Varying depositional environments of gypsum successions in the Upper Miocene Eskiehir-Sivrihisar lacustrine basin, NW Turkiye
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The basement rocks of Eskisehir-Sivrihisar basin are made of Upper Paleozoic metamorphic rocks and Upper Cretaceous ophiolite complex. These rocks are overlain by Middle Miocene sedimentary rocks the base. Upper Miocene-Pliocene stratigraphic succession consists of basal conglomerates, sandstones, claystones, limestones, dolomites and tuff's are thinly interbedded with cyclic evaporites. The main clay mineral assemblage consists of smectite, illite and sepiolite that are enriched in different percentage in different horizons. The Upper Miocene-Pliocene stratigraphic succession consists of alterations of gypsum, clayey carbonates and unconsolidated very soft dolomite beds, which are conformably underlain by sepiolite-bearing clayey carbonates and disconformably overlain by alluvium and conglomerate series of Pleistocene age. The Upper Miocene unit includes three different facies, based on sedimentary properties of gypsum from the base to the top; (1) laminated gypsum beds are thinly interbedded with dolomites and green clays; (2) gypsum rosettes within thin reddish and greenish clays and carbonates; and (3) scattered gypsum mainly within carbonates. Gypsums had been deposited in somewhat different depositional conditions in the same basin where multi-periodic evaporative episodes, ranging from saline to schizolialine environments, had been involved. During these episodes, under high Eh-pH intervals and high salinity conditions, three different types of gypsum formations had been developed in a meromictic-type lake under the influence of varying paleoclimatologic conditions and pore water chemistry of brines. All these gypsum morphologies respectively reflect the different conditions of environments of deposition in a Miocene playa lake basin: (1) subaqueous environments in playa lakes with short evaporation episodes; (2) marginal swamps with cyclic fluctuation of lake level; and (3) extended closed-lake with evaporatic lake water. These different gypsum morphologies are associated with distinctive sedimentary subfacies, which are also primarily related to cyclic climatic changes, alternating semi-arid and evaporite conditions, fluctuation of lake water level and sulphate input in the chemistry of lake water from runoff and hydrothermal source adjacent to basin.