Sedimentology and geochemistry of the middle Miocene playa lake evaporites in the Gurun Basin (S of Sivas), Central Anatolia, Turkey

The Gurun basin is a half graben filled by alluvial, fluvial and playa-lake deposits of the Gurun Formation accumulated under the N-S direction extensional tectonic regime and volcanic rocks. The Gurun Formation is divided into four members such as the Kavak, Gokpinar, Cayboyu and Terzioglu members, and the Cayboyu Member has two different evaporitic occurrences such as selenitic and satin spar. During the first phase of evaporite deposition, selenitic gypsum crystals in 5 to 15 cm thick layer within the dolomitic mudstones were formed. The second evaporitic phase consists of four different levels of cavity filling within the sandstones and shale alternation at the upper part of sequence originated as 5 to 10 cm sized satin-spar gypsum crystals with white colored, partly orientated, fibrous-radial shaped. XRF major, minor and some rare elements analyses (La, Ce, Ta, W and U) and low Sr-87/Sr-86 (parts per thousand) and delta S-34 (CDT) values indicate that meteoric and volcanic solution occasionally mixed with the lake water. In addition, low delta O-18 (SMOW) values in the continental evaporate could be interpreted as mixing of fresh water with the playa lake environment. Selenitic gypsum in the Cayboyu Member of the Gurun Formation was precipitated from ground water-brine water rich in humic acid during periodic desiccations of in the shore-lake plain.