Origin and palaeodepositional environment of evaporites in the Bala sub-basin, Central Anatolia, Türkiye

Bala is located in Central Anatolia and is a sub-basin of the Tuzgölü basin, which consists of many basins formed in response to the closure of the Neotethys ocean. The evaporites in Bala sub-basin formed under the influence of regional uplifting during the Middle Eocene period with gradual shallowing in response to the structural compressional regime. The Bala sub-basin comprises four units from bottom to top: the Lower Unit (BU1) consisting of mudstone, claystone with limestone layers; the Evaporites (BU2) containing dominant gypsum with anhydrite; mixed siliciclastic-carbonate-evaporitic deposits (BU3); and the Upper Unit (BU4) containing conglomerate-sandstone-claystone and siltstones. The evaporitic unit, which are the main subject of study consists of primary selenitic gypsum, anhydrite, locally celestite crystals and secondary gypsum formed due to the anhydritization of primary gypsum and then hydration of these anhydrites. The results obtained from mineralogical-petrographic, geochemical and isotopic (δ18O and δ34S) investigations indicate that these evaporites were deposited in shallow marine to sabkha environments dominated by arid conditions. The presence of primary selenite crystals, the anhydrite with nodular, chicken-wire, and mosaic structures support precipitation occurred in a partially saline shallow water. The excessive thickness of the evaporitic accumulation, chaotic masses including gypsum breccia, salt diapirs and dome structures are attributed to tectonic processes acting during the formation of the Central Anatolian basins.

巴拉盆地位于安纳托利亚中部，是图兹湖盆地（Tuzgölü basin）的次级盆地；图兹湖盆地由新特提斯洋（Neotethys Ocean）闭合过程中形成的诸多盆地共同构成。巴拉次级盆地内的蒸发岩（evaporites）形成于中始新世时期的区域隆升作用影响下，同时受构造挤压体制驱动，盆地逐渐变浅。巴拉次级盆地自下而上可划分为四个地层单元：下伏单元（BU1）由夹灰岩夹层的泥岩、黏土岩组成；蒸发岩单元（BU2）以石膏为主，伴生硬石膏（anhydrite）；陆源碎屑-碳酸盐-蒸发岩混合沉积单元（BU3）；以及上覆单元（BU4），其岩性包括砾岩、砂岩、黏土岩与粉砂岩。作为本次研究主要对象的蒸发岩单元，主要由原生透石膏（selenitic gypsum）、硬石膏、局部产出的天青石（celestite）晶体，以及因原生石膏发生硬石膏化作用后再经水化作用形成的次生石膏组成。通过矿物学-岩石学、地球化学以及氧同位素（δ¹⁸O）、硫同位素（δ³⁴S）分析获得的研究结果表明，该套蒸发岩沉积于以干旱条件为主的浅海至萨布哈（sabkha）环境中。原生透石膏晶体的产出、具结核状、鸡窝状及镶嵌状结构的硬石膏，均佐证了其沉积作用发生于半咸水浅水环境。蒸发岩堆积体的异常厚度、包含石膏角砾岩（gypsum breccia）、盐底辟（salt diapirs）与穹窿构造的杂乱岩体，均归因于安纳托利亚中部盆地形成过程中所经历的构造作用。