Abu Dhabi Sabkha Research Articles

Microbial and aeolian influences on coniatolite development near Jebel Dhanna, Abu Dhabi

Coniatoids and the more extensive coniatolites were originally described, respectively, as rough surfaced, aragonite coated grains and rough surfaced, aragonite encrusted beachrocks near Jebel Dhanna, Abu Dhabi. At that time, a microbial involvement in their origins was rejected in favour of supratidal physico-chemical precipitation and this study investigates the validity of those conclusions. Vadose aragonite cementation has certainly occurred but new evidence of microbial involvement is presented and an intertidal influence is suggested with due consideration given to Holocene eustatic sea levels. The surface roughness of particularly the coniatolites is largely due to wick-like movement of surface marine waters under the influence of prevailing northwesterly winds, including the Shamal. Leewards growth extensions of the microbial communities in the direction of aeolian-driven, marine surface water drainage were rapidly cemented by micritic aragonite which also engulfed carbonate grains. A similar aeolian wick-like precipitation process is illustrated in a halite deposit of an Abu Dhabi sabkha salina.

Late Holocene to Recent aragonite‐cemented transgressive lag deposits in the Abu Dhabi lagoon and intertidal sabkha

AbstractModern cemented intervals (beachrock, firmgrounds to hardgrounds and concretionary layers) form in the lagoon and intertidal sabkha of Abu Dhabi. Seafloor lithification actively occurs in open, current‐swept channels in low‐lying areas between ooid shoals, in the intertidal zone of the middle lagoon, some centimetres beneath the inner lagoonal seafloor (i.e. within the sediment column) and at the sediment surface the intertidal sabkha. The concept of ‘concretionary sub‐hardgrounds’, i.e. laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary, is introduced and discussed. Based on calibrated radiocarbon ages, seafloor lithification commenced during the Middle to Late Holocene (ca 9000 cal yr bp), and proceeds to the present‐day. Lithification occurs in the context of the actualistic relative sea‐level rise shifting the coastline landward across the extremely low‐angle carbonate ramp. The cemented intervals are interpreted as parasequence boundaries in the sense of ‘marine flooding surfaces’, but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited. Aragonite, (micritic) calcite and, less commonly, gypsum cements lithify the firmground/hardground intervals. Cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography, cathodoluminescence microscopy and Raman spectroscopy. The morphology of aragonitic cements changes from needle‐shaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar, lath and platy crystals in the inner lagoon. Precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology. Data shown here shed light on ancient, formerly aragonite‐cemented seafloors, now altered to diagenetic calcites, but also document the complexity of highly dynamic near coastal depositional environments.

Microbial dolomite in the sabkha environment of the middle Cambrian in the Tarim Basin, NW China

ABSTRACTIn the Tarim Basin, dolomite, which formed during the middle Cambrian associated with evaporites, has been attributed to the sabkha-style dolomite formed during the syndepositional period. The sedimentary microfacies suggests dolomite formation in the middle Cambrian is an ancient analogue of the sabkha of Abu Dhabi. Poorly crystallised dolomite spheroids or ovoids within or on the surface of dolomite crystals are a common phenomenon that can be widely observed in different stromatolites in the upper part of the intertidal zone and strongly resemble the morphology in modern sabkha dolomite-producing microbial mats and in microbial culture experiments. These lines of evidence suggest organic substrates for dolomite nucleation. Dolomite formation in the middle Cambrian in the Tarim Basin has been considered a classic analogue for carbonate and evaporate assemblages. The extent of microbial dolomite in ancient sabkha environments is proposed as an alternative model for dolomite formation, in which the mineral properties of organic substrates play a crucial role.

Holocene marine hardground formation in the Arabian Gulf: Shoreline stabilisation, sea level and early diagenesis in the coastal sabkha of Abu Dhabi

This study provides the first comparison between a seaward and a landward section of the same diachronous hardground surface observed in the coastal sabkha of Abu Dhabi. This hardground is described here in terms of its mode of formation, its diagenetic environment and its impact on shoreline stabilisation during transgression. The hardground is exposed in the intertidal zone and buried by a late Holocene prograding succession of carbonates, evaporites and microbial sediments in the supratidal zone. The hardground itself is composed of bioclastic grains, primarily of aragonitic composition, that originate from intertidal depositional environments. Aeolian silt to sand-sized quartz grains are also observed. Lithification occurred through the precipitation of pore-filling aragonite, high-Mg calcite and dolomite cements from sea and interstitial water that was marked by high salinities and temperatures, as confirmed by stable isotope analyses. High-Mg calcite and non-stoichiometric dolomite are also observed as secondary recrystallisation products. The formation of these two mineral phases as recrystallisation products was possibly microbially-mediated. Lithification progressed in two phases, the older phase of which is marked by higher amounts of non-stoichiometric dolomite and high-magnesium calcite as compared to the younger phase. Transgressive reworking of precursor siliciclastic sands was inhibited by the development of transgressive pore-filling gypsum cements in the supratidal zone.

The preservation potential of environmentally diagnostic sedimentary structures from a coastal sabkha

The Abu Dhabi sabkha constitutes the landward portion of the shallow-sloping carbonate ramp situated on the northwestern coast of the United Arab Emirates (UAE). The sabkha in this region is composed of a complex assortment of carbonate, organic, and evaporitic facies that constitute a regressive sabkha sequence. The surface features of the sabkha can be grouped into distinct, laterally continuous facies associations that are directly correlatable with elevation above present-day sea level. Previous investigations have suggested the employment of the Abu Dhabi sabkha as a modern depositional analogue to ancient carbonate-evaporite formations; however, a quantitative analysis of the relationship between sabkha surface and subsurface characteristics has not previously been undertaken. This study describes in detail the sedimentary structures and textures seen at the sabkha surface and compares them to the Holocene-age facies retained in the shallow subsurface. Of the 36 sedimentary structures observed at the sabkha surface, only seven (19%) are still identifiable in the early burial realm. The majority of the surface structures were lost during early burial as a result of microbial mat decomposition, destructive evaporite precipitation, and compaction. The loss of diagnostic sedimentary features during early burial as a result of fabric-destructive processes has considerable implications for the recognition and interpretation of ancient sabkha sequences. Documenting these processes from a range of modern sedimentary environments will aid in the identification of those diagnostic features that may be reliably employed during paleoenvironmental reconstructions.

Dolomite‐mediating bacterium isolated from the sabkha of Abu Dhabi (UAE)

Terra Nova, 24, 248–254, 2012AbstractThe study of microbe‐mineral interactions and the identification of biominerals in sedimentary rocks provide crucial information on the coevolution of life and Earth surface environments. Desulfovibrio brasiliensis, a sulfate‐reducing bacterium isolated from Lagoa Vermelha (Brazil), is known for its ability to precipitate dolomite, a mineral that is common in the geological record, but difficult to obtain in laboratory experiments simulating Earth's surface conditions. Here, we report on a new bacterial strain capable of precipitating Mg‐calcite and Ca‐dolomite, isolated from a microbial mat in the sabkha of Abu Dhabi (UAE). The 16S rRNA gene sequence of the isolate revealed a 99.6% identity (i.e. same species) with Desulfovibrio brasiliensis. The presence of the same microbial species at two geographically distant dolomite‐forming locations is difficult to explain in terms of pure coincidence. Rather, it suggests that the ability of precipitating dolomite may be a unique characteristic associated with specific strains.

Development and evolution of subaerial halite crust morphologies in a coastal sabkha setting

The halite crusts of the coastal Abu Dhabi Sabkha were monitored and described in order to assess their temporal evolution and their potential for preservation and recognition in ancient sabkha sequences. The position of polygon ridges is controlled by subtle differences in the sabkha surface topography – typically in the form of low lying mud ridges and exposed halite. Where this topography remains undisturbed, then successive generations of halite polygons were observed to form with borders in the same location. As each generation of halite crust effectively ‘rebuilds’ the controlling mud ridge, then this process can be seen as self perpetuating until disturbed by a major erosional event. In marginal marine settings, such as the Abu Dhabi Sabkha, the preservation potential of the halite crust is negligible. While associated sedimentary structures have been employed to infer the presence of an earlier halite crust in other geographic settings, these all require a net influx of sediment to the depositional system. The absence of such a sediment supply in the coastal-fringing Abu Dhabi Sabkha is inferred to be a major constraint to the preservation and recognition of halite crusts and their associated features into the sedimentary record.

MICROBIAL DOLOMITES FROM CARBONATE‐EVAPORITE SEDIMENTS OF THE COASTAL SABKHA OF ABU DHABI AND THEIR EXPLORATION IMPLICATIONS

Detailed petrographic studies of Recent dolomites from the coastal sabkha of Abu Dhabi using SEM and Energy Dispersive X‐ray (EDX) analyses indicate that most of the dolomite started to form in “micro‐niches” (i.e. small isolated pore spaces) within the carbonate sediments. These micro‐niches tend to retain connate waters, becoming anoxic and supporting specific microbial activity which ultimately results in the formation of dolomite. This novel concept may help to explain the origin of many dolomite types, since recently deposited carbonate sediments have high porosities (up to 60%) and the pore spaces may serve as nucleation sites or “stations” for dolomite formation. The combination of neighbouring micro‐niches in a porous carbonate may result 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 with evaporites; and dolomites associated with microbial mats and foraminiferal tests.

Dolomite formation within microbial mats in the coastal sabkha of Abu Dhabi (United Arab Emirates)

Microbial mediation is the only demonstrated mechanism to precipitate dolomite under Earth surface conditions. A link between microbial activity and dolomite formation in the sabkha of Abu Dhabi has, until now, not been evaluated, even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences. Such an evaluation is the purpose of this study, which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of Abu Dhabi, along a transect from the intertidal to the supratidal zone. This investigation revealed a close association between microbial mats and dolomite, suggesting that microbes are involved in the mineralization process. Observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats. In current models, microbial dolomite precipitation is linked to an active microbial activity that sustains high pH and alkalinity and decreased sulphate concentrations in pore waters. Such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone. By contrast, these models cannot be applied to the supratidal zone, where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity. As no abiotic mechanism is known to form dolomite at Earth surface conditions, two different hypotheses can reconcile this result. In a first scenario, all of the dolomite present in the supratidal zone formed in the past, when the mats were active at the surface. In a second scenario, dolomite formation continues within the buried and inactive mats. In order to explain dolomite formation in the absence of active microbial metabolisms, a revised microbial model is proposed in which the mineral-template properties of exopolymeric substances play a crucial role.

Quantification of Carbonate Ramp Sedimentation and Progradation Rates for the Late Holocene Abu Dhabi Shoreline--Discussion

The efforts of Lokier and Steuber (2008) to provide some further data on the Abu Dhabi sabkha is to be welcomed. This wonderful modern environment of carbonate–evaporite deposition produced, when first described, such a great effect on the interpretation of evaporitic sequences in the stratigraphic record, in large part due to the persuasive writings and lectures of D.J. Shearman (1966). However, there are several comments that need to be made on this manuscript. It is surprising that the authors have neither shown nor considered how their sampling line relates to the deflated beach ridges, which, together with the band of early Holocene marine sediment that occurs behind them, mark the effective limit of the Holocene marine transgression and are well developed in the area (Kirkham 1997). Furthermore, it is curious that they show their sampling sites to be located in the intertidal zone although the sediments contain anhydrite. Earlier workers showed that, although anhydrite may occur in small quantities in the sediments of the upper part of the intertidal zone, it only becomes an important constituent at approximately high-water spring tide in the zone best known as the supratidal zone, i.e., a zone which is affected by tides but is only rarely covered when the tidal heights are increased due to abnormal meteorological conditions such strong Shamal winds in this area (Evans et al. 1969; Kinsman and Park 1976; Patterson and Kinsman 1977, 1981). Generally, Lokier's and Steuber's interesting description of the sediments in their pits seems to confirm those already published by previous workers. However, their key is rather misleading inasmuch as it suggests …

Lithology, mineral assemblages and microbial fingerprints of the evaporite-carbonate sediments of the coastal sabkha of Abu Dhabi and their extraterrestrial implications

AbstractDeep-core and surface samples collected from the coastal sabkha of Abu Dhabi were subjected to a multi-proxy study, including petrographic, geochemical and spectroscopic analyses. The sediments studied are composed of biochemical carbonate-evaporite mineral suites, such as calcite, dolomite, aragonite and gypsum, as well as clastic minerals, such as quartz, feldspar and serpentine. These sediments were also strongly influenced by microbial activities as reflected by the presence of cyanobacterial mats, boring, gas bubble structures, pustular and other macro and micro textures. A combination of marine, fluvial, aeolian, and groundwater processes shaped the geomorphology of the area and led to the formation of such mineral suites, as well as their microbial contents. Data collected from Mars indicate that its surface regolith contains sandstone composed of siliciclastic basaltic debris, as well as carbonate (e.g. magnesite) and evaporite (e.g. jarosite and relics of gypsum) mineral assemblages. Additional data suggest the presence of geomorphic features, characteristic of an arid climate, such as sand dunes and desert varnish. The hydrological model for the Late Noachian-Hesperian period of the plant proposed the existence of a surficial layer containing endolithic and stromatolitic cyanobacterial lamina. The combination of the coastal sabkha of Abu Dhabi with its carbonate-evaporite mineral suites, the neighbouring sand dune fields of the Empty Quarter Desert and the basaltic sediments resulted from weathering the ophiolitic Northern Oman Mountains to form a candidate terrestrial geologic province that may explain the mineral association of Mars and its potential biosignatures. The lithological features and the mineral association of the sabkha can be recognized by the present day detection equipment used on Mars, and even if their biosignatures are degraded, their existence may be inferred from these features.

Quantification of Carbonate-Ramp Sedimentation and Progradation Rates for the Late Holocene Abu Dhabi Shoreline

Abstract We utilize new radiocarbon dates and detailed sedimentological logging of the Abu Dhabi sabkha to produce the first well-constrained, internally consistent rates for late Holocene carbonate-ramp sediment accumulation and progradation for the southern shore of the Persian Gulf. These data are applied to test the hypothesis that, in low-angle carbonate ramp systems, ramp geometry is the primary control on sedimentation and progradation rates and, therefore, estimates acquired from other depositional environments cannot be applied to low-angle carbonate ramp systems. Average calculated progradation rates of 0.75 m/yr are higher than progradation rates proposed for other late Holocene and Recent carbonate settings; a relationship that is explained by the lack of accommodation space in the low-angle laterally extensive ramp system. In the sedimentary record, such relatively high progradation rates would produce laterally extensive lithologically uniform units with little or no evidence of progradational features and would appear to be largely contemporaneous across their extent. The total average sediment accumulation rate of 0.029 cm/yr is lower than those proposed from most other late Holocene to Recent shallow-water carbonate depositional environments; again, a relationship interpreted to result from the limited accommodation space of low-angle carbonate ramp systems. Calculated sediment accumulation rates show little variation over the past ~ 1,400 years, yet the calculated progradation rates exhibit a sharp decline from a maximum of 1.03 m/yr before 677–946 cal BP to 0.70 m/yr afterwards. This is interpreted to reflect a rise in relative sea level or distal steepening of the ramp system. Applying the progradation rates proposed in this paper, the lagoon system in the study area would be completely infilled in less than 4,000 years. Calculated sediment accumulation rates for syndepositional and early postdepositional interstitial displacive evaporite formation are higher than those for carbonate and microbial mat intervals. Continued precipitation of evaporite minerals during burial would result in a further increase in their relative contribution to the sediment pile. Conversely, compaction, particularly of microbial-mat intervals, will result in a decrease in apparent sediment accumulation rates for non-evaporite intervals. These processes need to be considered when modeling the basin evolution of mixed evaporite–carbonate sequences in the stratigraphic record. During calculation of sedimentation rates it is important to consider both a correction for compaction and a correction for evaporite formation.

Source of solutes to the coastal sabkha of Abu Dhabi

An ascending-brine model is proposed to address the observed isotope geochemistry, solute composition, and solute and water fluxes in the coastal sabkha of the Emirate of Abu Dhabi. Mass-balance measurements document that >95% of the solutes are derived from ascending continental brines; minor amounts are derived from rainfall and from groundwater entering from up- gradient areas. Nearly 100% of the annual water loss is from evaporation and not lateral discharge. Direct rainfall on the sabkha and subsequent recharge to the underlying aquifer account for ∼90% of the annual water input to the system; the remaining 10% comes from both lateral and ascending groundwater flow. Thus, the water and solutes in this system are from different sources. Solute concentrations of conservative (i.e., nonreactive) elements in the coastal, sabkha-covered aquifer are consistent with the fluid pore volumes of ascending brine calculated from hydrologic properties. Calcium to sulfate ratios and sulfur isotopes are consistent with this source of solute from the underlying Tertiary formations. Recharging rainwater dissolves halite and other soluble minerals on the surface, causing the solution to become more dense and sink to the bottom of the aquifer where it vertically mixes with less dense ascending brines. Solutes are returned to the surface by capillary forces and recycled or lost from the system by eolian or fluvial processes. Thus, the system becomes vertically mixed, consistent with the presence of tritium throughout the aquifer; but there is essentially no horizontal mixing of seawater with groundwater. The observed seawater solutes in the supratidal zone come from interstitial seawater trapped by the rapid progradation of the sediments into the Arabian Gulf and are not refluxed or laterally mixed. The ascending- brine model contrasts significantly with both the seawater-flooding and evaporative- pumping models previously proposed as a source of solutes to the coastal sabkha of the Emirate of Abu Dhabi. Use of these earlier models leads to incorrect conclusions and raises serious questions about their applicability in the evaluation of sabkhat in the geologic record.

Carbonate-Evaporite Cycles in the Miocene to Holocene of Abu Dhabi, United Arab Emirates: ABSTRACT

The coastal sabkhas of the United Arab Emirates provide a Holocene analog for the study of evaporite formation. Carbonate-evaporite sequences are common throughout geologic history and, in the Arabian Gulf region in particular, create the reservoir-seal relationship of some of the most prolific hydrocarbon reservoirs in the world. Detailed core description, thin section study and geochemical analysis of Miocene to Holocene cores from the sabkha of Abu Dhabi have been performed in order to characterize modern sabkha diagenetic patterns. Two primary lithologies, dolomite and anhydrite were identified and subdivided into lithofacies. Based on these lithofacies, deposition is interpreted to have occurred in shallow open marine, lagoonal, tidal channel, tidal/algal flats and supratidal sabkha settings. The primary diagenetic effects are dolomitization, anhydritc formation and leaching. As anhydrite precipitated (in the form of gypsum), the Mg:Ca ratio increased to the point where rapid dolomitization of original limestone occurred. Leaching was pervasive, as subaerial exposure led to the formation of moldic porosity in dolomitized packstones and grainstones. Dolomitic cements in these pores, and leached zones in some of these crystals suggests that leaching continued after dolomitization. By comparing the Holocene sabkha sediments to ancient ones, insight may be gained into the extent ofmore » dolomilization both with depth and distance for in the high water mark, the zonation of the stratigraphy from upper supratidal to shallow shelf, the preservation potential of algal mats after burial, the compaction effects after shallow burial, and other diagenetic alterations.« less

Diagenetically altered sabkha‐type Pleistocene dolomite from the Arabian Gulf

ABSTRACTDiagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha‐related dolomite deposits, these deposits comprise a thick (>56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13 and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precursor.

New Interpretation of Vertically Aligned Gypsum Fabrics: Implications for Gypsum Depositional Environments and Diagenesis: ABSTRACT

Gypsum and anhydrite fabrics in trenches and deep 500-m cores from Bristol Dry Lake, California, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentologic evidence indicates that the gypsum formed in Bristol Dry Lake precipitated as a diagenetic displacive mineral within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater seepage gypsum). Evidence for displacive growth of gypsum comes from (1) the geometry of the deposit, (2) stable isotopic data and the water chemistry of the brine, and (3) inclusions of matrix that follow twin planes and completely surround crystals as they grow. Because the fabrics and textures of this diagenetic gypsum formed by groundwater in playa settings are similar to those of primary gypsum formed in a brine pond, it is necessary to refine the criteria for the distinction between subaerial, groundwater, and subaqueous gypsum. When compared to the features observed in a Holocene subaqueous gypsum deposit (Marion Lake, Australia) and a Holocene subaerial deposit (Abu Dhabi sabkha), the following criteria can be used to distinguish between the three different types of gypsum deposits: (1) amount of matrix, (2) geometry of themore » deposit, (3) gypsum crystal size and orientation, (4) distribution of matrix and fluid inclusions within individual crystals, and (5) fabric of the matrix around the gypsum. Distinguishing between subaqueous and groundwater gypsum may be difficult. However, sufficient mesoscale differences exist such that, in most cases, the different gypsum types can be resolved from core slabs and thin sections. These criteria may be applied in ancient examples even after gypsum has been converted to anhydrite.« less

Abu Dhabi sabkha, Persian Gulf, revisited: Application of strontium isotopes to test an early dolomitization model

A study of brines and diagenetically altered minerals from coastal sabkha of Abu Dhabi, Persian Gulf, was undertaken to test the flood recharge-evaporative pumping model for early dolomitization using strontium isotopes as a tracer. The {sup 87}Sr/{sup 86}Sr ratio of the sabkha brines defines a pattern of both horizontal and vertical mixing of coastal seawater and continental ground water consistent with a previously proposed model. The {sup 87}Sr/{sup 86}Sr ratio of the diagenetic dolomites indicates early dolomitization by marine recharge waters. The {sup 87}Sr/{sup 86}Sr ratio of gypsum from the dolomitization area (1.4 to 4.4 km from high-tide mark) and seaward reflects precipitation from marine brines, but more landward (6 to 9 km), gypsum and anhydrite record a continental signal in the minerals suggests a paragenetic evaporative sequence (dolomite {yields} gypsum {yields} anhydrite). The dolomitizing fluid is evaporated seawater that recharges the sabkha aquifer during supratidal flooding and is afterward pumped upward through the saturated zone by evaporative processes.

Sulfur and Oxygen Isotope Geochemistry of Abu Dhabi Sabkha: ABSTRACT

Sulfur and oxygen isotope data vary in a laterally systematic manner across the sabkha at Abu Dhabi. At least three different mechanisms are required to explain the variation of ^dgr34S and ^dgr18O of sulfate in evaporite minerals and interstitial brine. Gypsum is precipitating in the seaward zone, and extensive postcrystallization exchange maintains isotopic equilibrium between bulk gypsum and dissolved sulfate. Anhydrite is the dominant sulfate mineral in the central zone where bacterial sulfate reduction results in heavier ^dgr34S and ^dgr18O values. In the landward zone both minerals and brine reflect mixing with the isotopically distinct sulfate of continental brines. Laterally systematic isotopic variation supports Patters n's chemical discrimination between seaward, mixed and landward hydrologic regimes, and is consistent with vertical variation of ^dgr34S demonstrated by Butler et al for this regressive sedimentary sequence. Gypsum-anhydrite transitions do not have significant effects on the isotopic composition of sulfate. Some ancient marine evaporite formations consisting mainly of dolomite and anhydrite may have formed in environments similar to the seaward and central zones of the Abu Dhabi sabkha; considerable isotopic variation would probably accompany subaerial deposition of laterally extensive marine evaporites. End_of_Article - Last_Page 505------------

STRATIGRAPHY AND GEOLOGIC HISTORY OF THE SABKHA, ABU DHABI, PERSIAN GULF

SUMMARYThe sabkha of Abu Dhabi was formed during the past 7,000 years by wind erosion of pre‐existing dunes and progradation of subaqueous, intertidal, and supratidal carbonate sediments. Marine transgression began in this area about 7,000 years ago and reached an apparent high about 1 m above its present level somewhat prior to 4,000 years B.P. Since then progradation of intertidal and supratidal sediments has taken place; this began 3,750 years ago. Arid conditions over the sabkha have produced large amounts of gypsum and anhydrite and lesser amounts of dolomite, magnesite, celestite and halite.