Sedimentology, petrography and diagenesis of Eocene-Oligocene evaporites: the Tuzhisar Formation, SW Sivas Basin, Turkey

摘要

The study area is situated in the southwestern part of the Sivas Basin. Sediments of the Ortakoey Formation (Middle-Upper Eocene) of flysch facies were deposited in a shallow marine environment and consist of interstratified sandstone, shale and calcareous mudstone. The Karaziyaret basaltic dikes cut the Ortakoey Formation. The Ortakoey Formation is comformably overlain by the evaporite-dominated Tuzhisar Formation. Gradual transitions between these two stratigraphic units crop out between Yenikoey and Yenituzhisar villages. The Tuzhisar Formation is composed mainly of secondary gypsum, which originated from the hydration of precursor anhydrite rock. The formation has been subdivided into three members: Yenikoey, Kuellue and Selimiye members. Yenikoey (lower evaporite unit) and Kuellue (upper evaporite unit) members contain gypsum sequences up to 70 m thick. The Selimiye member (Lower-Middle Oligocene), which consists mainly of red clastic units interrupt and gradationally overlies (laterally and vertically) these two members. In the gypsum of the Yenikoey Member nodular, nodular-banded, laminated-banded lithofacies with enterolithic and chicken-wire structures are distinguished. These associations indicate a sabkha or shallow-water setting of deposition such as a lagoon. Secondary gypsum in the Tuzhisar Formation generally displays alabastrine and porphyroblastic textures with corroded anhydrite relics. Associated minerals are dolomite, calcite, celestite, and lutecite. The Kuellue Member is composed of irregular thick-bedded, massive, locally brecciated secondary gypsum, probably formed by multiple synsedimentary collapse after the dissolution of pre-existing soluble minerals and/or by paleoslumping related to synsedimentary tectonism. However, most of the evidence for the origin of the breccia has been destroyed by intense karstification.