Stratigraphy, mineralogy and depositional environment of the evaporite unit in the Aşkale (Erzurum) sub-basin, Eastern Anatolia (Turkey)

•Investigations of the Lower Miocene evaporites in the Aşkale basin (Eastern Anatolia).•Petrographic and mineralogical determinations of the facies.•Implications on the formation conditions and processes of gypsum.

The study area is situated in the Aşkale sub-basin where the Early-Middle Miocene aged Aşkale Formation was deposited in a shallow marine to lagoonal environment, and consists of interstratifications of clastic sediments, carbonates and evaporites. The successions of the Aşkale Formation can be divided into four main members interfingering with one another both vertically and laterally, and composed of the sandstone-mudstone-limestone member, the evaporite member, the gravelstone-sandstone-mudstone intercalations and the limestone member. The evaporite unit comprises of secondary gypsum lithofacies formed by hydration of precursor anhydrite, anhydrite, gypsum-bearing limestone and claystone in the form of wedges and lenses. Massive, nodular, nodular-banded, laminated and laminated-banded gpysum lithofacieses in addition to chicken-wire and rare entrolithic structures were described, indicating a sabhka or a shallow water depositional environment. Alabastrine and porphyblastic textures of gypsum were identified within the all lithofacieses with abundant amount of anhydrite relics. Additionally, saponite and illite/smectite, calcite and dolomite, celestite, epsomite were also observed. Successions of the Aşkale Formation were deposited in stable subtropical climatic conditions within rapidly subsiding sub-basin resulted in conversion of sub-basin to shallow platform and even in lagoon environment.