Resumen de Sedimentology and geochemistry of the middle miocene playa lake evaporites in the Gürün basin (S of Sivas), central Anatolia, Turkey
Mehmet Önal, Cahit Helvaci, Erdogan Tekin, Turhan Ayyildiz
The Gürün basin is a half graben filled by alluvial, fluvial and playa-lake deposits of the Gürün Formation accumulated under the N-S direction extensional tectonic regime and volcanic rocks.
The Gürün Formation is divided into four members such as the Kavak, Gökpmar, Çayboyu and Terzioglu members, and the Çayboyu 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 low87Sr/86Sr (%o) and d34S (CDT) values indicate that meteoric and volcanic solution occasionally mixed with the lake water.
In addition, low d18O (SMOW) values in the continental evaporate could be interpreted as mixing of fresh water with the playa lake environment. Selenitic gypsum in the Çayboyu Member of the Gürün Formation was precipitated from ground water-brine water rich in humic acid during periodic desiccations of in the shore-lake plain
