MICROBIAL DOLOMITES FROM CARBONATE-EVAPORITE SEDIMENTS OF THE COASTAL SABKHA OF ABU DHABI AND THEIR...

289Journal of Petroleum Geology, Vol. 33(4), October 2010, pp 289-298

© 2010 The Authors. Journal compilation © 2010 Scientific Press Ltd

MICROBIAL DOLOMITES FROM

CARBONATE-EVAPORITE SEDIMENTS OF THE

COASTAL SABKHA OF ABU DHABI

AND THEIR EXPLORATION IMPLICATIONS

F. N. Sadooni1*, F. Howari2 and A. El-Saiy3

Detailed petrographic studies of Recent dolomites from the coastal sabkha of Abu Dhabi usingSEM and Energy Dispersive X-ray (EDX) analyses indicate that most of the dolomite started toform in “micro-niches” (i.e. small isolated pore spaces) within the carbonate sediments. Thesemicro-niches tend to retain connate waters, becoming anoxic and supporting specific microbialactivity which ultimately results in the formation of dolomite. This novel concept may help toexplain the origin of many dolomite types, since recently deposited carbonate sediments havehigh porosities (up to 60%) and the pore spaces may serve as nucleation sites or “stations” fordolomite formation. The combination of neighbouring micro-niches in a porous carbonate mayresult in dolomitization of an entire stratum.

Six dolomite types were reported from the studied sediments in the Abu Dhabi sabkha.These were pustular and cluster dolomite; dolomites associated with clay minerals and withevaporites; and dolomites associated with microbial mats and foraminiferal tests.

1Environmental Studies Centre, Qatar University, PO Box2713, Doha, Qatar.2Environmental Science Programme, The University ofTexas of the Permian Basin, 4902 East University, Odessa,Texas 79762, USA.3Department of Geology, United Arab Emirates University,PO Box 17551, Al-Ain, United Arab Emirates.* author for correspondence, email:[email protected]

Key words: dolomite, sabkha, microbial sediments, AbuDhabi, The Gulf.

INTRODUCTION

The existence of dolomite in sabkha sediments wasfirst reported by Wells (1962) who documented thepresence of dolomite associated with aragonite andgypsum from nearshore and intertidal sedimentsaround the Qatar Peninsula. Curtis et al. (1963) madesimilar observations on a 20 cm core from the uppercoastal flats (sabkha) of Abu Dhabi. In these and laterstudies, the dolomite was found to be associated withevaporites. Illing and Taylor (1993) proposed thatdolomite in the sabkha sediments resulted from

changes in brine chemistry in favour of Mg ions. Theynoted that macro- and microscopic observationsindicated that aragonitic sediments may be convertedto dolomite which however retained traces ofdolomitized microbial laminations. This was supportedby SEM photos (Illing et al., 1965). Butler (1969)studied the geochemistry of coexisting brines fromthe sabkha of Abu Dhabi and suggested that thedistribution of evaporite minerals and dolomite iscontrolled by hydrological factors. He proposed thatthe equilibration between brines and host sedimentsis responsible for dolomitization of the carbonatesediment. Hsu and Siegenthaler (1969) conductedexperiments on hydrodynamic movements caused byevaporation related to dolomite formation, andsuggested that fluid flow through porous sedimentwas induced by upward decline in hydrodynamic

290 Microbial dolomites from the coastal sabkha of Abu Dhabi

potential during the process of evaporation. Theyproposed that such movement may be an effectivemechanism to move Mg-bearing solutions in thesediment of an arid coastal plain. Patterson andKinsman (1982) investigated the textural features ofthe Abu Dhabi sabkha dolomite, and suggested thatmost diagenetic dolomite forms at the expense ofprimary aragonite in a reducing environment inassociation with hydrogen sulphide formation. Arecent detailed geochemical and petrographic studyof three sites in the intertidal and supratidal zones ofthe coastal sabkha of Abu Dhabi was conducted byBontognali et al. (2010). They concluded thatauthigenic dolomite forms within the exopolymericsubstances (EPS) making up the microbial mats withinthe intertidal zone. For dolomite precipitating in thesuprtaidal zone where microbial activity is low, theysuggested that either this dolomite was formed in theintertidal zone during a previous sedimentary cycleor that dolomite formation continues within the EPSof buried and inactive mats.

Chafetz and Rush (1994) studied Pleistocenedolomite from boreholes in the offshore Al-Jubayl area,Saudi Arabia. They suggested that the observed markedchanges in δ13C values may be due either to the actionof descending meteoric waters beneath a soil horizon,or to diagenesis associated with bacterially mediatedsulphate reduction, with the latter as the favouredhypothesis. Vasconcelos and McKenzie (1997) studiedmodern dolomite from a shallow-water isolated coastallagoon to the east of Rio de Janeiro (Brazil). Theyshowed that Ca-dolomite forms here under anoxichypersaline conditions within a black sludge layerdirectly above the sediment-water interface. Theyreported that both the initial formation and thesubsequent “ageing” of the dolomite are stronglymediated by microbial activity. Furthermore, they wereable to generate a highly ordered dolomite in thelaboratory at low temperatures using sulphate-reducing bacteria cultured from the studied sediments(Warthmann et al. 2000). Teal et al. (2000) studiedHolocene shallow-marine sediments from NorthernBelize and found that dolomite mainly occurred in theform of cement with an average crystal size of 7 μm.Dolomitization was promoted by bacterial sulphatereduction and methanogenesis in environments withanoxic pore waters. Similar studies were wasconducted by van Lith et al. (2003), who concludedfrom a study on two lagoons in Brazil as well as fromexperimental work that sulphate-reducing bacteriafacilitate the formation of low-temperature Ca-dolomite under anoxic conditions. They suggested thatthe bacterial metabolism, activity and the rate ofmineral precipitation can affect the type of carbonatemineral formed. Wright and Wacey (2005) managedto precipitate dolomite in culture experiments that

simulated the microbiogeochemical conditions inhypersaline dolomitic lakes of South Australia. Theyconcluded that bacterial sulphate reduction controlsdolomite precipitation in both the laboratoryexperiments and the lake sediments by overcomingthe kinetic constraints on dolomite formation byremoving sulphates and releasing magnesium andcalcium ions from neutral ion pairs.

Other types of bacteria have also been reported tomediate in the formation of carbonate mineralsincluding dolomite. Rivadeneyra et al. (1993)succeeded in precipitating carbonate minerals bymoderately halophilic strains of Bacillus isolated froma saline soil. Bacillus sp. formed mineral crystals inall the media tested including magnesium calcite,aragonite, monohydrocalcite and dolomite, in varyingproportions depending upon the growth medium used.Methanogens were reported by Roberts et al. (2004)to colonize basalt in a freshwater aquifer and nucleatenon-stoichiometric dolomite on cell walls. Sanchez-Roman et al. (2008) demonstrated experimentally thattwo moderately halophilic aerobic bacteria mediatethe precipitation of dolomite at low temperature. Theyreported the nucleation of dolomite on nanoglobulesassociated with bacterial cell surface and suggestedthat these may represent the initial stage in dolomitenucleation and may be used as proxy for the detectionof ancient microbial dolomite. Culture experimentsusing the moderately halophilic aerobic bacteriaisolated from the uppermost part of the microbial matin Brejo do Espinho, Brazil, were conducted bySanchez-Roman et al. (2009). They demonstrated thatmicrobially mediated dolomite precipitation can occurunder ambient surface conditions in the presence ofoxygen. Their results add aerobic respiration as ametabolic activity to bacterial sulphate reduction andmethanogenesis, which have previously beenidentified with dolomite formation. They alsosuggested that the formation of carbonate mineralswith spherulitic structures in both the naturalenvironment and laboratory culture experimentssuggest microbial involvement, as is the case innumerous other modern environments and ancientsystems.

From another perspective, dolomite productionmay represent an important phase in microbialmetabolic activity in sabkha sediments. In a study onthe same sabkha sediments investigated in this workusing Raman spectroscopy, Edwards et al. (2006)suggested that extremophiles found in the sedimentsrequired the presence of dolomite or magnesite as asubstrate. Dolomite provided an Mg-rich base onwhich microbial activity could occur; also dolomiteformation increases the porosity of a sediment byaround 13% and hence creates space in whichmicrobial communities can develop.

291F N. Sadooni et al.

The aim of this paper is to investigate the role ofmicrobial mediation in dolomite formation and toinvestigate whether microbial activity may be able togenerate significantly thick dolomitic strata as opposedto being restricted to shallow coastal sediments, ashas hitherto been reported. This may have significantimplications for the exploration and development ofdolomite reservoirs.

MATERIALS AND METHODS

The study area is a remnant of the coastal sabkhainvestigated by Shearman (1963; 1966), and is locatedto the east of the city of Abu Dhabi (Fig. 1). Here, aseries of coastal lagoons and tidal channels arebordered by microbial-mat encrusted tidal flats,surrounded in turn by a supratidal zone. Three deepcores were cut in the supratidal and intertidal zonesand around thirty surface samples were collected (Fig.

1). Two cores (SBH-1 and SBH-2) were cut to a depthof 6 m; the third core, SBH-3, was only 1.5 m longdue to difficult ground conditions. Field measurementsof salinity, pH and Eh were also conducted at differenttimes on the coastal waters covering the intertidal andsubtidal zone of the sabkha area.

Standard thin sections were prepared from bothsurface and subsurface samples. All the samplesunderwent X-ray diffraction analysis (scanning at 1°/min in the range of 2θ from 3° to 65°) to determinetheir composition. The percentage of each componentwas calculated and a lithologic column for the twolonger cores was constructed. The collected sampleswere examined by Scanning Electron Microscope todetermine their crystal morphology and nano-scaletextural features. Energy Dispersive X-ray (EDX)analyses were also preformed on the samples underthe SEM.

Fig. 1. Location map showing parts of the coastal sabkha of Abu Dhabi. The red dots indicate the locations of

the cores.


RESULTS

Variations in core compositionA comparison of the mineral compositions in the two6 m cores from the supratidal (SBH-1) and intertidal(SBH-2) zones shows that most of the differencesoccur in the top 50 cm (Fig. 2). Supratidal zone coreSBH-1 is characterized by the presence of highproportions of calcite and quartz, and the occurrenceof anhydrite below about 50 cm. In core SBH-2, calciteand quartz occur in smaller proportions but there is amuch higher dolomite content. In this core, the contentof aragonite is relatively high in the upper few metres;it is lower but nearly constant throughout core SBH-1. Anhydrite is absent from core SBH-2.

Dolomite forms about 20% of the supratidal core(SBH-1) and up to 50% of the intertidal core (SBH-2).A characteristic feature of the intertidal zone was thepresence of microbial mats, and the relatively highdolomite content in the core may be associated withthis microbial material. There was, however, no clearexplanation for the relatively low quartz content in the

intertidal zone core (SBH-2), especially as feldsparhas a similar distribution in both cores. The absenceof anhydrite in the intertidal zone core may beattributed to the high water content of the sediments.

Below about 1 m depth, the two cores are ofsimilar composition, probably reflecting similarenvironmental conditions, with slightly higher quartzcontent in the supratidal core SBH-1.

In the shallow core (SBH-3) which was cut atthe tip of a tidal channel, calcite, quartz and dolomitewere found in similar proportions to those in thedeeper parts of the other two cores. The onlydifferences were the near-absence of gypsum andthe presence of higher amounts of aragonite. Thismay indicate that the core comprised sediments thatwere deposited under normal subtidal conditions.

Dolomite typesDetailed petrographic and SEM examination of thesurface and subsurface samples indicated thepresence of a wide variety of dolomite textures. Thefollowing types of dolomite were observed:

Fig. 2. Mineralogical compositions of cores SBH-1 (supratidal zone) and SBH-2 (intertidal zone) recovered fromthe Abu Dhabi sabkha.

Calcite Quartz Dolomite Plagioclase Gypsum Aragonite Serpentine Anhydrite

mineralsClay

10 20 30 40 50 60 70 80 90Interv

al0-

0.45

1.0-

1.45

2.0-

2.45

3.0-

4.45

4.0-

4.45

5.0-

5.45

6.0-

6.45

10 20 30 40 50 60 70 80 90Interv

al

Lithology Mineral % noitisopmoC

6.0-

6.45

5.0-

5.45

4.0-

4.45

3.0-

3.45

2.0-

2.45

1.0-

1.45

0.0-

0.45

SBH-1 SBH-2Mineral % noitisopmoCLithology


Pustular dolomite (Fig. 3A)This dolomite comprises rounded, fluffy crystalswhose surfaces have small nibbles (pits) andprojections. Some of the crystals have retained arhombohedral shape but others have no definitegeometric boundaries and are vascular or rounded.

The texture is probably a result of microbialdissolution and precipitation which modified theappearance of dolomite crystals. Edwards et al.(2006) suggested that extremophiles may convert ordestroy calcium and magnesium carbonates duringtheir metabolic activity.

Cluster dolomite (Fig. 3B)This type of dolomite is formed of clusters of dolomiterhombs of different sizes mixed with other carbonategrains. The clusters are separated from each otherby voids which are lined in some cases by dolomiterhombs. The aggregate texture of this dolomite typemay be a result of binding by endolithic organisms.Dissolution vugs may also be enhanced by microbialactivity.

Dolomite associated with clay minerals (Fig. 3C)This type of dolomite is usually composed of limpid,perfect rhombs of dolomite which appear to beentangled within a network of clay minerals. In somecases, dolomite rhombs appear to grow out from theclay mesh. Clay minerals may form a substrate formicrobial activity, or may provide a source of Mg fordolomite formation. Some of the clays are Mg-richand may act as nucleation sites. Mg ions may be

adsorbed to clay mineral surfaces and are henceincluded in the microbial film that surrounds the grains.Some of these filaments, however, may be partiallymineralized exopolymeric substances from themicrobial mats.

Dolomite associated with evaporites (Fig. 3D)Prominent seams of anhydrite crystals appear in someof the samples. Dolomite rhombs were observed tobe associated with these anhydrite seams. In a previousstudy of these sediments (Edwards et al., 2006),Raman spectroscopy of a large crystal of gypsumembedded in a halite matrix indicated the existence ofbeta-carotene, which may suggest biologicalcolonisation of part of the crystal. There was also awell-defined band that was assigned tohydromagnesite (hydrated magnesium carbonate)which may represent a transitional phase ofdolomitisation. Therefore, evaporites may have someinfluence on microbial colonisation and this couldenhance the dolomitisation process.

Dolomite associated with microbial mats (Fig. 3E)Dolomite was observed to grow from aragoniteneedles forming the microbial mats which occurwidely in the upper intertidal zone. The surfaces ofthese dolomites are covered with small pits and havea slimy appearance. In some cases, dolomite rhombsare mixed with aragonite needles derived from thedissociation of the algal mats. Microbial mats representa very favourable location for dolomite formation dueto microbial activity. Beneficial factors include the

Fig. 3. Dolomite types from the Abu Dhabi sabkha as they appear under the SEM:(A) pustular dolomite; (B) cluster dolomite; (C) dolomite associated with clay minerals or mineralized

exopolymeric substances ; (D) dolomite associated with evaporites; (E) dolomite associated with microbialmat; (F) dolomite associated with foraminiferal test.


retention of water within the extracellular polymericsubstances constituting the mats (even when the otherparts of the sabkha are dry), and the presence of Mgions.

Dolomite associated with organisms (Fig. 3F)A number of different types of dolomite occurredassociated with benthic foraminifera and gastropodsin the studied samples. The simplest relationship wasthe precipitation of dolomite rhombs on the unalteredsurface of an organism. In other cases, dolomiterhombs formed in natural perforations or voids on anorganism’s surface. In what is probably a later stageof dolomite development, dolomitization may dissolveparts of the organism. Some of the dolomite growthmay be associated with perforation of the organismsby microbial activity.

THE “MICRO-NICHE” CONCEPT

Detailed descriptions of the studied sediments usingSEM showed that dolomitization within the carbonatesediments appeared to begin in “micro-niches”. Amicro-niche may be considered as an isolated porewhich is capable of holding a volume of connate waterwhose retention causes the pore to become anoxic.Such conditions favour the metabolic activities ofmicrobes known to mediate dolomite formation (e.g.sulphate-reducing bacteria). Micro-niches can be foundwithin many types of carbonate sediments and mayprovide a locus of dolomite formation.

In the sediments investigated, micro-nichesincluded the pores of foraminifera and micro-depressions between clay mineral plates (Figs. 4A &B). Micro-depressions within cellular organisms wouldalso retain small volumes of water in a desiccatingenvironment. This water may become anoxic due to

the decay of the organic parts of the organism whichwould consume ambient oxygen. Evaporation wouldcause the salinity to increase gradually, and theresulting anoxic micro-environment would befavourable to microbial sulphate reduction.

Where a micro-niche is surrounded by a lesspermeable rock, dolomitization would be restrictedto the nucleation sites themselves, resulting in whathas been referred to as selective dolomitization in theliterature. In more porous sediments, the dolomiteswhich form in individual nucleation stations willspread and combine, destroying the original micro-pores which will no longer therefore bedistinguishable.

In some cases as shown in Fig. 5A, dolomiteappears to form in small “dish”-like depressions filledwith water. The underlying substratum, which is herea foraminiferal test, is impermeable. Dolomitizationcannot proceed outside the “dish” and the dolomiterhombs form a surficial crust. In Fig 5B, a micro-depression has formed within a compact limestone,but again the dolomite is restricted to the micro-depression and does not extend beyond it. If thesurrounding areas are more porous, however, thendolomitization will extend beyond the micro-niche andinto neighbouring areas.

The concept that dolomitization begins at localstations or nucleation sites had been proposed bymany previous researchers. According to Goodell andGarman (1969), the pelletal mud forming near to andto the west of Andros Island contains sufficient Mgions to dolomitize 8-9% of the sediments, and thismay occur by “cannibalization” of the local Mg-calcite. Murray (1960) and Murray and Lucia (1967)described a similar mechanism for dolomite formationin which a rhomb of dolomite begins to grow at theexpense of a carbonate grain. Sibley (1982) suggested

Fig. 4. Dolomite growing in micro-niches: (A) inside the pores of a foraminiferal test; (B) between two clay

mineral plates.


that dolomitization is controlled by many factorsincluding the presence of suitable nucleation sites. Suchsites may be skeletal grains composed of high-Mg calcitewhich trigger the dolomitization process. Selectivedolomitization is the result of the restriction of dolomiteto local patches, which were originally micro-depressions.

Mode of dolomite formationDolomitization is considered as a wet process thatrequires the presence of water (Bathurst, 1975).Previous models describing the origin of dolomite havedealt solely with the physico-chemical properties of thewater within which dolomitization is assumed to occur.Most of the proposed models had focused on propertiesof the water such as pH and Eh and the concentrationsof dissolved Ca and Mg ions, but ignoring possiblemicrobial activity which may occur in association withchanges in these physico-chemical properties.

The exact role of microbes in dolomite formation isnot fully understood, but it is suggested that duringbacterial sulphate reduction, organic matter is oxidizedand the alkalinity increases promoting dolomiteformation. As sulphate is consumed, MgSO

4 becomes

disassociated and Mg++ comes available for furtherdolomite formation (Vasconcelos et al., 1995;Vasconcelos and McKenzie, 1997). Metabolic productssuch as hydrogen sulphide and bicarbonate ions arereleased from bacterial cells. Saturation of bicarbonateand magnesium ions on a micro-scale around bacteriacreates favourable conditions for dolomite precipitation(Vasconcelos and McKenzie, 1997). Warthmann et al.(2005) isolated the sulphate-reducing bacteriumDesulfovibrio brasiliensis strain LVform 1 fromdolomite-bearing sediments from a coastal lagoon inLagoa Vermelha, Brazil. This bacterium mediateddolomite precipitation in culture experiments conducted

by the authors. Strain LVform 1 is sodium-dependantand flourishes at NaCl concentration of up to 15%.

In a microbial-rich aqueous envelopesurrounding a carbonate grain, the pH and the Ehmay define a surface in which chemical reactionsare controlled by microbial activity. Selley (1997)suggested that in stagnant conditions, an oxidizedzone (+Eh) will be superimposed on a reducing zone(-Eh). In the reducing zone, the anaerobic bacteriumDesulfovibrio will remove oxygen from the SO

4

generating H2S; while in the overlying oxidised zone,

the bacterium Thiobacillus will re-oxidize thehydrogen sulphide to sulphate:

(DesulfovibrioSO

4 ————————————— S+ 2O

2

Thiobacillus)

First-order calculations were conducted toexamine the biogeochemical conditions required forbacterial sulphate reduction. The purpose was toevaluate whether bacterial sulphate reduction andsubsequent formation of sedimentary sulphideminerals could generate sufficient alkalinity andchanges in Eh and pH to favour dolomitebiomineralization.

In the samples collected, sulphur occurs at anaverage concentration of less than 3.2*10-6 mol S/L ± 0.5*10-6 in pore waters, whose sulphateconcentration is 0.1*10-6 ± 0.02*10-6M, and thepH was about 7.6. These average values, togetherwith the environmental redox half-reaction givenby Stumm and Morgan (1981), were used to locatethe sedimentary conditions within a redox sequence.The analyses indicate that the sediment environmentis characterized by an Eh of 209-300 mV. At theseredox levels, SO

4 and to a lesser extent CO

2 are the

Fig. 5. Dolomite growing in shallow micro-niches:

(A, left) as superficial dolomite crust on a foraminiferal test;(B, right) dolomite growing in a shallow micro-niche surrounded by compact limestone.


most likely chemical species being used as oxidantsin the degradation of organic matter in a decayingmicrobial mat. H

2S would be used by anaerobic

bacteria in the same way that H2O is used by algae

and green plants, thus producing SO4 instead of O

2.

Under these conditions, organic matterdecomposition can be considered as a consequenceof sulphate reduction in sediments (see simplifiedEquation 1), explaining the decrease in sulphate whichis most likely due to bacterial reduction. However,hydrogen sulphide gas (H

2S) and bicarbonate

(alkalinity) are generated in this process:

Equation 1: SO

42- + 2(CH

2O) = H

2S + 2HCO

32-

The high alkalinity is typical when consideredtogether with significant bacterial sulphate reduction.If water is slightly alkaline, dolomite formation mighttake place via the reaction:

Mg2+ + HCO3- + CaCO

3 CaMg(CO

3)2 + H+,

with the necessary 2-mol of bicarbonate suppliedby bacterial sulphate reduction. Pore water alkalinityvalues support dolomite formation by this secondreaction, which also explains the low values ofbicarbonate.

Exploration ImplicationsMany previous publications have suggested thatmicrobial dolomite accounts for only a smallproportion of dolomite, and that it is restricted toshallow-water systems or protected lagoons andsimilar settings which represent favourable sites formicrobial activity. Our findings, however, do notsupport these restrictions.

The average porosity of recently depositedcarbonate sediments is around 60% (Bathurst, 1975).Pores or micro-niches in carbonate sediments are

interconnected due to the loose texture and are filledwith connate waters. Over time, water in these poreswill become anoxic and microbial activity will generatedolomite, as discussed above. In porous rocks, thesedolomitisation sites would become connected, leadingto the dolomitization of an entire stratum.

Of particular interest in an exploration context areTriassic and Jurassic carbonate reservoirs in theArabian Basin, e.g. the Kurra Chine Formation ofnorthern Iraq and the Arab Formation of the southernparts of the Arabian Basin (Sadooni, 1995; Warren,2006). These reservoirs are characterized by:

1. Thick dolomite successions;2. Thick evaporite sections, such as the anhydrites

in the Upper Triassic Kurra Chine Formation ofnorthern Iraq and the Arab Formation in the southernArabian Basin, and the halite in the Gotnia Formationin southern Iraq;

3. The absence of viable source rocks from theArab section of Saudi Arabia and from the Triassic ofthe northern part of the Arabian Basin, which has ledsome investigators to suggest that much of the ArabianJurassic oil was sourced from microbial mats (e.g.Friedman, 1982).

Part of the dolomite in these carbonate reservoirsmay be of microbial origin. If sulphate-reducingbacteria were involved in the mineralization process,this requires the existence of large-scale anoxicconditions. Leeder and Zeidan (1977) suggested thatthe Late Jurassic evaporitic sulphates and minorchlorides of Saudi Arabia, Iran, Iraq, Kuwait, theEmirates and Yemen were deposited in giant sabkhas(>2106 km2) on the southern Neo-Tethyan margin. Asabkha setting can be proposed for the Triassic basinin Iraq, Syria and the Levant (Sadooni, 1995; Sadooniand Alsharhan, 2004). These sabkhas may have beenanoxic over large areas, with microbial mats in whichmicrobial activity led to the formation of dolomite.Dolomitization may have destroyed the mats, or theorganic material was consumed to generate oil. This

Fig. 6. Concentration of dolomite rhombs in

the micro-niches of a foraminiferal test. As therhombs grow, they may unite and dolomitizethe whole rock.


may explain the presence of scattered dolomiterhombs within background evaporite (Fig. 7). It isunlikely that these rhombs were floating before theywere cemented, and so they were probably within amicrobial mat which was later destroyed. In Fig. 8,clusters of Triassic dolomite are distributed aroundvents while the original background material has beendissolved and replaced by granular evaporite (probablygypsum). Most of these are probably isolated micro-niches that survived later diagenetic processes suchas dissolution that removed the non-dolomitized partsof the groundmass.

CONCLUSIONS

Detailed petrographic analyses were conducted onRecent dolomites from surface and subsurfacesamples collected from the coastal sabkha of AbuDhabi. The study involved the use of SEM coupledwith Energy Dispersive X-ray (EDX) analyses toidentify the nano-scale features of these sediments.Dolomite is found to form around 20% of the totalsediments in the supratidal zone and up to 50% in theintertidal zone sediments. Six dolomite types wereidentified: pustular dolomite; cluster dolomite; dolomiteassociated with clay minerals or evaporites; anddolomites associated with microbial mats or with otherorganisms. According to the model proposed, mostdolomite would start to form in micro-niches withinthe carbonate sediments. These micro-niches or porespaces are filled with connate waters that becomegradually anoxic, favouring the activity of microbesknown to mediate dolomite formation. Combinationof neighbouring micro-niches in porous carbonatesediments would lead to dolomitization of an entiresection. This novel concept may explain the origin ofdolomitized sections in carbonate rocks.

ACKNOWLEDGEMENTS

This project is sponsored by the UAEU ResearchCouncil grant 201/203. The authors are grateful toTomaso Bontognali (ETH Zurich) for comprehensivereview of the original manuscript, and to Wajeh Kittanaand Omer Al-Basheer (Geology Department, UAEUniversity) for their help with field work.

REFERENCES

BATHURST, R.G.C., 1975. Carbonate sediments and theirdiagenesis. Elsevier Science, 660p.

BONTOGNALI, T.R.R., VASCONCELOS, C., WARTHMANN,R., BERNASCONI, S., DUPRAZ, C., STROHMENGER, C.J.and McKENZIE, J.A., 2010. Dolomite Formation withinmicrobial mats in the Coastal Sabkha of Abu Dhabi(United Arab Emirates). Sedimentology, 57, 824–844

BUTLER, C.P., 1969. Modern evaporite deposition andgeochemistry of coexisting brines, the Sabkha, TrucialCoast, Arabian Gulf. Journal of Sedimentary Petrology, 39,70-89.

CHAFETZ, H.S. and RUSH, P.F., 1995. Two-Phase Diagenesisof Quaternary Carbonates, Arabian Gulf: Insights from13C and 18O data. Journal of Sedimentary Research, 65a,294-305.

CURTIS, R., EVANS, G., KINSMAN, D. J.J. and SHEARMAN,D.J., 1963. Association of dolomite and anhydrite in theRecent sediments of the Persian Gulf. Nature, 197, 679-680.

FRIEDMAN, G.M., 1982. Evaporites as source rocks forpetroleum. In: C.R. HANFORD, R. G. L. LOUCKS andDAVIES, G.E. (Eds.), Depositional and diagenetic spectraof evaporites. SEPM Core Workshop, Calgary, 3, 385-396.

GOODELL, H.G. and GARMAN, R.K., 1969. Carbonategeochemistry of Superior deep test well Andros Island,Bahamas. AAPG Bull., 53, 513-536.

HSU, K.J. and SIEGENTHALER, C., 1969. Preliminaryexperiments on hydrodynamic movement induced byevaporation and their bearing on the dolomite problem.Sedimentology, 12, 11-25.

ILLING, L.V., WELLS, A.J., and TAYLOR, J.C.M., 1965.Penecontemporaneous dolomite in the Persian Gulf.

Fig. 7. Dolomite rhombs with “cloudy” centres,

which probably result from the inclusion of microbialmat material, cemented by evaporites. Triassic

strata from the northern part of the Arabian Plate –Kurra Chine Formation (Upper Triassic), well Butma-15, 3510 m.

Fig. 8. Cluster of dolomite around vents or micro-niches. The framboidal texture may suggest

microbial intervention. Triassic strata from of thenorthern part of the Arabian Plate – Kurra Chine

Formation (Upper Triassic), well Butma-15, 3653 m.


Society of Economic Paleontologists and Mineralogists,Special Publication 13, 89-111.

ILLING, L.V. and TAYLOR, C.M., 1993. Penecontemporaneousdolomitization in Sabkha Faishakh, Qatar; evidence fromchanges in the chemistry of the interstitial brines. Journalof Sedimentary Research, 63, 1042-1048.

MURRAY, R.C., 1960. Origin of porosity in carbonate rocks.Journal of Sedimentary Petrology, 30, 59–64.

MURRAY, R. C. and LUCIA, F.J., 1967. Cause and control ofdolomite distribution by rock selectivity. Geol. Soc. Am.Bull., 78, 2l-36.

PATTERSON, R.J. and KINSMAN, D.J.J., 1982. Formation ofdiagenetic dolomite in coastal sabkha along Arabian(Persian) Gulf. AAPG Bull., 66, 28-43.

RIVADENEYRA, M.A., DELGADO, R., DELGADO, G., DELMORAL, A., FERRER, M.R. and RAMONS-CORMENZANA,A., 1993. Precipitation of carbonates by Bacillus sp.isolated from saline soils. Geomicrobiology Journal, 11, 175-184.

ROBERTS, J.A., BENNETT, P.C., GONZ, LEZ, L.A.,MACPHERSON, G.L. and MILLIKEN, K.L, 2004. Microbialprecipitation of dolomite in methanogenic groundwater.Geology, 32, 277-280.

SADOONI, F.N., 1995. Petroleum prospects of Upper Triassiccarbonates in Northern Iraq. Journal of Petroleum Geology,18, 171-190.

SADOONI, F.N. and ALSHARHAN, A., 2004. Stratigraphy,lithofacies distribution and petroleum potential of theTriassic sediments strata in the northern parts of theArabian Plate. AAPG Bull., 88, 515-538.

SANCHEZ-ROMAN, M., McKENZIE, J.A., DE LUCAREBELLO WAGENER, A., RIVADENEYRA, M. A. andVASCONCELOS, C., 2009. Presence of sulfate does notinhibit low-temperature dolomite precipitation. Earth andPlanetary Science Letters, 285, 131-139.

SANCHEZ-ROMAN, M., VASCONCELOS, C., SCHMID, T.,DITTRICH, M., McKENZIE, J.A., ZENOBI, R. andRIVADENEYRA, M.A., 2008. Aerobic microbial dolomiteat the nanometer scale: Implications for the geologicrecord. Geology, 36, 879-882.

SANCHEZ-ROMAN,M., VASCONCELOS, C., WARTHMANN,R.J., RIVADENEYRA, M.A. and McKENZIE, J.A., 2009.Microbial Dolomite Precipitation under AerobicConditions: Results from Brejo do Espinho Lagoon (Brazil)and Culture Experiments. In: Perspectives in SedimentaryGeology: A Tribute to the Career of Robert NathanGinsburg. In: P. K. Swart, G.P. Eberli and J.A. McKenzie(Eds), IAS Spec. Publ., 41, 167-178.

SELLEY, R.C., 1997. Elements of petroleum geology. 2ndedition. Academic Press, UK, 470p.

SHEARMAN, D.J., 1963. Recent anhydrite, gypsum, dolomite,and halite from the coastal flats of the Persian Gulf.Proceedings Geological Society of London, 1607, 63-65.

SHEARMAN, D.J., 1966. Origin of marine evaporites bydiagenesis. Transactions of Institute of Mineralogy Metall. SectionB, 75, 208-215.

SIBLEY, D.F., 1982. The origin of common dolomite fabrics.Journal of Sedimentary Petrology, 52, 1087-1100.

STUMM, W. and MORGAN, J., 1981. Aquatic chemistry. JohnWiley & Sons, New York, 1040p.

TEAL, C.S., MAZZULLO, S.J. and BISCHOFF, D., 2000.Dolomitization of Holocene shallow-marine depositsmediated by sulfate reduction and methanogenesis innormal-salinity seawater, Northern Belize. Journal ofSedimentary Research, 70, 649-663.

VASCONCELOS, C. and McKENZIE, J.A, 1997. Microbialmediation of modern dolomite precipitation anddiagenesis under anoxic conditions (Lagoa Vermelha, Riode Janeiro, Brazil). Journal of Sedimentary Research, 67,378-390.

van LITH, Y., WARTHMANN, R., VASCONCELOS, C. andMcKENZIE, J.A., 2003. Sulphate-reducing bacteria inducelow-temperature Ca-dolomite and high Mg-calciteformation. Geobiology, 1, 71-79.

VASCONCELOS, C., McKENZIE, J.A, BERNASCONI, S.,GRUJIC, D. and TIEN, A.J., 1995. Microbial mediation as apossible mechanism for natural dolomite formation atlow temperatures. Nature, 377, 220-222.

WARREN, J. K., 2006. Evaporites: sediments, resources andhydrocarbons. Springer, pp. 721-733.

WARTHMANN, R., van LITH, Y., VASCONCELOS, C.,McKENZIE, J.A. and KARPOFF, A.M., 2000. Bacteriallyinduced dolomite precipitation in anoxic cultureexperiments. Geology, 28, 1091-1094.

WARTHMANN, R., VASCONCELOS, C., SASS, H. andMcKENZIE, J A., 2005. Desulfovibrio brasiliensis sp. Nov., amoderate halophilic sulfate-reducing bacterium fromLagoa Vermelha (Brazil) mediating dolomite formation.Extremophiles, 9, 255-261.

WELLS, A.J., 1962. Recent dolomite in the Persian Gulf. Nature,194, 274-275.

WRIGHT, D.T. and WACEY, D., 2005. Precipitation of dolomiteusing sulphate-reducing bacteria from the CoorongRegion, South Australia: significance and implications.Sedimentology, 52, 987-1008.

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