Abstract: Tokuş formation which is the basal sediments of the Sivas Tertiary Basin and play a key role on its geological evcfution, was first described by Yılmaz and Özer (1984). Its age was regarded as Lutetian-Priabonian and it was thought to be deposited in shallow marine environmental conditions. Detailed geological map of the units, two measured stratigraphical columnar section; stratigraphie, paléontologie and sedimentalogic features of the formation were examined, therefore new aged data and depositional conditions have been defined in this study. The fossils Alveolina cucumiformis Hottinger and Nummulites solitarius (A) de la Harpefound in the lower part indicate llerdian stage, fossils Alveolina cuspidata Drobne, Alveolina ruetimeyeri Hottinger, Nummulitesformosus de la Harpe Assilina cf. dandotica Davies indicate Cuisian stage. According to this data the age of the formation is yieldeddown to llerdian. The dominant fades of the unit from bottom to top and in a recurrent manner are alluvial fan, coastal, neritic andslope fades. Thickness of the formation shows thinning and thickening from place to place caused by the dominant regional thrust tectonics.