Sedimentology and petrography of Paleocene–Eocene evaporites: the Sachun Formation, Zagros Basin, Iran

Abstract

The study area is situated in the folded zone of the Zagros Basin where sediments of the Sachun Formation (Paleocene–Lower Eocene) were deposited in a shallow marine environment and consist of interstratified carbonate, shale and evaporite. The Sachun Formation is conformably overlain by the dolomite-dominated Jahrom Formation and unconformably downlaps the Tarbur Formation carbonates (Alavi in Am J Sci 304:1–20, 2004). The Sachun Formation consists mainly of secondary gypsum, which precipitated from the hydration of precursor anhydrite rock. The succession of the Sachun Formation has been divided into three units: the lower red clastic unit, the middle evaporite unit, and the upper carbonate unit. In the middle unit gypsum, several structures are distinguished; e.g. nodular, nodular-banded, and laminated-banded lithofacies with enterolithic and chicken-wire structures. These associations were deposited in a sabkha or shallow water type of depositional environment. Petrographic investigations confirm that the evaporites of the Sachun Formation are mostly composed of secondary gypsum, with rare anhydrite relics. Five microfacies were distinguished according to the gypsum fabrics. The textural relationships of the carbonate and sulfate minerals indicate that some of these anhydrites were formed as a result of a replacement processes of carbonate sediments associated with evaporites. The textures are considered to be a by-product of anhydrite hydration, which is in turn rehydrated from primary gypsum. The evaporites indicate deposition during a regressive lowstand systems tract, whereas the carbonates indicate deposition under shallow water marine conditions during a highstand systems tract. The Sachun Formation succession deposits are in a quite stable arid climate within a rapidly subsiding basin.