Post on 24-Jan-2023
Western Michigan University Western Michigan University
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Dissertations Graduate College
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.
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,
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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
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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.
Rivers,J.M.,andLarson,K.P.,2018,TheCenozoickinematicsofQatar:Evidenceforhigh-angle
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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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
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<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
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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
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easternSwabianAlb,
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1998 Sedimentary
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Maloneet
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Recrystallizationof
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Formation
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1994 Sedimentology Miocene Deep
>80
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1996 Journalof
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>80-
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Nielsenet
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Multiple-step
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ancientdolomite
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fromtheDinantianof
Belgium
1994 Sedimentology Mississippian Deep ~3
Lonneeand
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Pervasive
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hydrothermal
alterationinthe
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SlavePoint
Formation,British
Columbia,Canada
2006 AAPGBulletin Devonianto
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Deep >1 230-
267
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Isotopicevidencefor
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examplefromthe
2009 Cretaceous
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N/A
81
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Yes
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2008 Sedimentary
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2006 AAPGBulletin Jurassic Deep ~4
Zempolich
andHardie
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1997 AAPGMemoir
69
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84
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Basque–Cantabrian
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Haeri-
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al.
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geochemical
attributesof
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carbonatesandtheir
spatialdistribution
insouthwestern
Ontario,Canada
2013 Marineand
Petroleum
Geology
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N/A
Haeri-
Ardakaniet
al.
Petrologicand
geochemical
attributesof
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Testingthe
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2020 Sedimentology Jurassic Intermediate 0.4-
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86
asgeochemical
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MississipianAlida
Beds,Willistoin
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0.75-
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Correlating
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87
floweventsinthe
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Cairdetal. Ediacaran
stromatolitesand
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phosphoriteofthe
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Phosphogenesis
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2017 Sedimentary
Geology
Precambrian Deep
Zhengetal. Stratigraphicand
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2019 Minerals Permian Deep <1.5 <85
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1996 Carbonatesand
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Yes
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Multi-phase
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Kupeczand
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Progressive
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1994 IASSpecialPub Ordovician Shallow
Yes
89
LowerOrdovician
EllenburgerGroup,
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Kupeczand
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Progressive
recrystallizationand
stabilizationofearly-
stagedolomite:
LowerOrdovician
EllenburgerGroup,
westTexas
1994 IASSpecialPub Ordovician Deep
Greggetal. Earlydiagenetic
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Nicolaides Originand
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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).
103
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)
106
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
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2 0 .0
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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
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OM g
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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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50
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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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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
119
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
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PALYGORSKITE
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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
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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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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.
145
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
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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
165
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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