Fluvial, evaporitic and shallow-marine facies architecture, depositional evolution and cyclicity in the Sivas Basin (Lower to Middle Miocene), Central Turkey

Lower to Middle Miocene rocks in the Karayün region of the Sivas Basin are represented by the Agilkaya and Egribucak Formations, forming a 4 km thick continuous section. Each formation is subdivided into three members, representing similar lithofacies. The lower members are made up of fluvial sheet-sandstone and red mudstone that migrated laterally on a flood basin within a semi-arid fan system. The middle members are composed of bedded to massive gypsum and red-green mudstone of a coastal and/or continental sabkha environment. They are intercalated with lagoonal dolomitic limestone and bituminous shale in the Agilkaya Formation and fluvial red-pink sandstone-red mudstone in the Egribucak Formation. The upper members are made up of shallow-marine fossiliferous mudstone and sandy limestone. Rapid vertical and horizontal facies changes in both formations reflect the locally subsiding nature of this molassic basin. The overall section can be also subdivided into three orders of fining-upward cycles. Type I cycles correspond to the Agilkaya and Egribucak Formations (each around 2 km thick) and indicate two long-term transgressive phases, controlled by tectono-eustatic factors. Type II cycles (140 m thick on average) are formed by alternations of fluvial sheet-sandstone and red mudstone capped by up to 50 m thick red flood basin mudstone. A 400 ka period Milankovitch cyclicity related to astronomical climatic fluctuations is suggested as a governing mechanism, although subsidence cannot be excluded. The existence of laterally persistent, fluvial sheet-sandstone overlain by red mudstone that forms fining-upward Type III cycles (10 m thick on average) is attributed to changes in subsidence rate. Rhythmic alternations between gypsum beds and red-green mudstone and/or dolomitic limestone and bituminous shale, observed in the middle members, might be related to minor periodical climatic changes rather than to tectonics.