suggests an overall shallowing upward trend. This is further

supportedbyanincreasedpresenceofgypsumintheuppermostpartoftheformation(Fig.2;3D).

In contrast to the carbonate intervals, the clay-rich intervals are generallymuch thinner,

ranging from 0.05 – 5.0 m thick. These intervals are characterized as condensed argillaceous

carbonate intervals, containing anywhereup to1 – 26.5%clayminerals (Fig. 2). These clay-rich

intervals commonly contain sedimentary structures, such as extensive burrows (Fig. 3E), which

indicate a general shallowing from the underlying carbonate interval and are interpreted to

representdepositional cyclecaps.Multiplecycles, suchas theone justdescribedanddepicted in

Figure 3E, are identified throughout the Umm er Radhuma, with varying concentrations of clay

materialwithinthecyclecaps(Fig.2).

XRDmineralogy

AsshowninFigure2,themineralogyoftheUmmerRadhumaFormationisdominatedby

dolomite.Alsopresentinsignificantamountsaregypsum(0-22%),andavarietyofsilicateminerals.

The abundance of clays and quartz generally constitutes ≤ 26.5% and ≤ 5% of the bulk rock,

respectively.InsixofthetwelvecoreplugsexaminedwithSEM-EDS,palygorskitemakesup>90%

ofthetotalclayfractionintherock.Intheremainingsixcoreplugs,palygorskiteconstitutes≥60%

of the clays, with the remaining fraction consisting mainly of illite or sepiolite. Figure 4 is a

representativeXRDdiffractogramfromtheUmmerRadhumashowingamixtureof~76%dolomite

and~20%palygorskite. Themost intense XRD peak on the diffractogram is the (104) dolomite

reflection at 31.0° 2θ. Dolomite stoichiometry (i.e.,mole%MgCO3)was determined using the d-

spacing of the (104) reflection based on the empirical equation derived by Lumsden (1979).

Stoichiometryrangesfrom49.8to51.1mole%MgCO3(x=50.5%;s=0.44%).Additionally,the(104)

dolomitereflectionpeaksdisplaysignsofasymmetry.Thepresenceofthe(101),(015),and(021)

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reflectionsat22.0,35.3,and43.8°2θ,respectively,suggestthatthedolomiteisordered(Greggetal.,

2015;Kaczmareketal.,2017).Thedegreeofcationorderingrangesfrom0.36to0.95(x=0.72;s=

0.14).TheprincipalXRDpeakforpalygorskiteisthe(110)reflectionidentifiedat8.5°2θ,whichis

consistentwithpalygorskiteexhibitingad-spacingof10.4Å(VanScoyocetal.1979;Chisholm,1992;

Garcia-Romeroetal.,2007;Post&Heaney,2008).

Thinsectionpetrography

Thin section analysis was used to characterize the variousmineral phases and establish

parageneticrelationshipsbetweendolomiteandpalygorskite(Fig.5).Emphasiswasgiventocore

sampleslocatedbetween80and123m,wherepalygorskiteismostabundantbasedonXRD(Fig.2).

Dolomite in this depth range is characterized by planar-e and planar-s crystals, with diameters

ranging 30 - 100 µm (Fig. 5). Clear, inclusion-poor dolomite rims are commonly observed in

associationwithcloudyeuhedraldolomitecores(Fig.5Dand5E).Cloudydolomitecoreslackcalcite

inclusions,and inmany instancesarepartially tocompletelydissolved (Fig.5A-F).Below123m,

5 10 15 20 25 30 35 40 45 50

Degrees 2O

Rel

ativ

e In

tens

ity (l

og s

cale

)Dol(104)

*Dol(015)

*Dol(021)*Dol(101)

Pal(110)

Qtz(101)

Qtz(100)

Dol(113)

Dol(006) Dol(110)Dol(202)

Dol(012)

Figure4:PowderX-raydiffractionpatternofarepresentativesample(110.45mdepth)containingca76%dolomiteandca20%palygorskitefrom5°to55°2q.Thedolomite(104)peakisthedominantdolomitepeakobserved,anddolomiteorderingreflectionpeaks[(101),(015)and(021)]aredenotedbyanasterisk.Palygorskiteisidentifiedbythepresenceofthedominant(110)peak,andquartzisidentifiedbythe(100)and(101)peaks.Dol,dolomite;Pal,palygorskite;Qtz,quartz.Bottomlinerepresentsbackground.

107

dolomitesarecharacterizedbyextremelycoarse(100-200+µm)anhedraltoplanar-smosaics.Above

80m, fine(<10µm), fabric-preservingmimeticdolomitesareobserved(Fig.3D).Throughoutthe

UER,palygorskiteisidentifiedinthinsectionbyitslightgreencolor.Palygorskiteoccursasapore-

occluding, intercrystalline cementhostedbetweendolomite rhombs (Figure5). Palygorskite also

commonly occurs on the surfaces of partially dissolved dolomite crystals (Figs. 5A and 5F), and

withinpartiallytocompletelydissolveddolomitecores(Figs.5B,5D,and5E).

SEM-EDSanalysis

SEMmicrographsshowthepresenceofpalygorskitefibersanchoredtothecrystalfacesof

partially dissolved dolomite rhombs as well as palygorskite clusters nested within partially to

completelydissolveddolomite cores (Figs. 6, 7, and8).As shown in Figure6, partially corroded

planar-edolomitecrystalsaresurroundedbyamatrixofpalygorskitefibers.Highermagnification

SEMimagesshowfinepalygorskitefibersoccupyingdissolutionvoidsonpartiallycorrodedeuhedral

dolomite crystal faces (Figs. 6C and 6D). Elemental compositions of these features (Fig. 7) are

consistentwith dolomite andpalygorskite.Dolomiteswith cloudy core fabrics exhibit a partially

dissolvedrhombohedralcorefilledwithpalygorskitefibers(Figs.8Aand8B),andlessfrequently

dolomitecoresarecompletelydissolvedandfilledwithpalygorskitefibers(Figs.8Cand8D).The

curvedshapeofpalygorskitefibersneartheirendislikelyanartifactresultingfromdehydrationand

electronbeamdamage(Krekeler&Guggenheim,2008).

High-resolutionEDSspotdatashowthatdolomiterimsaregenerallymorestoichiometric

thandolomite cores.Data from thehighlypolished thin section is associatedwith a significantly

loweruncertaintythanthedatafromthestandardpetrographicthinsections.Thepolishedsample

(78.19m)yieldsanaverageMg/Caratioof0.88(s=0.05)forthecoresandanaverageMg/Caratio

of0.92(s=0.04)fortherims(Fig.9).Thestandardthinsectionfrom86.06yieldsaverageMg/Ca

108

ratiosof0.78(s=0.18)and0.85(s=0.19)forcoresandrims,respectively.Thestandardthinsection

from89.55myieldsaverageMg/Caratiosof0.77(s=0.17)and0.85(s=0.16),respectively.

BA

C D

E F

Figure5:High-resolution,planepolarizedlight(PPL)thinsectionimagesofdolomite(lightbrowntolightgreyrhombs)andpalygorskite(greenhaze)fromtheUmmerRadhuma.(A)Depthof122.94m.Planar-eandplanar-sdolomitewithintracrystalline(cores)andintercrystallineporosityfilledwithpalygorskite.(B)Depthof120.67m.Similardolomiteasshownin(A),butexhibitsaninnerdissolvedcoreandintactrim.Palygorskiteisobservedasanintercrystallinecement,as well as intracrystalline cement in the partially dissolved cores. (C) Depth of 110.45 m. Euhedral dolomite withintercrystallinepalygorskitecement.(D)Depthof99.10m.Cloudy-coreclear-rimdolomitecrystalsdisplayingpartiallydissolvedcoreswithpalygorskite fillingas intracrystallinecement.Palygorskite isalsoobservedasan intercrystallinecementbetweendolomitecrystals.(E)Depthof89.55m.Palygorskiteisobservedascementbetweendolomitecrystals.Dissolveddolomite coresare commonandpalygorskite isobservedwithin thedissolvedcores. (F)Depthof79.77m.Euhedraldolomitecrystalsdisplayingpartialdissolutionofcrystalfaces,inwhichpalygorskiteisobserved.Rarecloudy-coreclear-rimdolomiteswithdissolvedcoresarealsoobserved.Scalebarinallimagesrepresents100µm.

109

F

B

10 µm

10 µm

D

5 µm

10 µm

10 µm

A

5 µm

C

E

Figure6:Scanningelectronmicrophotograph(SEM)imagesofdolomiteandpalygorskitefromwelldepthof110.45m(A)to(D)and93.98m(E)and(F).(A)Euhedraldolomitecrystalengulfedinpalygorskite.Dissolutionfeaturesareobservedonthedolomitecrystalface.(B)Euhedraldolomitecrystalsurroundbypalygorskite.Notethemajordissolutionfeatureinthetopleftquadrantofthecrystal.(C)Magnifiedimageoftopleftcornerofdolomitecrystalin(A).Fibrouspalygorskitecrystalsareobservedextendingperpendicularfromthepartiallydis-solveddolomiteface.(D)Magnifiedimageofthedissolutionfeatureobservedintheupperleftcornerofthedolomitecrystalfacein(C).Notethepalygorskitefibresforminginandonthe dissolved surface. (E) Extremely corroded dolomite crystal displaying palygorskite fibre nucleations on the crystalsurface. Palygorskite is also observed as intercrystalline cement coating the dolomite crystal. (F) Partially dissolvedeuhedraldolomitecrystalwithassociatedpalygorskiteonthecrystalfaceandwithinintercrystallinepores.

110

Discussion

ArabianPeninsuladolomite&palygorskiteco-occurrence

Numerousregionalstudieshavedocumentedtheco-occurrenceofpalygorskiteanddolomite

inEocenecarbonatesontheArabianPeninsula(Shadfanetal.,1985a;Shadfanetal.,1985b;Holail&

Al-Hajari,1997;Çağatay,1988;Çağatay,1990;Draidiaetal.,2016).Althoughanumberofgenetic

modelshavebeenofferedtoexplainthis,thereisagenerallackofconclusivepetrologicalevidence.

Shadfanetal.(1985a),forexample,reportedontheco-occurrenceofpalygorskiteanddolomitein

thecarbonatesofthePaleocene-LowerEoceneUmmerRadhumaFormationandtheLower-Middle

EoceneDammamFormationofeasternSaudiArabia.Palygorskite fiberswerereportedtoexhibit

ke V0 .0 0 0 .5 0 1 .0 0 1 .5 0 2 .0 0 2 .5 0 3 .0 0 3 .5 0 4 .0 0 4 .5 0 5 .0 0

Co

un

ts[x

1.E

+3

]

0 .0

2 0 .0

4 0 .0

6 0 .0

8 0 .0

1 0 0 .0

0 0 3

C

O

M gA l

S i

SS

K KC aC a

ke V0 .0 0 0 .5 0 1 .0 0 1 .5 0 2 .0 0 2 .5 0 3 .0 0 3 .5 0 4 .0 0 4 .5 0 5 .0 0

Co

un

ts[x

1.E

+3

]

0 .0

5 .0

1 0 .0

1 5 .0

2 0 .0

2 5 .0

3 0 .0

0 0 2

C

O

M g

S i SS

C a

C a

ke V0 .0 0 0 .5 0 1 .0 0 1 .5 0 2 .0 0 2 .5 0 3 .0 0 3 .5 0 4 .0 0 4 .5 0 5 .0 0

Co

un

ts[x

1.E

+3

]

0 .0

1 0 .0

2 0 .0

3 0 .0

4 0 .0

5 0 .00 0 1

C

OM g

A l

S i

SS

C a

C a

001

5 µm

002

5 µm

003

2 µm

10 µm

Figure7:ElectrondispersiveanalysisofthesamedolomitecrystalasFig.6AandC.Point001wastakeninthesameareaasFig.6B.Theelementsidentifiedareindicativeofbothdolomite(Ca,Mg,CandO)andpalygorskite(Mg,Al,SiandO).Point002wastakeninthecentreofthepartiallydissolveddolomiteface.Theelementsidentifiedarecharacteristicofdolomite,withrelativelyminorcountsofSiandS.Notethenm-sizedpalygorskitecrystalsnucleatingonapartiallydissolved dolomite face. Point 003 was taken in the surrounding palygorskite matrix. The elements identified arerepresentativeofpalygorskite,withrelativelyminorcountsofC,S,KandCa.Notethatenergy-dispersivespectroscopygraphsmaydisplaytwopeaksforthesameelement.AllgreypeaksrepresentcountsofK-alphaX-rays,whilethecolouredpeaksrepresentcountsofK-betaX-rays.

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alignedorientationsintheUmmerRhaduma,whileintheDammam,palygorskitefibersdisplayed

uniformsizesandshapes(Shadfanetal.,1985).Theseobservationsandanabsenceofotherclays

werecitedbyShadfanetal.(1985a)asevidenceforanauthigenic,ratherthanadetritaloriginfor

thepalygorskite.TheauthorssuggestedthatthepHoftheformationwatercouldhaveincreasedin

responsetoshallowwaterconditionsandhigherthannormaltemperaturesduringtheEarlyEocene,

whichmayhavecausedanincreaseinphotosyntheticuptakeofCO2.Asaconsequenceofincreased

pH, it was argued, the solubility of silica would have also increased, thus providing a source of

dissolved silica forpalygorskite formation (Shadfanet al., 1985a).Theauthorsdidnot,however,

proposeasourceoftheMgnecessaryforpalygorskite,nordidtheyprovideanexplanationforthe

D

10 µm 5 µm

5 µm 2 µm

A B

C

Figure8:Scanningelectronmicrophotograph(SEM)ofdolomiteandpalygorskitefromwelldepthof89.55m.(A)Thedolomitecrystalissurroundedbypalygorskitecrystals.Withinthedolomiterimisapartiallydissolveddolomiterhomb,similartoobservationsinFig.5DandE.(B)Similarobservationtothatin(A),displayingacloudy-coreclear-rimdolomitecrystalwithpalygorskitefibrescoatingtheoutersurfaceoftherimandnucleatingonthepartiallydissolvedcore.(C)Asimilardolomitecrystaltothatin(A),butitisdenselycoveredbypalygorskitefibresanddisplaysamoresolublerhombiccore.(D)Highermagnificationofthevoidleftbythecorein(C).Theinnerrhombohedralcoreofthisdolomitecrystalisobservedtobecompletelydissolvedandfilledwithpalygorskite.

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co-occurrenceofdolomiteandpalygorskite.Theycommentedonly that thepresenceofdolomite

suggestedamarineoriginforpalygorskite(Shadfanetal.,1985a).

Çağatay(1990)investigatedcarbonatesoftheEoceneRusandDammamFormationsinSaudi

Arabia. Palygorskite was the most abundant clay mineral, with illite and smectite also present.

Clustersofpalygorskitefibersweredescribedasblanketssurroundingdiageneticdolomitecrystals

anddetritalanataseandfeldspar.Thefiberswerealsopresentas intercrystalline,pore-occluding

cementswithin the dolomitized intervals. The fine, delicate nature of the fibers and the fibrous

coatings,coupledwithpreservationoffinelaminaeintheshaleswereinterpretedbytheauthorsas

evidencetoprecludeadetritaloriginforthepalygorskite.Instead,Çağatay(1990)interpretedthe

observed textures as resulting from direct precipitation from Mg-rich solutions, a hypothesis

consistentwithtextural,sedimentological,andfossilevidencesuggestingdepositionoccurredina

semi-restricted,shizohaline,peripheralbasin.Itwasarguedbytheauthorthatsuchanenvironment

wouldhavelikelyexperiencedanaridtosemi-aridclimate,reducedsalinity,andanelevatedpH,all

conditionsthatareidealforprimarypalygorskiteformation(Çağatay,1990).

Based on the observation that palygorskite formed cements around dolomite crystals,

Çağatay (1990) concluded that palygorskitemusthaveprecipitated afterdolomitization. Çağatay

(1990)furthercontendedthatthepresenceofdolomiteisevidencethatMgconcentrationsinthe

pore fluids were high enough to support subsequent palygorskite precipitation, with Si and Al

suppliescomingfromthedissolutionofdetritalsilicatemineralsunderalkalineconditions. Inhis

model,Çağatay(1990)postulatedthatmarine-derivedporefluidscouldprovidethenecessaryMgto

formdolomiteandpalygorskiteinsequence.

A studyofEocene, palygorskite-bearing carbonates ofQatarbyHolail&Al-Hajari (1997)

showedthattheSimsimaMemberoftheDammamFormationcontaineddolomitizedshallow-marine

limestoneand4%clayminerals,whicharemainlypalygorskite.SEMobservationsrevealedbundles

of palygorskite fibers coating dolomite crystals, suggesting that palygorskite postdates

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dolomitization. Holail & Al Hajari (1997) also carried out geochemical analyses including X-ray

fluorescence and stable oxygen isotopes to further understand the nature and timing of the

palygorskite relative to the dolomite. Elemental analysis revealed an excess of CaO in the

palygorskite, interpretedasCa-carbonate inclusionswithintheclay.Oxygen isotopevaluesof the

palygorskite averaged +18.7‰ SMOW, while the dolomites range from +1.1 to +4.7‰ SMOW

suggestingthatthepalygorskitecoatingsformedfromdifferentwatersthanthedolomitecrystals

(Holail & Al-Hajari 1997). No discussion on the difference in oxygen fractionation between

palygorskite and dolomite was offered, however. The depositional environment of the Simisma

duringEocenetimeisinterpretedbyHolail&Al-Hajari(1997)asashallowmarineenvironmentwith

elevatedsalinityandanaridclimate.Suchanenvironment,itwasposited,wouldhaveprovidedideal

conditions for direct precipitation of palygorskite.Holail&Al-Hajari (1997) suggested that their

textural and geochemical observations support a model in which palygorskite formed after the

dolomite,andthattheenvironmentwasmoresuitableforprimaryprecipitationofpalygorskite,as

opposedtoformationviaalterationofsmectiteordepositionasadetritalmineral.

ThemodelofHolail&Al-Hajari(1997)differsfromthatofÇağatay(1990),inthatitsuggests

that dissolving dolomite provides the Mg reactants for the precipitating palygorskite, with the

necessarySiandAlderivedfromthedissolutionofphyllosilicatesandaninfluxofmeteoricwater

(Holail&Al-Hajari1997).WhileitislogicalthatdissolvingdolomitecouldcontributeMgtothepore

fluidsforpalygorskiteprecipitation,noevidenceofdolomitedissolutionwasprovided.Further,a

scenariowherebylarge-scaledissolutionofclaysprovidedanaqueousmediumofdissolvedSiand

Al rather than amodelwhereby small-scale dissolution-precipitation reactions are occurring on

precursorclaysisunlikely.Althoughtheauthorsreportanabsenceofsmectite,itispossiblethatit

hadbeenreplacedbypalygorskiteviaadissolution-precipitationreaction,ratherthanpalygorskite

formingasadirectprecipitate.Thiswouldbeanalogoustocompletelydolomitizedlimestones,which

canbe100%dolomiteand0%calcite(Sperberetal.,1984)

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SerialFormationofDolomite&Palygorskite

Thedolomiteandpalygorskitetexturesreportedheredifferfromthosereportedinprevious

studies.Weaver&Beck(1977),forexample,observedthindisksofpalygorskite,andsuggestedthat

thedisks formduringa co-precipitationreactionbetweenmontmorillonite clayanddolomite.As

evidencefortheirmodel,Weaver(1975)andWeaver&Beck(1977)citedtheSEMobservedtexture

of the dolomites,which they described as external plates and hollow cores. They attributed this

texturetoeithercompetitivegrowthbetweendolomiteandpalygorskiteorresultingfromwhatthey

cited as “the normal way dolomites form.” Re-examination of their published images shows

remarkablesimilaritywiththehollowdolomitetexturesobservedinthepresentstudy,whichare

interpretedhereaspartiallydissolvedcores.

ThedolomitecrystalsinFigures5and8aresimilarinappearancetocloudy-centerclear-rim

(CCCR)dolomites(Sibley,1982),inwhichtheinnerrhombohedralcoresareinferredtobelessstable

thanthesurroundingcleardolomiterim(Folk&Siedlecka,1974).Sibley(1982)arguedthatcloudy

coresaretypicallylessstablebecausetheycommonlycontainahighdensityofcalciteinclusions,and

asaresult,thecoresgenerallydissolveorgetfilledbycalciteduringsubsequentstabilization.This

modelisconsistentwithourobservationsofthedolomitesintheUER,whichshowdissolvedcores

andpristinerims.Noevidenceofcalciteinclusionsisobserved,however.

Analternativemodel,inwhichthestablerimsformattheexpenseofthelessstablecores,is

consistent with the stoichiometry and cation ordering observations in synthetic dolomite

experiments carried out by Kaczmarek & Sibley (2011; 2014). Kaczmarek & Sibley (2014)

demonstratedthatstoichiometryandcationorderingincreaseduringrecrystallizationoftheinitial

dolomitephase.CationorderingreferstothedegreeoforderingofCaandMginthedolomitecrystal

lattice(Goldsmith&Graf,1958).Dolomiteswithahigherdegreeofcationorderinghavealowerfree

energyofformationthandolomiteswithalowerornodegreeofcationordering,andthusaremore

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stable and therefore less soluble (Navrotsky&Capobianco, 1987).TheCCCR fabrics observed in

Figures5and8areconsistentwithaprocesswherebythedolomitecoresareformedduringinitial

replacementofthelimestoneprecursor,andcontainlessstable(i.e.poorly-orderedand/orCa-rich)

dolomite,whereastheclearrimsareamorestable(well-orderedand/orstoichiometric)productof

recrystallization (Kaczmarek&Sibley,2014).This interpretation is supportedbyhigh-resolution

EDSdata,whichshowthatdolomitecoresgenerallyarelessstoichiometricthanthedolomiterims

(Fig.9).ThesedataarealsoconsistentwithCCCRfabrics inotherdolomitestudies(Sibley,1982;

Jones, 2004; Jones, 2007). Powder XRD patterns of these samples are also characterized by

asymmetric peaks for the principle dolomite reflections [(104), (015), and (110)],which further

supportthehypothesisthatarangeofdolomiteMg/Cacompositionsarepresent.Itislikelythatthis

isaresultofacombinationoflessstoichiometric(Ca-rich)andpoorly-ordereddolomitecores,as

wellasmorestoichiometricandwell-ordereddolomiterims.

It is possible, however, that the clear dolomite rims formed as overgrowths on the cores

during a seconddolomitization event. Although thismodel is appealing in that the ionsreleased

during later dissolution of the core could be used entirely for precipitation of palygorskite, it is

unlikelyasit impliesthatthecleardolomiterimsprecipitateddirectlyfromsolution.Becausethe

dolomite cores must have formed before the rim, there would have been a limited supply of

20 µm

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0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1

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Mg/Ca

Dolomite RimsDolomite Coresn = 158

mean = 0.88ѫ = 0.05

n = 155mean = 0.92ѫ = 0.04

A B

Figure9:(A)BackscatterSEMphotomicrographshowingthelocationswhereenergy-dispersivespectroscopy(EDS)spotanalyseswerecollectedonthehighlypolishedthinsection(78.19m).Thecontrastofthephotomicrographwasadjustedtoaccentuatecrystalboundariesandtheepoxy.Whitedotsrepresentdolomiterimlocationswhileblackspotsrepresentcore locationswhereEDSdatawereacquired. (B)Frequencyhistogramshowing the compiled resultsof theEDS spotanalysesfrom313points.Thedatashowthatdolomitecoresaregenerallylessstoichiometricthanthecorrespondingrims.

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carbonatereactantsforlaterrimformation,assumingnearcompletedolomitizationduringthefirst

event (Sperberet al., 1984;Sibleyet al., 1987).Asa result, the clear rimswouldhave formedas

primaryprecipitants,ratherthanasareplacementofacarbonateprecursor.Itismorelikelybased

ontheobservationspresentedthattheclearrimsformedasareplacementofthecloudycoresduring

recrystallization,ratherthanasprimaryprecipitantovergrowths.Thismodelismoreconsistentwith

experimentalfindingsofKaczmarek&Sibley(2014),whichshowthatformationofmoreordered,

andthusmorestable,dolomitehappensduringrecrystallizationonlyafterthevastmajorityofthe

Ca-carbonateprecursorhasbeenreplacedbytheinitialdolomitephase.Itisworthnotingthatwhile

coredissolutionisnotnecessaryorubiquitousfortheformationofclearrims(Riversetal.,2012),it

ismostconsistentwiththeobservationspresentedhere.

Thin section (Fig. 5) and SEM images (Figs. 6 and 8) provide textural evidence that

palygorskitefibersformedduringdissolutionoflessstabledolomitephasesafterdolomitizationand

dolomiterecrystallization.Palygorskite fibers in theUERcommonlyoccurasan irregularcoating

arounddolomiterhombohedra(Fig.6),asclustersinsidedissolveddolomitecores(Fig.8),andon

thecrystalfacesofpartiallydissolvedeuhedraldolomite(Figs.6Cand6D).Figures8Aand8Bshow

palygorskite fibers within a partially dissolved dolomite core, suggesting that precipitation of

palygorskitewasconcurrentorpost-dateddissolutionofthedolomitecore.Figure8Cfurthershows

a completely dissolveddolomite core filledwith palygorskite fibers, suggesting that palygorskite

formedafterdolomitedissolution.ThismodelisattractiveinthattheMgreleasedduringdissolution

ofthedolomitecorescouldbeusedforpenecontemporaneousprecipitationofpalygorskite.

ThegeneticmodelpresentedinFigure10illustratesthetotaldiageneticprocessfrominitial

dolomitization,torecrystallization,topalygorskitenucleation,andfinallypalygorskitegrowthand

filling of the dissolved dolomite core. First, a small dolomite core is formed during the initial

replacement stage of dolomitization. Second, the clear rim formsduring a recrystallization stage

concurrentwithpartialdissolutionof the relativelypoorly-ordered,Ca-richdolomitecore.Third,

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dolomite dissolution at a time after the dolomitization and recrystallization reaction results in

further core dissolution (while the clear rim remains stable) and subsequent palygorskite

precipitation.Thisprocesscontinuesuntil thedolomitecore is completelydissolvedandentirely

filledbypalygorskite.

Theparageneticmodelpresentedisfurthersupportedbythepetrographicevidenceshowing

thatpalygorskiteformationoccurredaftertheclearrimdolomitegrowth.Assumingthattheclear

rimsformfromtheredistributedmaterialfromthedissolvedcores,itispossiblethatpalygorskite

andthedolomiterimscouldhaveformedcoevally.However,thedolomiterimsshowninFigure8

display no evidence of palygorskite inclusions, as would be expected if the minerals formed

concurrently.Thepalygorskitefibersareinsteadfoundonlyascoatingsandassinglefibersattached

totheoutersurfaceofthedolomitecrystalsorasclusterswithinthepartiallydissolvedcores.This

supportsthecontentionthatthepalygorskiteprecipitatedafterthedolomiterimswerefullyformed.

Thisalsosuggeststhatthedolomitecoresarestillsusceptibletochemicalreactionswithporefluids

Initial Replacement Primary Recrystallization Post-dolomitizationDissolution

Complete CoreDissolution

Non-stoichiometric, poorly ordered dolomite Stoichiometric, better ordered dolomite Palygorskite fibres

Figure10:Evolutionarymodelofthedolomitizationreaction,growthofcloudy-centreclear-rimdolomitefabricsandpost-dolomitizationdissolutionofdolomitewithsubsequentpalygorskiteformation.Theinitialreplacementstagecontainsnon-stoichiometric,relativelypoorly-ordereddolomitethatcontinuouslyreplacesthecalcitereactantuntil97%depletion.Once97%depletionismarked,thedolomitizationreactionenterstheprimaryrecrystallizationstage.Duringtheprimaryrecrystallizationstage,non-stoichiometric,relativelypoorly-ordereddolomiteisdissolvedwhilestoichiometric,relativelywell-ordereddolomiteisprecipitated.Itisinterpretedinthisfigurethattheclearrimismadeupofstoichiometric,relativelywell-ordereddolomiteresultingfromthedissolutionofthenon-stoichiometric,relativelypoorly-ordereddolomitecore.Afterthedolomitizationreactionhascommenced,post-dolomitizationfluidsintroducedtothesystemdissolvethelessstabledolomitecorewhiletheclearrimremainsstable.Palygorskiteformssubsequentlywiththispost-dolomitizationdissolutionstage,aspartialtocompletedissolutionofthelessstablecoreoccurs.Figure8Arepresentsthepost-dolomitizationdissolutionstage,whileFig.8Crepresentsthefinalproductofthemodelpresentedhere.

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afterthegrowthoftheclearrims,asputforthbySibley(1982),andsupportstheideathatsubsequent

formationofpalygorskitecanoccurwithinthedolomitecoresfromlaterfluidsundersaturatedwith

respecttothelessstabledolomitephase.

Geochemicalrelationshipbetweendolomite&palygorskite

Geochemical arguments also support the model of palygorskite genesis by dolomite

dissolution,andhighlightoneofthemajorlimitationsofthecoevalformationmodel.Thestabilityof

dolomite is a function of temperature and the [Mg2+]/[Ca2+] ratio of the fluid (Carpenter, 1980;

Machel&Mountjoy, 1986;Kaczmarek& Sibley2011;Kaczmarek&Thornton2017; andothers).

Dolomitization,theprocessbywhichcalciteisreplacedbydolomite,isrepresentedbythefollowing

equation:

2CaCO3+Mg2+ßàCaMg(CO3)2+Ca2+ (1)

TheGibbsFreeEnergyofReaction(DGrxn)andtheequilibriumconstant(K)forthisreactiondepend

on the [Ca2+]/[Mg2+] in solution. According to calculations based on this reaction, a fluid is

supersaturatedwithrespecttodolomiteatnormalearthsurfaceconditions(i.e.,25°Cand1atm)

when the [Mg2+]/[Ca2+] of the aqueous solution is greater than~1 (Sibley, 1982), although large

uncertainties exist in the standard free energy of dolomite (Carpenter, 1980). Therefore, the

presence of dolomite as evidence for elevated [Mg2+] in solution to form both dolomite and

palygorskite isnotnecessarilycorrect.Dolomitestabilitydependsonthe[Mg2+]relative to[Ca2+],

whilethestabilityofpalygorskiteformationisindependentofthe[Ca2+]asdiscussedbelow.

Empiricalobservationssuggestthatpalygorskiteformationisdependentonthe[Mg2+]/[Al3+]

insolution(Millot,1970; Isphording,1973;Singer,1979;Birsoy,2002),amongotherparameters

suchaspH,salinity,andH4SiO4.This isevident inmanysedimentarysuccessions,wherebymore

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aluminum-richclayssuchaskaoliniteandsmectitearefoundclosertothesourceofdetritalmaterial,

andtheMgO:Al2O3ratioincreasesintothebasinuntilMg-richclayssuchaspalygorskiteandsepiolite

areobserved(Millot,1970;Isphording,1973).Inpuresmectite,theoctahedralAl3+/Mg2+ratioison

theorderof7to9,whileinpurepalygorskitetheoctahedralAl3+/Mg2+ratioisbetween1and1.5,

correspondingtoaMg2+/Al3+ratioof0.67-1,whichisatleast4.8timesgreaterthanthatofpure

smectite (Singer, 1979). This implies that much more Mg is needed relative to Al in order for

palygorskitetoforminsteadofsmectite.Thus,fieldobservationsofpalygorskitecoupledwiththe

thermodynamicsofdolomitizationsuggestitisunlikelythatthesemineralswouldformcoevallyin

Mg-richwatersastheycompeteforMg2+ions.

ThermodynamicstudieshavebeendonetofurtherunderstandtheSiO2-MgO-Al-H2Osystem

as it pertains to the stability of palygorskite in various solutions.Many studies have formulated

stabilitydiagramswithvaryingparameterssuchaspH,Mg,Al,andH4SiO4(SingerandNorrish,1974;

WeaverandBeck,1975;Elprinceetal.,1979;Birsoy,2002).UnderconstantAlactivities,palygorskite

stability increaseswith increasing pH,Mg, andH4SiO4. Decreases or increases in one parameter,

however,affecthowmuchadjustmentintheremainingtwoisrequired.Forexample,atrelatively

highpHlevels(pH>9),palygorskiteisstableatlowH4SiO4if[Mg2+]ishigh,orathighH4SiO4if[Mg2+]

islow(SingerandNorrish,1974).AtlowpH(e.g.,pH<6)palygorskiteisstableonlyifboth[Mg2+]

andH4SiO4areelevated(Fig.11A).

Additionalworktocharacterizethestabilityboundarybetweensmectiteandpalygorskitein

naturalsystemshasbeencarriedouttounderstandtheimpactinvariationsofpH,Mg,Al,andH4SiO4.

InitialworkwasdonebyWeaver&Beck (1977)whoanalyzed themontmorillonite-palygorskite

systemat25°Candconstantlog[Al(OH)4-]of-5.5.Theresultsdemonstratedthat,similartothework

of Singer and Norrish (1974), low pH values (pH < 6) require elevated [Mg2+] and H4SiO4 for

palygorskitetobefavoredovermontmorillonite(Fig.11B).Weaver&Beck(1977)calculatedthat

palygorskitecandirectlyprecipitatefromasolutionatapH~8andlog[H4SiO4]=-5,withincreases

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ineitherparameterfurtherfavoringpalygorskitestability.Incontrast,decreasesinlog[H4SiO4]cause

both minerals to become unstable, while lower pH values favor montmorillonite (Fig. 11B). In

additiontodeterminingthestabilityofmontmorilloniteandpalygorskiteinsolution,Weaver&Beck

(1977)alsocalculatedthestabilityfieldboundaryforthetransformationbetweenmontmorillonite

andpalygorskite.Thisdemonstrated thatan increase inpH, [Mg2+],orH4SiO4 favorspalygorskite

overmontmorillonite(Eq.2).

log[Mg2+]+2pH+2log[H4SiO40]=5.75 (2)

The results of Singer & Norrish (1974) & Weaver and Beck (1977) are helpful in

understanding the stability of palygorskite in aqueous solutions, as well as the transformation

reactionbetweenmontmorilloniteandpalygorskite.ThefundamentalcontrolAlmayhaveonthe

systemislesswellunderstood,however.Weaver&Beck(1977)suggestedthattransformationfrom

6543

7 8 9

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log[H4SiO40]

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log[Mg2+]

PALYGORSKITE

MONT.

SOLU

TION

A B

Figure11:StabilitydiagramsofpalygorskiteasafunctionofpH,H4SiOH4,andMg.(A)Three-dimensionalstabilitydiagramofpalygorskiteandanaqueoussolution.DiagramismodifiedfromSinger&Norrish(1974).(B)Three-dimensionalstabilitydiagramdisplayingthestabilityfieldsofpalygorskite,montmorillonite,andanaqueoussolutionat25°Candlog[Al(OH)4-]=-5.5.ModifiedfromWeaverandBeck(1977).

121

phyllosilicatesotherthansmectitewouldrequirechemicalmodificationsenergeticallyunfavorable

atlowtemperatures,duetohighertetrahedralAloccupancy.

ExtensiveandmorerecentworkbyBirsoy(2002)hasshedlightontheimpactofpH,Mg,Al,

andH4SiO4onthestabilityofpalygorskiteandmineralscommonlyassociatedwithit,withspecific

emphasisonvaryingAlconcentrations.KeyresultsofBirsoy(2002)aresummarizedinFigure12.

TheyshowthatincreasingAlcausesanincreaseinthestabilityfieldofbothmontmorilloniteand

palygorskite. In all stability diagrams constructed, the solution is in equilibrium with

montmorilloniteandpalygorskiteatlog[aMg2+/(aH+)2]values<~12andlog[aAl3+/(aH+)3]£5.5,with

theexceptionof thosesystems that containquartz insteadofamorphoussilica.AsAl continually

increases,however,thereactionmustproceedfirstthroughmontmorillonitebyincreasingH4SiO4,

andpalygorskitestabilityisonlyreachedthroughfurtherincreasingH4SiO4orbyincreasingMg(Fig.

12;Birsoy,2002).Thus,inafluidexhibitingrelativelyconstantH4SiO4concentrations,palygorskite

is more stable thanmontmorillonite at elevatedMg/Al ratios, supporting the field observations

discussedabove.

As increases in either pH, Mg, or H4SiO4 favor the stability of palygorskite, how they

individuallyaffectthestabilityofdolomitemustbeassessed.Becausepalygorskiteanddolomiteare

bothstableatsimilarpHvaluesclosetoseawater(~8),itisunlikelythatchangesinpHalonecan

cause both undersaturation with respect to dolomite and supersaturation with respect to

palygorskite.DespitethefactthatdolomiteandpalygorskitemustcompeteforMg2+ionsiftheyform

coevally,Figure12clearlyindicatesthatbothmineralscantheoreticallyformtogetherinsimilarMg-

richfluids.ThestabilitydiagraminFigure12,however,showsthatitispossiblethatfluctuationsin

Mg could cause variations in stability betweendolomite andpalygorskite. The dolomite stability

boundaryliesatlog[aMg2+/(aH+)2]=~12,abovewhichitisunstable,whilepalygorskiteremainsstable

withincreasingMgaslongaslog[H4SiO4]valuesaregreaterthanapproximately3.Thus,itispossible

thatfluidscontainingexcessivelyhighMgconcentrationscouldberesponsibleforcausingdolomite

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tobecomeunstable andpalygorskite to form.With regards toH4SiO4 levels, dolomite stability is

unaffected, but they must remain elevated in order for palygorskite to form. Therefore, the

theoreticalstabilitydiagramsindicatethataslightlyalkalinefluidthatcontainselevatedlevelsofMg

and H4SiO4 may be responsible for both undersaturation with respect to dolomite and

210-1-2-3-4-5-6-7-89101112131415161718192021

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A B

C D

Figure12:StabilitydiagramsmodifiedfromBirsoy(2002)displayingthestabilityboundariesofvariousmineralswithina seven component MgO-CaO-Al2O3-SiO2-H2O-CO2-HCl system. Each diagram is constructed at 25°C and 1bar withlog[aH2O]=1,log[aCO2]=-3.5,andlog[aCa2+/(aH+)2]=13.06.A)Stabilitydiagramconstructedatlog[aAl3+/(aH+)3]=4.5andsuppressionoftalc,kerolite,andamorphoussilica.B)Samediagramas(A),butincreasedAlatlog[[aAl3+/(aH+)3]=5.5. C) Same diagram as (A), but with suppression of talc, kerolite, and quartz. D) Same diagram as (B), but withsuppressionoftalc,kerolite,andquartz.

123

supersaturationwithrespecttopalygorskite.Thiscausesissues,however,assuchasystemstillfalls

inthedolomitestabilityfieldatlog[aMg2+/(aH+)2]values<12(Fig.12).

Onepossibilityisthatallthreeparameters(pH,Mg,H4SiOH4)didnotevolvesimultaneously,

butweresequentiallydevelopeduntilthesystemwasundersaturatedwithrespecttodolomiteand

subsequentlysaturatedwithrespecttopalygorskite.Meteoricwaterisalikelycandidateresponsible

for starting a series of reactions and has been interpreted as the source of silica or responsible

solution for palygorskite formation in other localities (Singer and Norrish, 1974; Soong, 1992;

Draidiaetal.,2016;Kadiretal.,2016;Kadiretal.,2017).Suchafluidcouldcontainelevatedlevelsof

H4SiO4(Knauth,1979;Bennet&Siegel,1987)aswellashighpCO2(James&Choquette,1990).Atthe

earlieststage,theelevatedlevelsofH4SiO4alonemaynotcausepalygorskitetoprecipitateduetolow

pHlevelsfromthedissolvedCO2.DissolvedCO2would,however,causethemeteoricfluidtobecome

slightlymoreacidic,andthuscorrosivetocarbonateminerals(James&Choquette,1990;Liuetal.,

2010).Anobviouscandidateforpreferentialdissolutionisthelessstable,Ca-richandpoorly-ordered

dolomitecores(Figs5and8).Asthedolomitedissolves,itwouldbufferthesolutionpHbyforming

HCO3-ions(Rau&Caldeira,1999).AssumingthattheconcentrationofH4SiO4inthemeteoricsolution

ishighenough,palygorskitewill thenbegin toprecipitateonce thepH iselevated. Inaddition to

bufferingthesolutionpH,dolomitedissolutionwillalsoreleaseMg2+ionsintosolution,andfurther

push the fluid into the stability fieldof palygorskite (Fig. 12).Therefore,H4SiO4-rich fluids likely

initiatetheprecipitationofpalygorskite,butdolomitedissolutionplaysavitalroleinmaintaininga

suitablesolutionpHandincreasingtheMgwhichfurtherfavorstheformationofpalygorskite.

Finally, the process(es) responsible for the intercrystalline palygorskite cement may be

similar to the process responsible for the intracrystalline palygorskitewithin dolomite cores. As

H4SiO4likelyprovidesthedominantcontrolonpalygorskiteformationinthisstudy,theformationof

palygorskitewithintheintercrystallineporesmayoccurasaresultoftheinitialstagesofdolomite

dissolution,causingthesolutiontobufferand increasingtheMgcontentof the fluid.Asdolomite

124

continues to dissolve, the dissolved cores would provide conduits for fluid flow within the

intracrystalline pores. The saturation state of the fluid with respect to palygorskite would also

continuetoincreasewithfurtherdolomitedissolution,increasingthereactionrate.Thisprocessmay

resultintheprecipitationofintracrystallinepalygorskitewithinthedissolvedcores,asobservedin

Figure8.

Precursorconditionsforpalygorskiteformation

There are two principal hypotheses for palygorskite formation in shallow-marine

environments:thetransformationofsmectite,orasadirectprecipitatefromaschizohalinesolution

with elevatedMg, Si,Al, andpHbetween8 and9.Asdescribedabove, the fluids responsible for

dolomitedissolutionwouldcontainsomeamountofH4SiO4,anddolomitedissolutionwouldbuffer

thesolutionpHinadditiontoprovidingMgtotheporefluids.Itisunlikely,however,thatthesefluids

alonewouldprovidetherequisiteconcentrationsofSiandMgforpalygorskiteformation.Thus,a

substantialsourceSi,Mg,andAlarerequired,whethertheyoriginatefromaprecursorclay,suchas

smectite,orasolutionwithampleamountsofdissolvedSi,Mg,andAl.Usingthisinformation,both

hypothesescanbeevaluatedtofurtherelucidatethegenesisofpalygorskiteintheUmmerRadhuma

ofQatar.

Theinterpretationthatpalygorskiteformedthroughdiageneticreplacementofsmectiteis

problematicfortworeasons.First,thereisnopetrographic,mineralogical,orgeochemicalevidence

ofsmectiteintheUmmerRadhuma.XRDanalysisfailedtoidentifysmectite,andSEM-EDSandthin

sectionpetrographicobservationsalsoyieldnoevidenceofsmectite.However,theargumentthat

transformationdidnotoccurbecausesmectiteisnotpresent(Isphording,1973;Singer,1979;Holail

&Al-Hajari,1997)istenuous.Asdiscussedearlier,manydolomitesconsistof100%dolomiteand

0%calcite,yet it ishypothesizedthatmostdolomite formsbyreplacementofcalcitethroughthe

dolomitizationreaction(Land,1985;Machel,2004).Thus,itisnotaccuratetoclaimthatsmectite

125

wasnot replacedbypalygorskite just because smectite is not observed.Additionally, smectite is

abundantintheoverlyingRusFm.,associatedwithpalygorskite(Fig.2).Thissuggeststhatsmectite

may have possibly been present in the Umm er Radhuma but was completely replaced by

palygorskite.

Secondly,duetothefailuretoreplicatethereactioninthelaboratory,thereisagenerallack

of understanding about the thermodynamics and kinetics of the smectite to palygorskite

transformation(Singer,1979).Althoughthetransformationfrompalygorskitetosmectitehasbeen

observedinnature(Krekeleretal.,2005)andinthelaboratory(Goldenetal.,1985;Golden&Dixon,

1990),thereversereactionofsmectitetopalygorskitehasbeenunsuccessful,thusmakingitdifficult

toevaluatetheprecisereactionmechanisms.Despitetheabsenceofexperimentalsuccess,authigenic

palygorskitefibershavebeenobservedontheedgesofsmectiteplatesinnaturalsettings(Chenet

al., 2004; Yeniyol, 2012; Xie et al., 2013). To explain this, Chen et al. (2004) proposed amodel

wherebyMg2+enterssmectiteandreorganizesthepositionsofboththeoctahedralandtetrahedral

sheetsandreversesthepositionsoffreeoxygensastoattainthepalygorskitestructure.

Despitethelackofsmectiteinthesamplesanalyzedandfailuretosynthesizethereactionin

laboratorysettings, thedepositional(Fig.3E)anddiagenetic(Fig.5)relationships in theUmmer

Radhumasuggestthereplacementreactionbetweensmectiteandpalygorskiteismorelikelythan

directprecipitationfromsolution.TheUmmerRadhumaformationwasdepositedasaperitidalto

subtidallimestone(Bou-Rabee&Burke1987;Pollittetal.,2012),withnumerousclay-richintervals

capping the subtidal sequences in Well RR-01 of the present study (Figs. 2 and 3E). Extensive

bioturbationthroughsuchintervals,suchasthatexhibitedinFigure3E,provideevidencethatthese

clay-rich intervals are likelydepositional in origin.Basedon theparagenetic sequencediscussed

above, however, palygorskite fibers postdate both dolomitization and subsequent dolomite

dissolution.Therefore,itishighlylikelythataprecursorclaywasdepositedinthemudflatcyclecaps

andwaslateraltereddiagenetically.It ispresumedthatsmectitewastheprecursorclay,asother

126

phyllosilicatescontaintoomuchtetrahedralAlthatwouldrequireexcessivelyhightemperaturesfor

transformation to palygorskite (Weaver & Beck, 1977). In the absence of direct evidence of a

smectite-palygorskitetransformation,thismechanismisnotcertain.Itissimplythemostreasonable

onebasedonliteraturefocusedonthetransformationofclaymineralstopalygorskite(Weaver&

Beck,1977;Chenetal.,2004).

Shadfanetal.(1985b)positedthattheoccurrenceofpalygorskiteintheArabianPeninsula

inEocenerocksisadirectprecipitatefromsolution.TheArabianShieldrepresentsthehighlandson

thewesternhalfoftheArabianPeninsula,whiletheArabianShelfiscomposedofmarinesediments

fromcyclesofperitidaltoclosed-basinsettingsintheeast.Shadfanetal.(1985b)suggestedthatthe

weatheringoftheArabianShieldcouldprovideenoughdissolvedSiandAltothebasinintheeastto

precipitatepalygorskitedirectlyfromsolution.Inorderforthisprocesstooccur,however,ample

amounts of dissolved Si and Al would need to flow into themarinewaterwithout transporting

detritalmaterial for a replacement reaction. Itwould also require elevated temperatures and an

evaporativeenvironment(Singer,2002).Suchconditionswerepresentduringdepositionandearly

diagenesisoftheUmmerRadhuma,asindicatedbythetemporalproximityofthesedepositstothe

Paleocene-EoceneThermalMaximum(PETM)(Zachosetal.,2001;2005;Pollittetal.,2012)andthe

presenceofdiageneticgypsumtowardsthetopoftheformation(Fig.2).

Explainingthepalygorskitecyclecapsviaprimaryprecipitationisproblematic,however.As

the burrows presented in Figure 3E are observed within the palygorskite cycle caps, and the

parageneticsequencesuggeststhatpalygorskiteformedafterdolomite,thenthebioturbationmust

haveoccurredafterextensivediageneticalterationfollowingtheprimaryprecipitationhypothesis.

While this may be possible, it is more likely that the burrows were preserved through post-

depositionaldiagenesisinvolvingthetransformationofaprecursorclay.

Another issue with direct precipitation is based on the solubility relationship between

dissolvedAlandpH.Ingeneral,thesolubilityofAldecreaseswithincreasingpH.Aluminumisalso

127

relativelyinsolubleinsolutionsinthepHrange6–8,withincreasingsolubilityaspHlevelsattain

values<6or>8(Driscoll&Schecher,1990).Asaresult,solutionsinthe7–9optimalpHrangefor

palygorskite formation likelydonot containampleamountsofdissolvedAl.Thus, aprocess that

wouldtransportslightlybasicfluidswithdissolvedAlthroughthesectionisnotsostraightforward

andisthermodynamicallyunfavorable.

AlthoughthesourceofSi,Mg,andAlcannotbedirectlyidentified,evidencefromtheUmmer

Radhuma suggests that dolomite dissolution can play an important role in the formation of

palygorskite. In the absence of elevatedMg or H4SiO4 concentrations in slightly basic solutions,

smectitewillremainstableandpalygorskitewillnotlikelyform(Fig.12).Therefore,anyprocessthat

increases[Mg2+],H4SiO4,andpHinasolutionwillpromotepalygorskiteformation(Singer&Norrish,

1974; Weaver & Beck, 1977; Singer, 2002). The combination of meteoric fluids and dolomite

dissolutionwouldprovideadditionalH4SiO4,bufferthesolutionpH,andincreaseMg,promotingthe

formationofpalygorskiteoversmectite(Weaver&Beck,1977;Birsoy,2002).

Conclusions

This study provides petrographic and geochemical evidence for an alternative model to

explain the co-occurrence of dolomite andpalygorskite in theEocene carbonates of theUmmer

RadhumaFormation.Thismodelisbasedontheobservationthatpalygorskitepost-datesbothan

initialphaseofdolomitizationaswellasasubsequentrecrystallizationeventwherebytheunstable

cores are dissolved at the expense of more stable rims. The observations presented preclude a

detritalorigin,directprecipitationfromsolution,orconcurrentprecipitationwithdolomite.Rather,

theevidenceprovidedhereismoreconsistentwiththeinterpretationthatpalygorskiteformsasa

diagenetic product concurrent with dolomite dissolution. Thin section and scanning electron

microscopeimagesdocumentauthigenicpalygorskitefibersgrowingbothontheoutersurfacesof

partiallydissolveddolomitecrystals,andfillingthevoidsformedbydissolutionofthedolomitecores.

128

Although the precise mechanism by which palygorskite replaces smectite is still unclear, these

observations suggest that dissolution of metastable dolomite plays an important role in the

formation of palygorskite by buffering solution pH and releasing Mg2+ ions into solution. This

evidenceprovidesanewmodeltoexploretherelationshipbetweendolomiteandpalygorskiteinthe

geologic record as well as the geochemical and diagenetic conditions responsible for their co-

occurrence.

Acknowledgements

ThisprojectwassupportedbyaresearchgrantfromExxonMobilUpstreamResearchQatar

(awardedtoSEK).WethankCameronMancheandHannaCohenforcommentsonearlierversionsof

thismanuscript.CameronMancheassistedinthedraftingofFigure1andinXRDanalysisofFigure

4.Thequalityofthefinalmanuscriptwasmuchimprovedbydetailedreviewsfromtwoanonymous

reviewers,andsubstantialcommentsandsuggestionsfromtheAssociateEditor,Dr.NickTosca.We

arealsogratefultoExxonMobilforpermissiontopublishtheresultsofthisstudy.

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137

CHAPTERV

MULTIEPISODICRECRYSTALLIZATIONANDISOTOPICRESETTINGOFDOLOMITESINNEAR-SURFACESETTINGS

BrooksH.Ryan1,StephenE.Kaczmarek1,JohnM.Rivers21Department of Geological and Environmental Sciences, Western Michigan University, Kalamazoo,Michigan49008,U.S.A.2QatarCenterforCoastalResearch(QCCR),ExxonMobilResearchQatar,P.O.Box22500,QatarScienceandTechnologyPark-Tech,Doha,QatarRyan,B.H.,Kaczmarek,S.E.,andRivers,J.M.,inreview,JournalofSedimentaryResearch.

Abstract

TheLowerEoceneRusFormationinQatarreflectscarbonatedeposition inarestrictedto

semi-restrictedmarine setting on a shallow ramp. Petrographic, mineralogical, and geochemical

evidence from three research cores show early diagenesis has extensively altered nearly every

petrologicalattributeoftheserocksdespitenothavingbeendeeplyburied.InsouthernQatar,the

lower Rus (Traina Mbr.) is comprised of fabric-retentive dolomite intervals that preserve

wackestone-packstonetexturesthatareinterbeddedwithdepositionalgypsumbeds. Innorthern

Qatar,thesamememberisdominantlycomprisedoffabricdestructive,planar-edolomiteandlacks

evaporites.InbothnorthernandsouthernQatar,theupperRus(AlKhorMbr.)iscomprisedoffabric-

retentive dolomite intervals as well as limestone intervals rich withMicrocodium textures that

display evidence of dedolomitization. Geochemical analysis reveals that the limestones have an

average δ18O of -10.73‰ VPDB and δ13C of -7.84‰ VPDB, whereas average dolomite δ18O is

significantly higher (-1.06‰VPBD)but dolomite δ13Cvalues (-3.04‰VPDB; range -10 to0‰)

overlapwithlimestoneδ13Cvalues.Additionally,dolomiteδ13Ctrendstowardnormalmarinevalues

withdepthawayfromthecalcite-dolomitecontactinallthreecores.Thesegeochemicalobservations

suggestthatthelimestoneswerefirstrecrystallizedinmeteoricfluidsresultinginnegativeδ18Oand

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δ13C in association with Microcodium textures, and subsequently dolomitized in marine fluids

resulting in relatively higher dolomite δ18O valueswith retention of the negative δ13C signature

associatedwithinitialmeteoricdiagenesis.Givenpetrographicobservationsthatdolomitecrystals

arecommonlyincludedincalciteandpartiallytocompletelyreplacedbycalciteintheseintervals,

however, the paragenetic sequence suggests that dolomite formed prior to calcite in the

Microcodium-bearingintervals.Furthermore,thedolomitesarecommonlycementedbygypsumin

theTrainaMbr. insouthernQatar, suggestingdolomitizationmayalsohaveoccurredprior to,or

concurrentwith,beddedgypsumformation.Insum,theseobservationsindicatethatdolomitization

occurredearly.Followingthis,thedolomiteswerereplacedbyMicrocodium-bearinglimestonesat

and immediately below paleo-exposure surfaces, and at greater depths recrystallized in mixed

marine-meteoricfluids,producinganegativeδ13Csignaturethattrendstowardmorepositivevalues

away from the limestone-dolomite contact. Lastly, the dolomites underwent another phase of

recrystallizationineithermarine-dominatedfluidsorpossiblyawell-mixedaquifersetting,resulting

inaninvariableδ18Osignaturebutretainingthenegativeδ13Csignatureandcausingthecontrastin

dolomiteδ18Oandδ13Cobserved.Thisstudythushasimplicationsforhowcarbonatediagenesisis

interpreted on a global and temporal scale, as it suggests early diagenesis of geologically young

carbonates can be extremely complex, resulting in multiple stages of mineral replacement and

isotopicexchangeinmeteoricandshallowmarinefluids,priortosignificantburial.Furthermore,this

study posits that dolomitization of a limestone may not necessarily prevent additional early

diagenesis.Rather, thisstudy indicates thatdolomite isunexpectedlyreactive in thenear-surface

environment, especially in response to high water flux coupled with extreme water chemistry

variability.

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Introduction

Carbonate diagenesis occurs in a diverse array of settings that differ in their biological,

chemical,andphysicalprocesses(McIlreathandMorrow1990;TuckerandBathurst1990;Tucker

and Wright 1990; Tucker 1993; Flugel 2004; Swart 2015). Early diagenesis of shallow-marine

carbonaterocksiscommonlyconsideredtoincludeallprocessesoccurringimmediatelyaftergrains

aredepositedontheseafloor,usuallyinvolvingcementation,micritization,andboring(Tuckerand

Bathurst1990;TuckerandWright1990;Tucker1993).Aftercementationandlithificationonthe

seafloor,however,carbonateplatformsmaybesubjectedtofardifferentconditionsinthemeteoric

andburialrealms.Thesedifferentenvironmentsresultindiageneticfeaturesthatarebothtexturally

andgeochemicallydiscernable fromoneanother,which isuseful for studiesaiming todocument

diageneticalterations.

A common diagenetic alteration impacting marine carbonates is the process of

dolomitization.Thereplacementofmassivelimestonerockbodiesbydolomiteisinterpretedtobe

commonin therockrecord(Machel2004),althoughthere ismuchdebateontheoriginsofmost

naturaldolomite(VanTuyl1916;Land1985;Budd1997;Warren2000;Machel2004;Greggetal.

2015; Kaczmarek et al. 2017). Despite the inability to synthesize dolomite under natural earth

surface conditions (Land, 1998), manymodels have been put forth to explain dolomitization of

limestones (Morrow1982;Land1985).These includedolomitizationbyseawater (SassandKatz

1982; Carballo et al. 1987; Land 1991; Budd 1997; Manche and Kaczmarek 2019), hypersaline

seawater(AdamsandRhodes1960;Deffeyesetal.1965;Warren2000;DravisandWanless,2018),

mixed freshwater-seawater (Badiozamani 1973; Humphrey and Quinn 1989), and burial fluids

(Mattes and Mountjoy 1980; Barnaby and Read 1992; Ryb and Eiler 2018). Ultimately, all

dolomitization models are based on dolomite mineralogy, texture, and geochemistry, with the

geochemistryofthedolomitesinterpretedtoreflectthenatureofthedolomitizingfluids(Warren

2000;Machel2004).

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Stable oxygen (δ18O) and carbon (δ13C) isotopes, for example, have long been used to

interpretthepaleoenvironmentalconditionsofcarbonaterocks(Urey1947,Ureyetal.1951,and

O’neiletal.1969).Meteoricwater,forexample,isgenerallynegativeinδ18O,andcarbonaterocks

forminginsuchfluidsrecordthesenegativeδ18Ovalues(Swart2015).Marinewaters,incomparison,

havehigherδ18Ovalues,commonly³0‰,withmorepositivevalueswithincreasingevaporation

(Swart2015;Riversetal.2019b).Incontrast,carbonated13Cvaluesaretemperature-independent,

butareinsteadinfluencedbythed13Cofdissolvedinorganiccarboninthecrystallizationfluids,which

islargelyafunctionofbiologicalprocessesbecauseCO2isfixatedduringphotosynthesis(Lohmann

1987; Swart 2015). Environmental and process interpretations based on themeasured isotopic

compositionof carbonaterocks is,however,a functionofprimarydepositionconditions,and the

culmination of all subsequent diagenesis. Thus, stable isotope proxies interpreted without

petrologicalcontextmayyieldambiguous interpretations.Forexample,d18Ovalues incarbonates

formed in meteoric environments can be similar to 18O values in carbonates formed in high

temperaturefluids(Swart,2015).Complicatingpaleoenvironmental interpretationsofcarbonates

furtheristheprocessofdolomitization,whichisthoughttocausea~+3‰fractionind18Ofromlow-

Mgcalcite formedunder the sameconditionsat25°C (FritzandSmith1970;MatthewsandKatz

1977; Vasconcelos et al. 2005; Horita 2014). Thus, there is much ambiguity in environmental

interpretationsforcalcitesanddolomitesbasedsolelyond18O.Incontrast,d13Cmeasurementsof

carbonatesedimentandrocksaregenerallyutilizedasstratigraphicmarkersofglobalandtemporal

changesintheearthcarboncycle(Hayesetal.1999).However,ithasbeendemonstratedthatthe

depositional d13C signature of carbonate rocks can be altered and reset through later diagenetic

processes(Lohmann1987;SwartandKennedy2012;OehlertandSwart2014;Swart2015).This

addscomplexitytotheobservationthatd13Ccanvarywidelyeveninoneenvironment(Swart2015).

Therefore, utilizing only d18O and d13C values of carbonate rocks can lead to ambiguous

141

interpretations, and must be integrated with other petrological datasets, such as mineralogy,

petrography,andtraceelementgeochemistry.

TheLowerEoceneRusFm.isashallowmarinecarbonateunitthatconstitutesadiversesuite

ofcarbonateminerals,textures,andd18Oandd13Csignatures(Riversetal.2019a).Al-Saad(2003)

dividedtheRusintothelowerTrainaMbr.andupperAlKhormember.TheTrainaMbr.isdominated

byinterbeddeddolomiteandgypsuminsouthernQatarbutrelativelypuredolomitesinnorthern

Qatar.TheAlKhorMbr.acrossallofQatarischaracterizedbylayeredcalcitesanddolomites,with

sharpandabruptvertical texturalandgeochemical changesbetween them.Notably,whereas the

d18Oofthecalciticintervalsdifferfromtheanddolomiticintervalsinallcores,thed13Cisrelatively

similar. Rivers et al. (2019a) reported limestones that had been dolomitized and dolomitized

limestonesthathadbeendedolomitized, leadingthemtohypothesizethatrecrystallizationof the

rocksismulti-generational.Thepresentstudybuildsuponpreviousworkinordertoanswerspecific

questionsabouttheseshallowlyburiedcarbonates.First,howwastheRusdolomitized?Secondly,

haveRusdolomitesbeenrecrystallized, and if so,underwhat conditions?And lastly,what is the

paragenetic and geochemical relationship between the dolomites and calcites? A detailed

parageneticsequenceusingpetrographiccross-cuttingrelationshipscoupledwithhigh-resolution

mineralogicaldataisusedtodeterminetherelativetimingandcrystallizationofcalciteanddolomite.

Stableisotopedataisalsousedtoelucidatethediageneticenvironmentsresponsibleforeachphase.

Ultimately, this study demonstrates that a complex diagenetic history resulting in multiple

recrystallizationevents and isotopic alterationshas impacted these relatively youngand shallow

carbonate rocks,with implications for interpreting evaporite associateddolomites, recrystallized

dolomites,andcarbonatediagenesisbasedonisotopicdatawithoutpetrography.

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GeologicBackground

TheLowerEocene(middle to lateYpresian)RusFm. inQatar is interpretedtohavebeen

depositedinavariablyrestrictedshallowmarineenvironmentalsetting(Cavelier1970;Ecclestonet

al. 1981; Al-Hajari and Kendall 1992; Al-Saad 2003; Rivers et al. 2019a). The Rus Formation

unconformablyoverliestheUmmerRadhumaandunconformablyunderliestheDammamFm.(Fig.

1; Cavelier 1970; Rivers et al. 2019a). Predominantly observed in the subsurface, the Rus only

outcropsatthesurfaceinnorth-centralQatar(Cavelier1970).TheRusisformallydividedintothe

lowerTrainaMbr.andupperAlKhorMbr.,whichvarygeographically(Powersetal.1966;Cavelier

1970; Eccleston et al. 1981;Al-Hajari andKendall 1992;Al-Saad2003;Rivers et al. 2019a). The

Traina is composed of intercalated gypsum and dolomitic limestone in southern Qatar, but

predominantlydolomiteinnorthernQatar(Al-HajariandKendall1992;Al-Saad2003;Riversetal.

2019a).Incontrast,theAlKhorisprimarilycomposedofdolomiticlimestoneacrossmostofQatar

(Al-Saad2003;Riversetal.2019a).ThestratigraphicthicknessoftheRusalsovariesbetween20and

110mthick(Cavelier1970;Abu-Zeid1991;Al-HajariandKendall1992;Al-Saad2003;Riversand

Larson 2018; Rivers et al. 2019a), an observation attributed to structural highs in central Qatar

associatedwithhigh-angle, syndepositionalnormal faults that resulted inmore restriction in the

southandamoreopenmarineenvironmentinthenorth(Cavelier1970;Ecclestonetal.1981;Rivers

andLarson2018).

MuchworkwithregardtofaciesanalysisandstratigraphyhasbeencarriedoutontheRus

andoverlyingDammam(Al-HajariandKendall1992;Al-Saad2003;Al-Saad2005).Morerecentwork

byRiversetal.(2019a),however,hasfurtherrefinedthesedimentologyandstratigraphyofQatar

withrespecttotheRus,synthesizedinthefollowinglines.TheRusacrossthreeshallowresearch

coresconsistspredominantlyofthreedepositionallithofacies,witha4thfaciesonlypresentinthe

southernhalfofQatar.Thesefaciesincludeprotectedinnerrampfacies,tidalflatfacies,subaqueous

salina facies, and calcitic paleosol facies. In southern Qatar, the Traina is largely composed of

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interbedsof innerrampandsubaqeoussalinafacies,withfewthin intervalsof tidal flat facies. In

contrast,thesubaqueoussalinafaciesisabsentintheTrainainnorthernQatarwhereitisdominated

byinnerrampfacies,althoughfabric-destructivedolomitetexturesgenerallyobscuretheprecursor

fabric.TheAlKhorinallthreecoresisdominatedbytidalflatfaciesandcalciticpaleosolsrichwith

clay minerals, with few intervals of inner ramp facies. Two calcitic paleosols consisting of

recrystallizedMicrocodium-bearinglimestoneswereidentifiedandcorrelatedacrossallthreecores

andinterpretedtoreflectmajorexposureevents.

Core 1

Qatar

SaudiArabia

Persian Gulf

Kilometres

Core 2

Core 3

Geologic Age Stratigraphic Unit

Lithology

PA

LE

OG

EN

E

Pale

ocen

eEo

cene

Early

Mid

dle

Rus Fm.

Umm er

Radhuma Fm.

DammamFm.

Dolomite Limestone Gypsum

Arabian Peninsula

Figure1:AgeneralizedstratigraphicsectionofthePaleoceneandEocenesedimentsofQatar,aswellasalocationmapoftheArabianPeninsula,Qatar,andthethreeresearchcores(modifiedfromRyanetal.2020).1

144

MaterialsandMethods

RockcorewasrecoveredfromthreeresearchboreholesdrilledincentralandnorthernQatar

(Fig.1), fromwhichallpetrographic,mineralogical, andgeochemicaldatawereacquired for this

study.Detaileddescriptionsofthemethods,sedimentology,andgeneralstratigraphyofthesecores

arereportedinRiversetal.(2019a).CorerecoverythroughtheRuswas100%inCores1and3,but

approximately 5 intervals from 50-62 m depth in Core 2 that are < 3 m thick consisted of

unconsolidated material due to karsting (Rivers et al. 2019a). In all cores, the Rus Fm. is

approximately45mthick(Fig.2).306cylindricalside-wallcoreplugs(~2.5cmdiameter,~3.8cm

long)weretakenat0.02-2mverticalspacingfromtheRus,andsamplepowdersfromplugswere

usedforX-raydiffractionandstableisotopeanalysisasdescribedbelow.

Depth (m)

20

30

35

25

Depth (m)

40

45

15

50

60

Depth (m)

Clastics

Dolomite

Calcite

Gypsum

Mineralogy0% 100%

S N

Core 1

Core 2

Core 3

Mineralogy0% 100%

Mineralogy0% 100%55

20

30

35

25

40

45

15

50

60

55

Al-K

hor

Trai

na

Al-K

hor

Trai

na

15

25

30

20

35

40

10

45

50

Al-K

hor

Trai

na

Figure2:Percentmineralogyforcores1,2,and3fortherecoveredRusFm.basedonXRDgeneratedbyCoreLaboratories.Claysandchalcedonycomprisetheclasticcomponent,andallcalciteislow-Mgcalcite.CoresaredatumedtotheboundarybetweentheTrainaandAlKhormembers.

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X-raydiffractionmineralogy

AllcoreplugpowderswereanalyzedformineralogybyX-raydiffraction(XRD)methods.Bulk

mineralogywasquantifiedbyCoreLaboratoriesutilizingtheirstandardprocedure.Eachsampleis

dispersed in a dilute sodium hexa-meta phosphate solution, then centrifugally size-fractioned in

ordertoisolateclay-sizedparticles(<2-4µm).Samplesarethenair-driedandexposedtoethylene

glycol forat least2hoursat60°C,beingmeasuredbothbeforeandafterexposure.APanalytical

automated diffractometer equippedwith an X’celerator linear detector usingReal TimeMultiple

Strip Technology (RTMS), a graphitemonochromator, Ni-Filter, and 40 kV Cu source is used to

analyze the samples. Bulk samples are scanned in the 2θ scan range of 4° to 70° at a rate of

4.2°/minute,whereasclaysamplesarescannedinarangeof2.5°to40°atarateof6.4°/minute.

Becausemanyofthesamplescontainamixedmineralogyofcalciteanddolomite,additional

XRD analysis was undertaken at Western Michigan University in order to determine percent

dolomiterelativetocalcite,dolomitestoichiometry,anddolomitecationorderingon68ofthe306

coreplugs.StandardXRDprocedureswereutilizedandsampleswereanalyzedwithaBrukerD2

PhaserDiffractometerwithCuKαradiation.Powderswereextractedfromcoreplugsusinganelectric

rotarydrill,groundbyhandwithmortarandpestleforhomogenization,andmountedonaBoron-

dopedsiliconP-typezerobackgrounddiffractionplate.Mountswereanalyzedundera2θrangeof

20to40°withastepsizeof0.01°andacounttimeof1.0sperstep.ThemethodofRoyseetal.(1971)

wasemployedinordertocalculatedolomitepercentages(relativetocalcite),whichtakestheratio

ofthedolomited(104)peaktothesumofthecalcited(104)anddolomited(104)peak.Themol%MgCO3

ofthedolomite(i.e.stoichiometry)wascalculatedbyusingthed-spacingofthedolomited(104)peak

(Lumsden1979).BecauseReederandSheppard(1984)demonstratedthattheequationofLumsden

(1979) can lead to inaccuracies of up to 3 mol% CaCO3 in Ca-poor dolomites, several internal

standardswereusedtoverifypeakpositions.Cationorderingofthedolomiteswasdeterminedby

takingtheratioofthedolomited(015)peaktothedolomited(110)peak(GoldsmithandGraf1958).

146

Thinsectionpetrography

Allthinsectionswerepreparedfromtheendsofthe306coreplugsanalyzedandimagedin

avarietyofdifferentways.ThinsectionsweremadeandprovidedbyCoreLaboratoriesforallcore

plugsfromeachofthethreewells.Thinsectionsampleswereimpregnatedwithblueepoxy,cutfrom

theendsofthecoreplugs,andstainedwithAlizarinRedS(ARS)todifferentiatedolomitefromcalcite

(Dickson1965).AnalysisandimagingofallthinsectionswasdoneonaZeissAxioplanMicroscope.

Additionally,CoreLaboratoriesprovidedplane-light,high-resolutiondigitalscansofallthinsections

fromCore1.

Stableoxygenandcarbonisotopes

Atotalof211bulkrocksampleswereextractedfromtheRusacrossall3cores(n=51,98,

and62forCores1,2and3,respectively)andanalyzedforδ18Oandδ13C.Datareportedherewere

first reported in Rivers et al. (2019a). Data were yielded from the Center for Stable Isotope

Biogeochemistry(CSIB),UniversityofCalifornia,Berkeley.SampleswerereactedwithH3PO4at90°C

for10minutesandanalyzedbyaGV ISoPrimemassspectrometerwithDual-InletandMultiCarb

systems.Sampleweightrangedfrom10to100µg,andinternationalstandardNBS19aswellastwo

labstandardsweremeasuredwitheachrun.Analyticalprecisionisapproximately±0.05‰forδ13C

and±0.07‰forδ18O.ValuesforallisotopiccompositionsarereportedrelativetotheViennaPeedee

Belemnite(VPDB)standard.

Results

BulkX-raydiffractionmineralogy

ThemineralogyoftheTrainaandAlKhormembersvariesspatiallyandtemporallywithin

Qatar(Fig.2).Incore1,theTrainaiscomposedofsequencescharacterizedbybeddedgypsumand

dolomitecappedbyclay(Fig.2).Mostbeddedgypsumintervalsare£5mthickandcontain>95%

147

gypsum,withminoramountsofdolomiteandclays.Similarly,beddeddolomiteintervals(>80%)

aregenerally£ 3mwithminor (£ 20%)amountsof gypsumandclay (Fig.2).Theclay capsare

typically£1mthick. Incores2and3, theTrainadiffers fromcore1 in that it lacksgypsumand

containspredominantlydolomiteandcalcite,withminoramountsofsilicaandclayincentralQatar.

In core 2, the lower part of the Traina contains low-Mg calcite (LMC) and dolomite,withminor

amountsofsilicaandclay(Fig.2).Inmixedcalcite-dolomitesamples,calcitecancompose>88%of

thebulkrock,whereasothersamplesaredominatedbydolomitecomprising>98%ofthebulkrock.

Theaveragesilicacontentinthesebulksamplesis4.2%(range0-70%).Averagebulkclaycontentin

themixedcalcite-dolomitesamplesis2.2%(range0-17%).Dolomitedominatesintheupperpartof

theTrainaincore2,andtheentireTrainaincore3,comprising>75%ofthebulkrockinallsamples,

andaveraging>95%.Theremainingfractionconsistsofsilica(1.24%)andclayminerals(3.3%).

ThemineralogyoftheAlKhorisgenerallyconstantacrossQatar(Fig.2).Themaindifference

betweentheAlKhorandtheunderlyingTrainaisthatm-scaleintervalsofLMCoccurinallthree

cores,samplescomprisedofmixeddolomite-calcitearerare,andthereisagreaterpresenceofclays

(Fig.2).Thecalcite-dominatedintervals(<90%)oftheAlKhoralsohostclayminerals(ave~12%)

anddolomite(<2%).Thedolomite-dominated intervals,bycontrast,hostsimilaramountsof the

claysandcalciteisminor(<1%).Incores2and3,theabundanceofdolomiteincalcite-dominated

intervals and fractions of calcite in dolomite-dominated intervals are higher than in core 1. For

example,dolomitemakesupto7%ofbulkrockincalcite-dominatedintervals,andcalciteupto20%

indolomite-dominatedintervals.Averageclaycontentisalsohigherincores2and3thanincore1,

averaging18.8%and22.8%ofbulk-rockmineralogy,respectively.

Thinsectionpetrography

Dolomitecrystalsizeandtexturevarybylocationanddepth.Incore1,theTrainaMember

(29.4-61.3m)consistsofmicrocrystalline(crystalsize£10µm)fabric-preservingdolomite(Fig.3A).

148

Miliolidsandothersmallbenthicforaminifera(SBF)arecommonlypreservedincore1(Fig.3A).In

the dolomite-dominated intervals in core 1, dolomite crystals are included in clay (Fig. 3B) and

gypsum(Fig.3C). Incores2and3,bycontrast, theTrainaMember ischaracterizedbyrelatively

coarse(crystalsize£100µm)planar-edolomiteshavecloudytopartiallycorrodedcoresandclear

rims(Fig.3D).Althoughthedolomitesincores2and3aregenerallyfabricdestructive,largebenthic

foraminifera(LBF)andechinoderms(Figs.3E,F)arerare.Incontrast,dolomiteintheAlKhorofall

coresispredominantlymicrocrystallineandgenerallypreservestheprecursorlimestonefabric(Fig.

4).

Thedolomite-calciteboundaryissharp(Fig.5A),althoughlarge(>200µm)replacivecalcite

crystalsareobserved(Fig.5B).Sparrycalcitecementsthatreplacedolomitecrystals(Fig.5C)and

arevoid-filling(Fig.5D)arealsoobservedinthedolomiteunderlyingthedolomite-calcitecontact.

Theoverlyingcalciteintervalsexhibitawiderangeofcrystalsizesandmorphologies.IntheTraina

ofcore2,theonlycorewherecalciteisobservedintheTraina,calcitecrystalsarelarge(>100µm)

andconstitutepoikilotopiccementsthatengulfplanar-edolomitecrystals(Fig.6A).IntheAlKhorof

core2,poikilotopiccementsarealsodominant(Fig.6B),whereasMicrocodiumstructuresaremore

prevalentincores1and3(Figs.6C,D).Microcodiumstructuresincludethe“typicalMicrocodium”as

describedbyKabanovetal.(2008)(similartotype(a)ofEsteban,1974)whichexhibitscorncobor

lamellarcolonies(Fig.6C).Additionally,Microcodiumtype(b)ofEsteban(1974)isalsoobserved,

withsmaller,subquadrangularprisms(Fig.6D).Calciteisalsopresentasacementthatbothfillsand

partiallyreplacesdolomitecrystalcores(Fig.6E).Inallsamplesinwhichcalciteanddolomiteco-

occur,dolomiteisincludedincalcite.

In theTrainaofcore1,and lesserextent incore3,clay isobservedasan intercrystalline

cement (Fig. 7A) between dolomite crystals. In the Al Khor of all cores, clays are observed as

intercrystalline cements in calcite intervals (Fig. 6C, D), and form aggregates around which

Microcodiumstructureshaveformed(Fig.7B).

149

200 µm 100 µm

200 µm 100 µm

200 µm 100 µm

200 µm 100 µm

A B

C D

E F

Figure3:PlanepolarizedlightthinsectionimagesofvariouspetrographicfeaturesofthedolomitesintheTrainaMbr.A)Microcrystalline dolomite that has preserved the precursor limestone fabric, displaying dolomitized small benthicforaminifera(core1,depth44.28m).B)Microcrystallinedolomite(redarrows)cementedbypalygorskite(green)(core1,30.8m).C)Microcrystallinedolomite(redarrows)encasedingypsum(white)(core1,55.68m).D)Planar-edolomitedisplayingcloudytopartiallydissolvedcoresandclearrims(core2,53.5m).E)Planar-edolomitewithpreservationofalargebenthicforaminfer(core3,46.2m).F)Planar-edolomitewithpartialpreservationofanechinoderm(redarrow)(core3,48.65m).

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Figure4:AplanepolarizedlightthinsectionimageofadolomitizedmiliolidpackstonewiththefabricpreservedfromtheAlKhorMbr.(core2,20.65m).

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Calcite

Dolomite

Figure5:Featuresofthedolomite-calcitecontactobservedat~28mdepthincore1.A)Imageofasliced-corehandsampledisplayingthesharpcontactbetweencalciteanddolomite.B)Plane-polarizedthinsectionimageofcalcite(pink)anddolomite(gray),exhibitingthereplacivenatureofcalcite.C)Cross-polarizedphotomicrographshowingsparrycalcitecementthathasincludedandpartiallyreplaceddolomitecrystals.D)Cross-polarizedimagedisplayingvoid-fillingsparrycalcitecementwithinthedolomiteintervaldirectlybelowthedolomite-calcitecontact.

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200 µm

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500 µm 500 µm

200 µm

A B

C D

E

Figure6:IntheTrainaMbr.ofcore2,large(>200µm)poikilotopiccalcitecrystals(pink)engulfplanar-edolomitecrystals(A)(core2,55m).However,intheAlKhorMbr.,numerouscalcitefeaturesareobservedasfollows.B)Poikilotopiccalcitecements(pink)engulfingmicrocrystallinedolomite(core2,15.5m).C)Corncob,lamellarcoloniesofMicrocodium(pink)andclay(darktan/green)(core1,24.12m).D)SubquadrangularprismsofMicrocodium(pink)andclay(green)(core1,24.64m).E)Calcitecement(pink)thathasfilledorpartiallyreplacedthedissolvedcoresofdolomitecrystals(grey)(core3,8.64m).Allimagesareplanepolarizedlightphotomicrographs.

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Both chalcedony and quartz are observed in intervals that are dominated by calcite.

Chalcedonyisfrequentlyobservedaspartialtocompletereplacementofcalcitecrystals(Fig.8A).

Quartzcrystalsarecommonlycoarse(>200µm),euhedraltosubhedral,andoccurwithincalcite

mosaics(Fig.8B).

100 µm100 µm 500 µm

A B

Figure7:A)PlanepolarizedlightimageoftheTrainaMbr.,exhibitingdolomitecrystals(tan)cementedbypalygorskite(darkgreen)(core3,51.33m).B)PlanepolarizedlightimageoftheAlKhordisplayingcalcitecrystals(Microcodium;pink)includedwithin,aswellassurrounding,clayaggregates(green)(core1,24.64m).

200 µm 500 µm

A B

Figure8:Planepolarizedlightimageofchalcedony(white)thathaspartiallyreplacedcalcitecrystals(pink)(A)andacrosspolarizedlightimageofeuhedralquartzcrystals(greyandblack)includedincalcitemosaics(pink)(B)incore2(56.5m).

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Dolomitemineralogy

Rus dolomites are near stoichiometric and relatively well-ordered (Fig. 9). Dolomite

stoichiometry ranges 48.5-51.0%, averaging 50.05 ± 0.45% and cation ordering ranges 0.4-0.9

averaging 0.62 ± 0.11. The Rus Formation displays no, or very weak, trends of increasing

stoichiometrywithdepthinallcores.Cationorderingincreaseswithdepthincores2and3(R2of

0.18and0.75,respectively).NodepthtrendisobservedwithcationorderinginCore1(R2=0).

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Figure9:Plotsofstoichiometrywithdepthforcores1(A),2(B),and3(C).Alsodisplayedareplotsofcationorderingasafunctionofdepthforcores1(D),2(E),and3(F).GreydatapointsareAlKhorsamples,whileblackdatapointsareTrainasamples.

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Somewhatstronger,althoughinconsistent,trendscanbebediscernedinspecificintervalsin

theTrainaandAlKhormembers,however.Forexample,incore1,veryweaktrendsareobserved

forstoichiometryandcationorderinginboththeTrainaandAlKhor.TheTrainaexhibitsaveryweak

increaseofstoichiometrywithdepth(R2=0.03),withnotrendobservedincationordering(R2=0).

Stoichiometry and cation ordering increase with depth in the Al Khor (R2 = 0.05 and 0.12,

respectively).However,astrongincreaseinstoichiometrywithdepthisobservedinthecontinuous

dolomiteintervalfrom~16-21m(R2=0.79).TheTrainaincore2,hasdecreasingstoichiometry(R2

=0.48)andcationordering(R2=0.63)withdepth,whereastheoverlyingAlKhordisplaysincreasing

stoichiometry(R2=0.24)andcationordering(R2=0.14)withdepth.Incore3,theoppositetrendis

observedfortheTraina,withstoichiometryandcationorderingincreasingwithdepth(R2=0.42and

0.76,respectively).However,intheAlKhorincore3,stoichiometrydecreaseswithdepth(R2=0.44),

whereascationorderingdisplaysaveryslightincreasewithdepth(R2=0.04).Overall,theTraina

dolomites are generally more Mg-rich and well-ordered (50.14% ± 0.49% and 0.67 ± 0.12,

respectively)thantheAlKhorMember(49.98%±0.41%and0.59±0.08,respectively).

Oxygenandcarbonisotopes

Theaverageδ18Oandδ13Cvaluesdifferbetweencalcite,dolomite,andmixedcalcite-dolomite

samples (Fig. 10). Calcite averages δ18O and δ13C values of -10.73‰ and -7.84‰ (VPDB),

respectively, whereas dolomite averages -1.06‰ and -3.04‰ (VPDB), respectively. Samples

composedofmixedcalcite-dolomiteyieldedδ18Obetween-6.4‰and-3.5‰,andδ13Cfrom-4.3‰

to-3.6‰.Incore1,calciteδ18Ovaluesaremorepositivewithdepth(R2=0.73),whereasδ18Ovalues

aremorenegativewithdepthincore3(R2=0.79)(Fig.11).Notrendinδ18Oisobservedincore2

(R2 = 0). For dolomite, δ18Oweakly increaseswith depth in cores 1 and 2 (R2 = 0.07 and 0.20,

respectively)butdecreaseswithdepthincore3(R2=0.39)(Fig.11).Thecalciteintervalfrom24-29

mexhibitsastrongincreaseinδ13CwithdepthwithR2=0.79(Fig.12).Similarly,thedolomite(30-

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62m)belowthiscalciteintervalexhibitsincreasingδ13Cwithdepth(R2=0.44).Incore2,dolomite

δ13Cbecomesmorenegativewithdepthuntilamajorcalciteintervalat27-29m(R2=0.33).Under

thecalciteintervalat27-29m,dolomiteδ13Cincreaseswithdepth(R2=0.55).Incore3,dolomite

δ13Cincreaseswithdepth(R2=0.45)belowa~1m-thickcalciteinterval.

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Figure10:Crossplotofbulkdolomiteδ18Oandδ13CvaluesforallRussamples.Bluecirclesarecalcitesamples,blacksquaresaremixedcalcite-dolomitesamples,andpurplediamondsaredolomitesamples.

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Figure12showsthatcalcitevalueshavethemostnegativeδ18Oandδ13Cvalues.Thecalcite

δ18O and δ13C values exhibit weak covariance (R2 = 0.18). Dolomite δ18O is more positive,

predominantlybetween-2‰and+1‰,whereasδ13Cvaluesspanamuchwiderrangefrom-10‰

to+1‰.Mixedcalcite-dolomitesamplesexhibitδ18Oandδ13Cvaluesbetweenthoseofpurecalcites

anddolomitesanddisplayastrongpositivecovariancebetweenδ18Oandδ13C(R2=0.9).

TheindividualTrainaandAlKhormembersoftheRusexhibitminordifferencesindolomite

δ18Obutmajordifferencesindolomiteδ13C.Trainadolomitesδ18Oaverage-0.86±0.61‰compared

to-1.06±0.77‰intheAlKhor.IncontrasttotheTrainadolomiteδ13Caveraging-1.98±1.92‰,

theAlKhordolomiteδ13Cvaluesarerelativelymorenegativeδ13Caveraging-4.62±2.12‰.With

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Figure11:Bulkrockδ18OfromtheRusplottedasafunctionofdepthforcores1(A),2(B),and3(C).Bluecirclesarecalcitesamples,blacksquaresaremixedcalcite-dolomitesamples,andpurplediamondsaredolomitesamples.

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theexceptionofonesample,allcalcitesoccurexclusivelywithintheTrainaandcontainrelatively

similarδ18Oandδ13Cvaluesacrossallthreewells(Figs.10and11).

Discussion

Parageneticsequence

TheproposedparageneticsequencefortheRusFm.ispresentedinFigure13.Basedoncross-

cuttingrelationships,dolomitizationisinterpretedtobetheearliestdiageneticprocess.IntheTraina,

dolomite is included either in palygorskite clay or gypsum cement (Fig. 3), indicating that

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Figure12:Bulkrockδ13CfromtheRusplottedasafunctionofdepthforcores1(A),2(B),and3(C).Bluecirclesarecalcitesamples,blacksquaresaremixedcalcite-dolomitesamples,andpurplediamondsaredolomitesamples.

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dolomitization occurred prior to precipitation of these minerals. In the Al Khor, dolomite is

commonly cemented or partially to completely replaced by calcite (Figs. 5 and 6), indicating

dolomitizationalsooccurredpriortocalcitecrystallization.Theobservationthatdolomitecrystals

included in thesemineralphasesarepartiallycorroded(Figs.5-7)suggests thatpartialdolomite

dissolutionprecededorwasconcurrentwithprecipitationofclaysandcalcite.

Dolomitization in theRus is followedbyclayandgypsumcementation. Insectionswhere

dolomite and clay are present, clay occurs as intercrystalline cement that surrounds dolomite

Early LateDiagenetic Event

Dolomitization

Dolomite dissolution

De-dolomitization via Microcodium

Clay cementation

Gypsum cementation

Palygorskite cementation

Replacement of calcite by chalcedony

Euhedral quartz

Dolomite recrystallization 2 (?)Dolomite recrystallization 1 (?)

Figure 13: Paragenetic sequence of the relative timing of diagenetic events impacting the Rus in Qatar, based onpetrographiccross-cuttingrelationships.

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crystals (6A), the inference being that depositional clays were diagenetically altered after the

dolomitization (e.g., Ryan et al., 2019). In the Traina in Core 1, gypsum is observed as a pore-

occluding cement that surrounds dolomite (Fig. 3C), which suggests that gypsum precipitation

postdates dolomitization. Following formation of dolomite, clay, and gypsum is the formation of

euhedral quartz crystals included in calcite crystals, suggesting either growth of quartz prior to

calcite,orco-precipitationofquartzandcalcite.

FollowingtheprecipitationofquartzcrystalsistheformationofMicrocodiumthatcommonly

includesandreplacesdolomitecrystalsandformspoikilotopiclow-Mgcalcitecements.Giventhat

dolomitecrystalsareincludedwithinandpartiallyreplacedbycalcite,calcitemusthaveformedafter

dolomitization. The process of dedolomitization, i.e. dolomite replaced by calcite, suggests that

dolomitedissolutionhappenedconcurrentwithprecipitationofcalcite,asobservedinmanyofthe

calcitizedsections(Figs.5and6).Basedontheobservationthatcalcitecrystalsarepartiallyreplaced

by chalcedony in some intervals (Fig. 8A), chalcedony is interpreted to have formed after the

Microcodium.Calcitecrystalsinthecalcite-dominatedintervalsarealsosurroundedbyclaycement

(Fig.6D),suggestingthatsomeclaymayalsohaverecrystallizedafterthecalcitewasemplaced.

Dolomitizationinterpretation

Dolomitization is interpretedtohaveoccurredearly in theparageneticsequence,but it is

important tonotethatalthoughdolomite iscementedbygypsum,relatively large(>5m)bedsof

depositionalgypsumareinterbeddedwithdolomitizedtidalflatfaciesintheTrainaofcore1(Rivers

etal.2019a).Dolomitizationbyhypersalinefluidsrelatedtogypsumhasbeencommonlyinvokedto

explaintheformationofmassiveevaporite-associateddolomiteplatforms.Itisgenerallyunderstood

thatsuchfluidshavehighMg/CaratiosconducivefordolomitizationduetotheremovalofCa2+by

gypsumprecipitation,andarealsodenseandthushaveadownwardhydrologicdrive(e.g.,Adams

and Rhodes 1960; Deffeyes et al. 1965;Moore et al. 1988; Ruppel and Cander 1988; Saller and

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Henderson1998;Cantrelletal.2004;KaczmarekandSibley2011;DravisandWanless2018;and

manyothers).ThisinterpretationhasalsobeensupportedbyRTMstudiesthatsuggestseepageof

hypersalinebrines isaviablemechanismforplatform-scaledolomitization(JonesandXiao2005;

Garcia-FrescaandJones2011;Al-Helaletal.2012;Garcia-Frescaetal.2012).Theobservationthat

dolomite iscementedbygypsumdoesnotpreclude thepossibility that thedepositionofgypsum

happenedfirst,raisedtheMg/Caratioofthefluidswhichresultedindownwarddolomitization,and

excessCa2+releasedthroughdolomitizationcausedsubsequentgypsumcementation.Studiesusing

reactivetransportmodeling(RTM)havedemonstratedthatthissequenceofeventsisplausibleand

canexplainwhydolomitesthatarecementedbygypsumarelocatedbelowdepositionalgypsumbeds

(JonesandXiao2005;Al-Halaletal.2012).

Whetherornotdolomitizationoccurredinevaporativefluidsrelatedtodepositionalgypsum

canbetestedbycomparingtheobserveddolomiteδ18Ovaluestohypotheticaldolomiteδ18Ovalues

thatwouldbeexpectedifdolomiteformedfromevaporativefluids,however,assumingthatdolomite

δ18Oisreflectiveoftheprimaryconditionsofdolomitization.GiventheobservedTrainadolomite

δ18Oaverage-0.86‰,itwouldbeexpectedthattheδ18Oswwouldbe-3.8to+0.7‰basedonawide

rangeofnaturaltemperatures(20-40°C)(Horita2014).However,seawatersaturatedwithrespect

togypsumhasδ18Oswbetween+7to+10‰(KnauthandBeeunas1986;Riversetal.2019b),which

ismuchhigherthantherangecalculatedbasedontheaverageTrainadolomiteδ18O,althoughthe

effectsonδ18Oswduetoevaporationdifferinresponsetohumidityvariability(e.g.Budd1997).Rus

dolomites would have formed at temperatures of 75°-98°C (Horita 2014) under these fluid

chemistries,which ismuchwarmer thanmodern sabkha surfacewater temperaturesof 20-35°C

(McKenzie 1981; Rivers et al. 2019b). Furthermore, co-occurring aragonite and early-formed

dolomitefromevaporiticsettingshavebeenshowntohaveδ18Ocarbvaluesbetween+4‰and+8‰

(Bellanca and Neri 1986), considerably higher than the dolomite δ18O values of the evaporite-

associatedRusdolomitesinthepresentstudy.

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Two interpretations are presented to explainwhy evaporite-associated Traina dolomites

havemuchlowerδ18Ovaluesthanareexpectedifformedinfluidsevaporatedtothepointofgypsum

saturation.Thefirstinterpretationisthatthedolomitesdidnotformingypsum-saturatedfluids,and

ratherformedinnear-normalmarineseawaterrelativelyearlyafterthedepositionoftheintertidal

sediments but prior to significant evaporation and deposition of gypsum. This interpretation is

consistentwiththeobservationthatdolomiteiscommonlycementedbygypsumdirectlybelowthe

gypsumdepositsaswellasthedolomiteδ18Osuggestinganon-evaporativeorigin.Ifthedolomites

formedbynear-normalseawater,itislikelythatthisseawaterwasmesohaline(slightlyevaporative

but not to the point of gypsum saturation) and had elevated temperatures slightlywarmer than

expected for Eocene sea surface temperatures (>35°C; Zachos et al. 1994; 2006) due to the

abundanceofinnerrampandtidalflatfaciesandinterpretedsemi-restrictedsetting(Riversetal.

2019a).Forexample,fluidswithδ18Oswof+0.7‰at40°Ccanproducedolomiteswiththeaverage

Traina dolomite δ18O (-0.86‰).A similar dolomite δ18O is attained as temperatures continue to

increaseandδ18Oismorepositiveduetoevaporation(e.g.45°Cand+1.7‰).Iftheaverageδ18Oof

evaporite-associateddolomites(i.e.Trainaincore1)isutilized(-0.42‰),slightlymoreevaporative

fluidsareneededatthesametemperature(e.g.δ18Osw+1.2‰at40°C).Inbothcases,however,the

calculatedδ18Osw reflectsmesohaline fluidsandnot seawater thathasevaporated to thepointof

gypsumsaturation(KnauthandBeeunas1986;Riversetal.2019b).Fluidvalueswithδ18Osw³~+1‰

areconsistentwithslightevaporationofnormalEoceneseawaterwhichisinterpretedtohaveδ18Osw

of -0.98‰ (Zachos et al. 1994). Small-scale flushing of mesohaline fluids due to fluid density

contrastshasbeenshowntobeaviablemechanismforplatform-scaledolomitizationinRTMstudies

(JonesandXiao2005),andthusitmaybetheresponsiblemechanismfordolomitizationoftheTraina

dolomites.Thepetrographicobservationsindicatingdolomiteformedpriortogypsum,aswellasthe

dolomite δ18O that suggest a fluid origin uninfluenced by evaporation to the point of gypsum

saturation,isalsoconsistentwiththisinterpretation.

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ThesecondinterpretationtoexplaintherelativelylowTrainadolomiteδ18Ovaluesisthat

thedolomiteshavebeenrecrystallized,andthepresentdolomiteδ18Ovaluesreflectthemostrecent

fluidsresponsibleforstabilization.Inthisinterpretation,itispossiblethatinitialdolomitizationdid

occurinfluidsthatwereevaporatedtothepointofgypsumsaturation.Followingthepossibilityof

dolomitization by hypersaline brines is the requirement that the dolomites are recrystallized by

fluidswithnear-normalmarineδ18Ocompositions inordertoproducethepresentdolomiteδ18O

values.Itiscommonfordolomiteδ18Otobecomelowerwithprogressiverecrystallization(Mazzulo

1992;Maloneetal.1996),anditispossiblethatthedolomiteswereexposedtomorenormal-marine

dominatedfluidsasthedepositionalsystembecameincreasinglytransgressiveduringdepositionof

theAlKhor(Cavelier1970;Riversetal.2019a).AverageSSTestimatesduringtheEoceneare25-

35°C (Zachos1994;2006),whichwould requiremarine fluidswithδ18Osw between -2.6‰and -

0.3‰toproducetheaverageTrainadolomiteδ18Oof-0.86‰(Horita2014).Theseδ18Oswestimates

arewithin range of the estimated Eocene normalmarine δ18Osw of -0.98‰ (Zachos 1994),with

decreasesinδ18Oswpossiblyindicatingaslightinfluenceofmeteoricfluids.Thus,recrystallizationof

thesedolomitesinnear-normalmarinefluids,possiblywithaslightmeteoricinfluence,followingany

initialdolomitizationisaplausibleexplanationfortheobserveddolomiteδ18O.

A slightly different explanation is required for the dolomites of the Al Khor which have

relativelysimilardolomiteδ18OvaluesastheTrainadolomites,butarenotobservedinassociation

withgypsuminanyof the locales included inourstudy.TheAlKhordolomitesaverageδ18Oof -

1.06‰whichsuggeststhatincomparisontotheTrainadolomites,AlKhordolomitesformedeither

inslightlywarmer fluids, fluidswithslightlymorenegativeδ18O,wererecrystallized toagreater

extent, or a combination of all three (Land 1980;Mazzullo 1992). Fluids required to produce a

dolomiteδ18Oof-1.06‰overthetemperaturerangeof25-35°Cwouldhaveaδ18Oswbetween-2.8‰

and-0.5‰,withinrangebutslightlylowerthantheexpected-0.98‰(Zachos1994;Horita2014)

and not precluding the possibility of some meteoric influence. If it is interpreted that Al Khor

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dolomiteδ18Ovalues reflect initialdolomitization conditions, thennear-normalmarine fluidsare

likelyresponsiblefordolomitization.Althoughtheinterpretationofdolomitizationbynear-normal

marineseawaterisnotuncommon(Land1991;VahrenkampandSwart1994;Mazzulo1995;Budd,

1997;MancheandKaczmarek2019;Ryanetal.2020),eitherhydrologicdriversorkineticenhancers

mustfacilitatedolomitization(Land1985).SimilartothemodelproposedfortheTrainadolomites,

small-scalerefluxofmesohalinefluidsisapossiblemechanismfordolomitizationfortheAlKhor

dolomites.However,giventhe lackofgypsumandthe interpretationthatdepositionalconditions

were transitioning fromrestrictedevaporitic settings tomoreopen-marinesettings (Riversetal.

2019a),small increasesinsalinitymaybeunlikely.Anothercommonmodelfordolomitizationby

near-normalseawaterisgeothermal(Kohout)convection(WhitakerandXiao2010).However,this

model is based on convection of seawater entering an isolated carbonate platform,whereas the

EocenecarbonatesofQatarareinterpretedtohavebeendepositedonaprotectedramporlagoon

(Riversetal.2019a).Tidalpumpingandwind-drivencirculationofseawaterthroughsedimentsis

alsoaviablemechanismofnormalseawaterdolomitization,althoughsuchquantitiesofdolomiteare

usuallyvolumetricallysmall(<5%;Carballo1987;Mazzulloetal.1995).

Alternatively,andsimilartotheTrainadolomites,theAlKhordolomiteδ18Ovaluesmaynot

presentlyreflecttheinitialdolomitizationconditions,butrathertheconditionsofrecrystallization.

Also similar to that proposed for the Traina, the dolomite δ18O values suggest that the

recrystallizationfluidswereeithermarinefluidswithelevatedtemperaturesormarinefluidsslightly

dilutedbymeteoricwater.Thevariousdolomitizingandrecrystallizationfluidsareassessedbelow

in the context of the petrographic relationships between dolomite and calcite, as well as the

geochemicalrelationshipswithregardstoδ13C.

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Geochemicalrelationshipbetweendolomiteandcalcite

Despitetheirclosestratigraphicandpetrographicrelationships,thestableisotoperesultsin

Figures 10-12 suggest that the calcites and dolomites in the Rus formed in distinctly different

environments.Theextremelynegativeδ18Oaverage(-10.73‰)ofthecalcitessuggestsformation

eitherinveryhotfluids(Urey1947;Epsteinetal.1953;O’Neiletal.1969;Swart2015)orinmeteoric

fluids(AllanandMatthews1982;Lohmann1988;JamesandChoquette1990;HaysandGrossman

1991; Swart2015), bothofwhich tend tohave stronglynegativeδ18O.Given that the associated

Microcodiumfeaturesobservedinthecalciteintervalsaregenerallyinterpretedtoreflectbiogenic,

non-marine, pedogenic environments (Klappa 1978; Kabanov et al. 2008), it is more likely that

negativeδ18OvaluesintheRuscalcitesarerelatedtometeoricfluids.Previousworkhasshownthat

pedogenic environments commonly produceMicrocodiumwith δ18Ovalues as negative as -10‰

(Kabanovetal.2008).Incontrast,thedolomiteshaveδ18Ovaluesthataresuggestiveofeitherinitial

dolomitization by normalmarine Eocene fluids, near-normalmarine Eocene fluids with a slight

meteoricinfluence,orrecrystallizationinsuchfluidsasdiscussedabove.

A shared δ13C trend in the calcites and dolomites complicate interpretations about the

chemistry of the diagenetic fluids. The calcite δ13C values are fairly straightforward to interpret.

Average calcite δ13C of -7.84‰ is also consistent with a biogenic and pedogenic origin for

Microcodium(Kabanovetal.,2008).Microbial-derivedsoilCO2commonlyexertsastrongerinfluence

onMicrocodiumδ13Cthanhostrockderivedcarbon,resultinginmorenegativeδ13Cthanthehost

rockitreplaced(Kabanovetal.2008).Furthermore,negativeshiftsinδ13Carecommonincarbonates

alteredbymeteoricfluids(AllanandMatthews1982;JamesandChoquette1990).

Thenegativedolomiteδ13Cvaluesaremorecomplicatedtoexplain.Althoughthedolomite

δ18Ovaluesareconsistentwithnear-normalmarinefluids,highlynegativedolomiteδ13Cvalues(-

10‰)suggestanalternativemechanismfordolomitization.Furthermore,dolomitesdirectlybelow

themajorcalciteintervalintheAlKhordisplaysimilarlynegativeδ13Cvaluestothecalcite,andtrend

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positivewithdepthawayfromthecalciteinallthreecores(Fig.11).Itispossiblethatdolomitization

tookplaceinevaporativefluidsinthepresenceofsulfatereductionandoxidationoforganicmatter,

whichcancausethedissolvedCO2intheporefluidstobecomedepleted,andsubsequentlyresultin

negative dolomite δ13C values (Swart 2015). However, modern sabkha settings have carbonate

sediments with δ13C ranging from -1‰ to +5‰ (Rivers et al. 2019b), implying that highly

evaporative fluids may not be the culprit for the negative Rus dolomite δ13C values. Another

possibility is that dolomitization occurred in fluids associated with formation of the overlying

meteoriccalcites.Theobservationthatdolomiteδ13Cismorepositivewithdepthunderthecalcite

interval is consistent with a model whereby the meteoric fluids responsible for the calcite

crystallizationcontainadecreasingpercentageofsoil-relatedCO2withdepth(JamesandChoquette

1990). Such trends in δ13Cwith depth is common formixing-zone systems (Allan andMatthews

1982). This results in dolomitization of the underlying strata coincidingwith increasing δ13C as

seawateristheendmemberfluidatthebottomofthesection.Althoughthisscenariocanexplainthe

isotopedataandtrends,thepetrographicrelationships,whichconsistentlyshowdolomitecrystals

includedinorreplacedbycalcite,suggestthatinitialdolomitizationoccurredpriortocalcitization.

The observation that the average dolomite δ18O is -1.06‰, significantly higher than would be

expected if thedolomites formed fromsimilar fluidsas the calcites, alsoarguesagainstmeteoric

fluids.Ifthedolomitesformedundersimilarconditionsasthecalcites,dolomiteδ18Owouldbeabout

-6.9‰ based on the ~+3.8‰ fractionation from calcite to dolomite (Land 1980). However, if

dolomitizationoccurredprior to the formationof themeteoriccalcites,andthen fluidsrelatedto

calciteformationmixedwithmarinewatertorecrystallizetheunderlyingdolomites,thenalterations

indolomiteδ18Oandδ13CmayresultintheobservedRusdolomiteδ18Oandδ13Cvalues.

166

Mixing-zonedolomiterecrystallizationmodel

A mixing-zone recrystallization model posits that the petrographic and geochemical

observationscanbeexplainedbydolomiterecrystallizationinamixedmarine-meteoricfluidrelated

to theoverlyingmeteoriccalcites. In thismodel,earlydolomitizationofmarinesedimentsoccurs

untilthemiddleoftheAlKhor,whichischaracterizedbyam-thickcalciteintervalrepresentinga

majorexposureevent.Thisexposurecauseddedolomitizationoftherocksbelowandtheformation

ofMicrocodium at the exposure surface, resulting in the negative δ18O and δ13C observed in the

calcites.

Below themajor exposure surface lies the rest of the Al Khor and the Traina dolomites,

suggestingthatthecontactbetweenthecalciteanddolomitemayreflectdifferencesinthepore-fluid

chemistry. Specifically, that the observed δ18O and δ13C values in the overlying calcites reflect

meteoricfluids,whereasthoseintheunderlyingdolomitesreflectrecrystallizationinafreshwater-

marinemixing-zone.Thismodelisalsoconsistentwiththepetrographicrelationshipsobserved,in

whichthedolomitesarepresentasinclusionswithincalcitecrystalsandthusformedpriortocalcite,

butsubsequentlyrecrystallizedinfluidsrelatedtocalciteprecipitation.Thisscenariopositsthata

rangeof ratios ofmeteoricwater andmarine seawater contain amixed-fluid δ18O thatproduces

dolomiteswiththeobserveddolomiteδ18Orange.

Asastartingpointforcalculatingthefluidmixingratios,itisreasonabletoassumethatthe

end-memberfreshwatercomponentissimilartothefluidsresponsiblefortheMicrocodiumcalcites,

andthattheend-memberseawatercomponentissimilartoEoceneseawater.Basedonanaverage

Ruscalciteδ18Oof-10.7‰,theδ18Owofthefreshwatercomponentwouldbe-8.4‰at25°Cusing

theequationofKimandO’neil (1997).TheEoceneseawatercomponentwouldhaveaδ18Ow of -

0.98‰basedonZachosetal.(1994).Inordertoproducedolomiteinthetemperaturerangeof25-

30°CwithisotopiccompositionssimilartotheaverageRusdolomites(δ18O=-1.06‰),afluidwith

δ18Ow -2.8‰to -1.6‰(Horita2014) is required.Using thesevalues inamassbalanceequation

167

wherethemixingendproductδ18Owisaresultofthevolumetricallyweightedaverageoftheδ18Ow

ofthefreshwaterandseawatercomponents,itispossibletocalculatetheratiosofthemixedfluids

(Eq. 1; e.g. Rohling 2013). The seawater and freshwater fractions required to produce dolomite

recrystallization fluids with δ18Ow between -2.8‰ to -1.6‰ are 75% -92%, and 25% -8%,

respectively.However,whetherthesesameratioscouldproducedolomiteswithδ13Caslowas-10‰

isunknown.δ13Cincarbonaterocksislargelycontrolledbytheδ13Cofdissolvedinorganiccarbon

(DIC),whichincludesCO2(aq),CO32-,andHCO3-(Swart2015).BecausetheDICinfreshwaterfluids

isusuallypresentasCO2(aq)whereasDICinthemarinerealmisusuallypresentasHCO3-(Swart

2015),asimplemassbalanceequationcannotbecarriedoutfordolomiteδ13C.ThisisbecauseHCO3-

is 8‰ enriched in δ13C compared to atmospheric CO2 (Vogel et al. 1970), and the fractionation

betweencarbonatematerialandCO2istemperaturedependent(Romaneketal.1992).

(X)*(freshwaterδ18Ow)+(Y)*(seawaterδ18Ow)=δ18Ow(mixed) (1)

WhereXisthefractionoffreshwaterandYisthefractionofseawater,andX+Y=1

Resultsin(X)*(-8.4‰)+(1-X)*(-0.98‰)=(-1.6‰to-2.8‰) (2)

Someaspectsofthismodelaresupportedbymineralogicalobservations.Thedolomitesof

theRusarestoichiometric(50.05%)andrelativelywell-ordered(0.62),consistentwiththeideathat

recrystallizeddolomitesarecommonlymorestoichiometricandwell-ordereddolomite(Carpenter

1980;NavrotskyandCapobianco1987;Sibley1990;Maloneetal.1996;KaczmarekandSibley2014).

Secondly, the trends in dolomite crystal size are consistent with dolomite recrystallization in a

mixing-zone.TheAlKhorincores2and3consistsoffinelycrystalline(£10µm),fabric-preserving

dolomites,whereastheunderlyingTrainaconsistsofrelativelycoarser(30-100µm)planar-efabric

168

destructivedolomites.Thischangeintextureandincreaseincrystalsizewithdepthisconsistent

withdolomiterecrystallization(Mazzulo1992;Greggetal.1992).

Despitethetheoreticalandempiricalargumentsagainstmixingzonedolomitization(Hardie

1987;MachelandMountjoy1990;Melimetal.2004),dolomiterecrystallizationcanoccurinmixing

zones (Hardie 1987; Machel and Burton 1994), though recrystallized dolomites limited in their

spatialextent(WardandHalley1985;Humphrey1988;Cander1994).Gaswirthetal.(2007)showed

thatmixingzonesformedupto13.5km3ofdolomiteintheUpperEoceneOcalaLimestoneand2.2

km3intheearlyOligoceneSuwanneeLimestoneofsouthwestFlorida.Basedon22-45mthickness,

anapproximateareaofQatarof11,500km2,andanaverageRusdolomiteporosityof30%(Riverset

al.2019a),thevolumeofdolomiteintheRusisconsiderablyhigher(between~180and360km3).

Further criticism of this mixing-zone dolomite recrystallization model lies in the

interpretationthatseawaterfractionsbetween75%and92%arenecessarytoformRusdolomites

withtheobservedδ18O.ComparingthisrangetotheoreticalcalculationspresentedinHardie(1987),

suchafluidwouldbesupersaturatedwithrespecttocalcite,idealdolomite,andnon-idealdolomite,

andthusfalloutsideofthe“DoragZone”whichischaracterizedbyfluidssupersaturatedwithrespect

todolomitebutundersaturatedwith respect to calcite. Comparing the saturation states for ideal

dolomite and non-ideal dolomite in Hardie (1987), it is suggested that 75-92% seawater is

supersaturatedwithrespecttobothidealandnon-idealdolomite,implyingsuchamixedfluidwould

providenothermodynamicdriveforrecrystallization.Ofcourse,thisinterpretationrestsonmany

assumptionsincludingthatthestartingwatercompositionsaresimilartothoseusedbyBadiozami

(1973),whichincludesmodernYucatangroundwaterandmodernaverageseawater.

A mixing-zone dolomite recrystallization model is also inconsistent with the observed

increaseinδ13CwithdepthindolomitesbelowtheMicrocodium-bearingexposuresurface,butthe

lackofatrendinδ18O.Inatypicalmixingzone,δ13Candδ18Ocovarysuchthatmineralsthatformin

themeteoricendmembercontainmorenegativeδ13Candδ18Othanmineralsthatforminthemarine

169

endmember(AllanandMatthews1982).Whereasthedolomiteδ13Cfollowsthetrendofincreasing

δ13CwithdepthbelowtheMicrocodium-bearingexposuresurface,notrendisobservedintheδ18O.

Thissuggeststhatmixing-zonedolomiterecrystallizationmaynotberesponsibleforthedolomite

δ13Candδ18Otrends,andthatitisprobablethatamorecomplexdiagenetichistoryhasimpactedthe

Rusdolomites.Aplausibleexplanationisthatmixing-zonedolomiterecrystallizationoccurredafter

theformationofMicrocodium,resultingintheincreaseinδ13Cawayfromtheexposuresurface,as

detailedabove.However,another,subsequentdolomiterecrystallization(R2)inmarine-dominated

fluidsresetthedolomiteδ18Otopresentvalueswhereasdolomiteδ13Cwasretainedfromtheinitial

mixing zone recrystallization. This multi-recrystallization interpretation would explain both the

observed trend indolomiteδ13C,aswellas the lackofa trend indolomiteδ18Oandnear-normal

marinerangesofdolomiteδ18O.

Itmaybepossible that thedolomiteswererecrystallized,either initiallyorduringR2,by

entirely meteoric-dominated fluids rather than marine-dominated fluids. This model may also

provideanexplanationforsomeaspectsthatareinconsistentwithamixing-zonemodelfortheinitial

recrystallization,mainly thedrastic differences in δ18Oandδ13C.Themodel is predicatedon the

moderngroundwaterchemistryofthefluidswhichcurrentlyresideintheRusandunderlyingUmm

er Radhuma formations, which make up Qatar’s groundwater aquifers (Eccleston et al. 1981).

Ecclestonetal.(1981)reportedthattheδ18Oofthemodernaquiferfluidsrangefrom-3‰to-1‰,

andthattheδ13Cofthefluidsrangefrom-12‰to-5‰.Otherthanthegeneralobservationthat

dolomiteδ18Oislowerindolomitesthathavebeenrecrystallized(Mazzulo1992;Maloneetal.1996),

thereiscurrentlynopublisheddataonthesubjectofisotopefractionationfromunstabledolomite

(non-stoichiometricand/orpoorly-ordered)tostabledolomite(stoichiometricandwell-ordered).

Assuch,thismodelassumesthatasdolomiterecrystallizes,itincorporatestheδ18Oandδ13Cvalues

oftherecrystallizingfluidswithlittletonochange.However,thiswouldalsoassumethatdolomite

does not retain the precursor δ13C signature, as is widely assumed (Swart, 2015). Another

170

assumption of this model is that predominantly meteoric waters are saturated with respect to

dolomite, causing the dissolution and re-precipitation of a more stable dolomite phase. This

assumption may not be unreasonable, as data show that the modern Qatar aquifer waters are

marginallysupersaturatedwithrespecttodolomite(F.Whitaker,pers.comm.).

Incorporatingtheaquiferδ18Owvaluesof-3‰to-1‰andanear-surfacetemperatureof

25°C,dolomitesprecipitatingfromsuchfluidsshouldhaveδ18Odolomitebetween-1.3‰to+0.7‰,well

withinrangeoftheaverageRusδ18Odolomiteof-1.06‰.Inawell-mixedaquiferwithincreasingrock-

waterinteractionwithdepth,itwouldnotbeunlikelyforcarbonateδ18Ovaluestoremainrelatively

constantwhile δ13C values are higherwith depth as a result of increased rock-water interaction

(Lohmann,1987).SuchaprocesscouldberesponsibleforthetrendsobservedinRusdolomiteδ18O

andδ13C.Theextremelynegativedolomiteδ13Cdirectlyundertheexposuresurfaceisconsistentwith

highfluxesofvadoseCO2whichhasrecentlybeenconsideredmoreimportantforthedissolutionof

limestoneincavesthanthemixingoffreshwaterandseawater(Gulleyetal.2014),andmayhave

implicationsforthedissolutionofmetastabledolomiteinsimilarenvironments.However,apotential

downside to thismodel it is unlikely that themodernQatar aquifer values are representative of

Eocenemeteoric fluids,asregionalspeleothemstudieshaveshownthatmeteoriccalcitecements

haveδ18Ovaluesbetween-12‰and-4‰duringinterglacialperiods(Fleitmannetal.2004).These

valuesaresimilartothemeteoriccalcitesinthisstudyandsuggestthatδ18Oofmeteoricfluidswere

significantlylowerinthisstudyareainthegeologicalpast,andnotwithinrangeofthemodern-day

aquiferwaterδ18Ovalues.Thus,utilizingthemodernaquiferδ18Oanddolomitesaturationindices

maynotbeindicativeofprocessesthataffectedtheserocksinthepast.

Complicating all interpretations about the recrystallization of Rus dolomites in meteoric

fluidsrelatedtothecalciteexposuresurfacearethepetrographicobservationsshowingdolomite

crystals included incalcitecrystals (Figs.5and6), suggesting thatdolomites formedprior to the

precipitationofcalcite.Furthermore,dolomitecrystalsthatare includedincalcitecrystalsdonot

171

differ in texture compared to dolomites surrounding the calcite intervals (Figs. 5C; 6A and 6B),

suggesting thatcalcitesdidnotpreservean initialdolomitephase that isuniquelydifferent from

underlyingdolomitesinterpretedtoberecrystallized.Fromapetrographicstandpointitisrelatively

easy to discern that dolomitization and subsequent recrystallization occurred prior to meteoric

exposure, but the stable isotope data clearly indicate a vastly more complex diagenetic history

includingpost-exposurerecrystallization(Fig.13).

Implications

RecentstableisotopicstudiesofQatar’smodernevaporativemarinesettings(Riversetal.

2019b)presentaconundrumintheinterpretationofthelocalCenozoicrockrecord.Modernwaters

andsedimentsfromrestrictedevaporativelagoonsshowsignificantlyelevatedδ18Oandδ13Cvalues

relative to unevaporated seawater and associateddeposits (Rivers et al. 2019b). In spite of this,

underlyingEocene-agecalciticanddolomiticrocksassociatedwithbeddedgypsumdepositspointto

formation inwaters of either near-normalmarine ormeteoric affiliation (Ryan et al. 2020; this

study).Withinthiscontext,thecurrentreportdemonstrateshownear-surfaceandrelativelyyoung

carbonatedepositsaregeochemically reset to thedegree thatallprimary isotopic signaturesare

likelymasked.Morespecifically,ourfindingsdemonstratethatearlydiagenesisofrelativelyyoung,

shallowly buried carbonate rocks can be extremely complex and result in multiple stages of

crystallization, recrystallization, isotopic inheritance, and diagenetic resetting. Despite the

documentedresetting,amajorfindingofthisstudyisthatnoneofthepetrographicrelationshipsor

petrologicalcharacteristicsoftheRusdolomitesreflectdolomitizationbyhypersalinefluids,despite

theintimaterelationshipwithtidalflatfaciesandoverlyingbeddedgypsum.Thus,thepresentstudy

addstoagrowingbodyofliteraturesuggestingthatitmaybemorecommonthanpresentlyaccepted

thatdolomitescappedbyevaporitesmaybegeneticallyunrelatedtotheoverlyingevaporativefluids

(Newportetal.2017;MancheandKaczmarek2019;Ryanetal.2020).

172

Anotherimplicationofthisstudyisthatdolomitesaresusceptibletoextensive,andmulti-

episodic recrystallization in shallow burial to near-surface conditions. Such extensive

recrystallization has not been previously documented in young, shallowly buried dolomites.

Published reports of extensivemineralogical andgeochemical resetting are invariably associated

withgeologicallyolderdolomitesthathaveundergonedeep(>1000m)burial(GreggandShelton

1990;GaoandLand1991;MontañezandRead1992;KupeczandLand1994;Maloneetal.1994;

Machel2004).Incontrast,relativelyyoungdolomitesandthosethathaveundergoneonlyshallow

burialgenerallydisplayminoralterationsingeochemicalparameters(McKenzie1981;Greggetal.

1992; Veillard et al. 2019). For example, Veillard et al. (2019) analyzed Miocene dolomites in

Australia and interpreted a shallowburial recrystallization trendbased on increases in clumped

isotope crystallization temperatures and δ18Ow with depth, coupled with mottled CL signatures,

increasingcrystalsizeandslightly increasingstoichiometrywithdepth.However,suchdolomites

weregenerallycalcium-rich(commonly<47mol%MgCO3)andpoorlyordered(0.2-0.3)(Veillardet

al.2019)comparedtothedolomitesinthisstudy(average50.1%and0.62,respectively).McKenzie

(1981)interpretedrecrystallizationofdolomitesinAbuDhabibasedonincreasesincrystalsizeand

cationorderingwithdepth.However,thesedolomitesweremainlynon-stoichiometric(45-48mol%

MgCO3), unusual for extensively recrystallized dolomites (Mazzulo 1992; Kaczmarek and Sibley

2014). In another study of Holocene dolomites from Belize, Gregg et al. (1992) interpreted

recrystallizationofdolomitesbasedonsubtleincreasesincrystalsize(0.4µmatthetopto1.0µmat

thebase) and cationordering (0.7-1.0)within~30 cm, although thesedolomiteswere alsonon-

stoichiometric (40-46 mol% MgCO3). The present study thus builds upon previous studies and

demonstratesthatrelativelyyoung,shallowburialdolomitescanbeextensivelyrecrystallizedwith

respect tobothmineralogy(dolomitestoichiometryandcationordering)andgeochemistry(δ18O

andδ13C)multipletimesintheirearlydiagenetichistory.

173

OurfindingsbroadlysupporttheconclusionsofSwartandKennedy(2012)andOehlertand

Swart (2014) who cautioned against using carbonate δ13C as a proxy for global biogeochemical

events in Earth history. As shown in the present study, post-depositional diagenesis can exert a

strongcontroloncarbonateδ13C.Although, itshouldnotbemistakenthatδ13Cisalwaystheend

resultofpost-depositionalalterations.RecentstudiesinthePaleozoicstrataoftheMichiganBasin

have shown that depositional δ13C trends can still be traced spatially and temporally through

lithologicalchangesbetweencalciteanddolomite(Caruthersetal.2018).Thismeansthatcarbonate

δ13Cmaybealteredafterdepositionduetoextensivediagenesis,butthatitalsodependsonother

factors. Lastly, this study adds to a large body of literature that demonstrates that carbonate

geochemistryalonemayleadtoinaccurateinterpretationsabouttheparageneticsequenceofarock.

Instead,geochemicalproxiesmustbeintegratedwithpetrographicobservationsinordertoassess

thecompletediagenetichistoryofcarbonaterockunits(SchlagerandJames1978;Melimetal.1995;

FrankandBernet2000;Melimetal.2002;Gischleretal.2013).Iftheisotopictrendspresentedin

Figures 11 and 12 were assessed without petrographic context, the interpretation that marine

dolomitizationpost-datedexposureeventsrelatedtoMicrocodiumwouldbelogicalgiventhatthe

dolomiteδ13Cisincreasinglypositivedeeperfromtheexposuresurface(i.e.retainsmeteoric-derived

δ13C), but dolomite δ18O is invariable. The onlyway to eliminate this as a geologic possibility is

throughdetailedpetrographicanalysis.

Conclusions

Early and extensive near-surface diagenesis has significantly altered the textures,

mineralogy,andisotopicsignatureoftheEoceneRusFormationofQatar.Majoralterationsinclude

early dolomitization, dolomite recrystallization, and exposure dedolomitization byMicrocodium.

Petrographicevidenceindicatesthatdolomitizationoccurredearlyandpriortocalcitization,gypsum

precipitation,andclaydiagenesis.Thestableisotopiccompositionsofdolomiteandcalcite,however,

174

suggest a rich and complex history involving multiple recrystallization events tied to exposure.

Dolomites invariably exhibit near-normal marine δ18O values (ave -1.06‰ VPBD) but strongly

negativeδ13Cvalues(-10to0‰;ave=-3.04‰VPDB),despitebeingcloselyassociatedwithbedded

evaporites.Calcite intervals, incontrast,arecharacterizedbystronglynegativeδ18Ovalues(ave-

10.73‰ VPDB) but similar δ13C (ave -7.84‰ VPDB) to the dolomites. The trend in increasing

dolomiteδ13Cwithdepthawayfromthecalcite-dolomitecontact,coupledwithsimilarcalciteand

dolomite δ13Cvalues, suggest thatdolomiteswere recrystallized in fluids related to formationof

Microcodium, despite evidence showing replacement of dolomite by Microcodium. The model

presentedheretoexplainpetrographicallyearlydolomiteswithgeochemicalcharacteristicsoflater

calcitephasesisasfollows.EarlydolomitizationoftheRustookplaceeitherinhypersalinefluids

related to gypsum precipitation (Traina) or near-normal marine fluids (Traina and Al Khor).

Followingthis,majorexposureeventsresultinginmeteoriccalciteandMicrocodiumresultedbothin

dedolomitizationaswellassubsequentdolomiterecrystallization,resettingboththedolomiteδ18O

and δ13C in a mixed freshwater-seawater system. Following this, the dolomites were possibly

recrystallizedoncemoreeitherinnear-normalmarinefluidsorpossiblyawell-mixedaquifersetting,

resultinginthepresentdolomiteδ18Obutretainingtheinitialrecrystallizationfreshwaterinfluenced

δ13Csignatureorrecordingtheaquiferδ13Csignature,withmorepositiveδ13Cwithincreasedrock-

water interaction.Ultimately, this study sheds lighton i) theoriginof someevaporite-associated

dolomites,ii)thedegreetowhichearlyformeddolomitescanrecrystallizeundershallowburialand

in near-surface fluids, and iii) interpreting geochemical relationships without incorporating

petrographicobservations.

Acknowledgements

WewouldliketothankExxonMobilmanagementforpermissiontopublishthisstudy.Wewouldalso

liketothankSabrinaSkeatforherlogisticalsupportandhelpwithdatatransfer.

175

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