Geochemistry and Sr, S, and O stable isotopes of Miocene Abu Dhabi evaporites, United Arab Emirates
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This study investigates for the first time the subsurface Miocene evaporite facies (Gachsaran Formation) in Abu Dhabi, United Arab Emirates. Forty-five evaporite rock samples were selected for petrographic, mineralogical, and geochemical investigations and stable isotope analyses to decipher their origin and constrain their age. Secondary gypsum with anhydrite relics dominates the investigated evaporitic rocks, with minor amounts of clays, dolomicrite, Fe/Ti oxides, and celestite. These samples are characterized by their excellent purity and low variability in geochemical composition. The distribution of trace element concentrations is significantly influenced by continental detrital intake. The main focus of the study is to determine the strontium, sulfur, and oxygen stable isotope compositions. The measured 87Sr/86Sr values of 0.708411–0.708739 are consistent with Miocene marine sulfates and indicate ∼21.12–15.91 Ma (Late Aquitanian-Burdigalian). The δ34S and δ18O values are 17.10‰–21.59‰ and 11.89‰–19.16‰, respectively. These values are comparable to those of Tertiary marine evaporites. The relatively low values of δ34S suggest that non-marine water possesses little influence on S distribution. The geochemical composition and Sr, S, and O isotope distributions of the Abu Dhabi gypsum facies from the Gachsaran Formation reveals that their source brines were marine (coastal saline/sabkha) with subordinate continental input.Keywords:
δ34S
Sabkha
Anhydrite
Isotope Geochemistry
Most efforts in the study of sea-marginal sabkhas have concentrated on the Persian Gulf, but little is known about the sediments and mineralogy of sabkhas marginal to other seas. The purpose of this paper was to present some geochemical and mineralogical observations in a recent sabkha on the coast of Sinai along the Gulf of Suez. The sabkha is composed of coarse clastic sediments with marine-derived groundwater at depth of about 1 m. The general morphology, climate and water salinity of the Gulf of Suez resemble those of the Persian Gulf, despite the fact that the content of authigenic evaporites in this sabkha is more sparse. The evaporite minerals accumulated only in the upper 30–40 cm of the sabkha, below that and down to the groundwater table, there is no accumulation of evaporites. Laterally, the salinity of the groundwater in the sabkha and the concentration of evaporites in the sediments above it increase constantly with distance from the shore. In contrast to the Persian Gulf where anhydrite is a major evaporite mineral, in Belayim gypsum is the only calcium sulphate mineral in the recent sabkha. Anhydrite is found only in an old elevated sabkha where it recrystallized from gypsum. The gypsum occurs as interstitial crystal concentrations or lithified horizons almost exclusively at the depth of 20–40 cm below the sabkha surface. Above that, in the uppermost horizons, there is in situ accumulation of interstitial halite crystals. The total concentrations of gypsum and halite are almost equal in this sabkha. The sea water recharge in El Belayim is almost exclusively by seepage through the sabkha sediments and not by flooding. The groundwater under this sabkha is only slightly more saline than the Gulf water, thus, not heavy enough for extensive downward refluxing. The major hydrodynamic process must be upward migration of the brines from the groundwater, precipitating on the way gypsum and later halite with some magnesite. Since the sediments of the sabkha are too coarse to support extensive capillary movement, the brines must, therefore, migrate upwards due to ‘evaporative pumping’.
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Anhydrite
Halite
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Lithification
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Anhydrite
Halite
Dolomitization
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Modern sabkhas are recognized as analogues to ancient evaporitic reservoirs and as Earth analogues to Martian paleoenvironments. Sabkhas are normal marine coastal sediments modified by groundwater precipitation of evaporites and carbonates. Previous work on Holocene sabkhas has focused largely on dolomitisation in carbonate-evaporite systems. Little attention has been given to understanding the origins of evaporites in mixed clastic-carbonate systems and their influence on reservoir quality. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework within which to understand processes controlling the origins of evaporites, their spatial distribution and likely evolution through time. Mesaieed sabkha is a 4-6 km wide coastal plain which consists of an onlap wedge of Holocene sediments some 3-6 m thick reaching a maximum of 15 m, which onlaps onto Eocene bedrock. Within the sabkha, gypsum is the most abundant diagenetic mineral, reaching 20-50% of the sediment volume over several square kilometres, with minor calcite, dolomite, anhydrite and halite. Gypsum cementation is pervasive above and below the water table in the proximal sabkha, in sediments dated c.6,000 years before present (yr BP), whilst in the central part (c. 4,000 yr BP) gypsum is restricted to surface crusts and water table cements, and is largely absent in the distal (coastal) sabkha (≤ 2,000 yr BP). Preliminary analysis of hydrological and geochemical data suggests evaporative pumping of groundwater from the underlying aquifer is an important source of solutes in the upper part of the sabkha, whilst seawater recharges the lower sabkha via the porous and permeable Eocene carbonates. Evaporation close to the water table results in fluids reaching gypsum saturation, and active precipitation of gypsum is evidenced by depletion of calcium and sulphate in the shallow brines. This is most marked in the middle part of the sabkha where salinity is highest. These increased density fluids reflux downwards from the Holocene, to mix within the Eocene aquifer, where reaction with the Eocene carbonates results in relative enrichment of calcium.
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Halite
Dolomitization
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Anhydrite
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Abstract Modern sabkhas are recognized as analogues to ancient evaporitic reservoirs and as Earth analogues to Martian paleo-environments. Sabkhas are normal marine coastal sediments modified by groundwater precipitation of evaporites and carbonates. Previous work on Holocene sabkhas has focused largely on dolomitisation in carbonate-evaporite systems. Little attention has been given to understanding the origins of evaporites in mixed clastic-carbonate systems and their influence on reservoir quality. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework within which to understand processes controlling the origins of evaporites, their spatial distribution and likely evolution through time. Mesaieed sabkha is a 4–6 km wide coastal plain which consists of an onlap wedge of Holocene sediments some 3–6 m thick reaching a maximum of 15 m, which onlaps onto Eocene bedrock. Within the sabkha, gypsum is the most abundant diagenetic mineral, reaching 20–50% of the sediment volume over several square kilometres, with minor calcite, dolomite, anhydrite and halite. Gypsum cementation is pervasive above and below the water table in the proximal sabkha, in sediments dated c.6,000 years before present (yr BP), whilst in the central part (c. 4,000 yr BP) gypsum is restricted to surface crusts and water table cements, and is largely absent in the distal (coastal) sabkha (= 2,000 yr BP). Preliminary analysis of hydrological and geochemical data suggests evaporative pumping of groundwater from the underlying aquifer is an important source of solutes in the upper part of the sabkha, whilst seawater recharges the lower sabkha via the porous and permeable Eocene carbonates. Evaporation close to the water table results in fluids reaching gypsum saturation, and active precipitation of gypsum is evidenced by depletion of calcium and sulphate in the shallow brines. This is most marked in the middle part of the sabkha where salinity is highest. These increased density fluids reflux downwards from the Holocene, to mix within the Eocene aquifer, where reaction with the Eocene carbonates results in relative enrichment of calcium. Introduction Many ancient sedimentary systems, particularly those deposited at low latitude, include units deposited in non-evaporitic settings but within which evaporitic minerals occlude significant volumes of porosity such as the Permian Zechstein Formation, the Permo-Triassic Khuff Formation, and the Jurassic Arab Formation. Much of the pore-filling or nodular anhydrite that is common at a wide range of burial depths may be secondary, precipitated from pore fluids rich in Ca2+ and SO42- (Kendall and Walters, 1978). Anhydrite is the product of dehydration of a gypsum precursor, which is the most abundant primary CaSO4 mineral (Warren, 2006). However, the sources of solutes forming these evaporites, flow pathways of fluids transporting these solutes and the stability of resulting evaporite precipitates has received little attention. Studies of ancient rock seldom attempt to distinguish between CaSO4 that was precipitated as gypsum prior to dehydration and CaSO4 precipitated at depth as primary anhydrite. Hence by using a modern arid environment to develop a better understanding of the distribution of early diagenetic CaSO4, and thus contribute to reconciling potential sources of secondary anhydrite formed during burial.
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ABSTRACT The Upper Jurassic Buckner Formation contains evaporites (anhydrite) buried to depths of up to nearly 6,000 m in southwestern Alabama. Although these evaporites and the associated sediments have been deformed and mineralogically altered, some primary depositional textures and fabrics remain. This establishes the Buckner Formation as the most deeply buried succession of evaporites from which primary features have been reported. A cored section of the Buckner Formation contains repetitive depositional sequences, from 1.5 m to 5.5 m in thickness, of 1) dolostone facies (shallow, restricted lagoon) overlain by 2) nodular anhydrite facies (sabkha). Within the nodular anhydrite facies, layers containing nodules surrounded by siliciclastic sediment and finely crystalline dolomite are interpreted as having formed by displacive growth of gypsum or anhydrite at or above a sabkha water table. Other intervals within the nodular anhydrite facies consist of vertically oriented and elongated nodules thought to be pseudomorphs after gypsum that originally formed by subaqueous crystal growth. Each of the Buckner Formation sequences represents the outbuilding of an arid shoreline with progradation of sabkha evaporites and siliciclastics over lagoonal carbonates in a manner similar to the Holocene sequences of the Persian Gulf and the northwest Gulf of California. The Buckner sequences that contain the vertically elongated anhydrite nodules differ from the normal Persian Gulf record of supratidal sabkha deposition in that these pseudomorphs are interpreted as having formed as gypsum in temporary ponds of a sabkha proper, a gypsum pan.
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Anhydrite
Dolostone
Siliciclastic
Halite
Pseudomorph
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This study investigates for the first time the subsurface Miocene evaporite facies (Gachsaran Formation) in Abu Dhabi, United Arab Emirates. Forty-five evaporite rock samples were selected for petrographic, mineralogical, and geochemical investigations and stable isotope analyses to decipher their origin and constrain their age. Secondary gypsum with anhydrite relics dominates the investigated evaporitic rocks, with minor amounts of clays, dolomicrite, Fe/Ti oxides, and celestite. These samples are characterized by their excellent purity and low variability in geochemical composition. The distribution of trace element concentrations is significantly influenced by continental detrital intake. The main focus of the study is to determine the strontium, sulfur, and oxygen stable isotope compositions. The measured 87Sr/86Sr values of 0.708411–0.708739 are consistent with Miocene marine sulfates and indicate ∼21.12–15.91 Ma (Late Aquitanian-Burdigalian). The δ34S and δ18O values are 17.10‰–21.59‰ and 11.89‰–19.16‰, respectively. These values are comparable to those of Tertiary marine evaporites. The relatively low values of δ34S suggest that non-marine water possesses little influence on S distribution. The geochemical composition and Sr, S, and O isotope distributions of the Abu Dhabi gypsum facies from the Gachsaran Formation reveals that their source brines were marine (coastal saline/sabkha) with subordinate continental input.
δ34S
Sabkha
Anhydrite
Isotope Geochemistry
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