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.