Geology and trona potential of the middle Miocene Gurun (Sivas) Basin, central Anatolia, Turkey

Gurun (Sivas) basin, extending along east-west direction,contains a volcano-sedimentary sequence of up to 1222 m thick. The middle Miocene sedimentary sequence rest with angular unconformity on the basement rocks, which consist of limestone and flysch sediments ranging in Triassic-Jurassic-Cretaceous and middle-upper Eocene. The sequence has been subdivided into Gurun Formation and Karada (g) over tilde Volcanics. The Gurun Formation consist of four members in ascending order: Kavak, Gokpinar, Cayboyu and Terzioglu members. Trona occurences are placed within the Terzioglu member, alternating with bituminous shale, mudstones and tuffs; whereas the lignite deposits occured in the Cayboyu member of the Gurun Formation. The Miocene sedimentary rock units are laterally and vertically gradational. Karada (g) over tilde Volcanics, located in the eastern and northern parts of basin, interfinger with the upper parts of the Terzioglu member, and also cut the whole rock units. The thickness of the sequence increases towards the northern and eastern sides where the basin was faulted. According to spor and pollen fossils, the age of sedimentary rock units is middle Miocene. The rock units were deposited in alluvial, fluvial, lacustrine and playa-lake environments. The middle Miocene sedimentary sequence, which were deposited under the control of a tectonic regime. In the region, gravity tectonic, which is characterized by the growth faults, began to developed in the beginning of middle Miocene. The growth faults, which were developed at the northern margin of the basin, controlled the deposition during the middle Miocene. Gurun basin was affected by the extensional tectonic regime changed to a compressional regime during the late Miocene time. During this new tectonic phase, NE, SW compressional regime occuring in the region probably originated from the movement of the East Anatolian Fault. The bituminous shales of the Terzioglu member contains starkeite, hexzahydrite, dolomite and calcite. Na-carbonate (trona) leachings are rather common within the lower and middle parts of the Terzioglu member. Sodium carbonate minerals occuring in the basin are nahcolite, pirssonite, gaylusite and shortite. The beds consisting of the Na-carbonate minerals grade into mudstones and clayey limestones towards the edges of the basin. The Gurun middle Miocene basin is mainly filled by clastics and products of adjacent volcanic activity which was centered at the east and north of basin. The most likely sources of sodium for the formation of Na-carbonate (trona) accurences are thermal springs, tuff interbedded with the sediments, and extensive the middle Miocene volcanic rocks interfingering with the sedimentary rocks in the eastern and northern parts of the basin.