Evidence of synsedimentary tectonics in the Northern Tabas Block, East-Central Iran: The Callovian (Middle Jurassic) Sikhor Formation

摘要

The Sikhor Formation (new) is a predominantly siliciclastic sediment package intercalated between the marly-silty Baghamshah Formation (below) and the calcareous Esfandiar Limestone and Qal'eh Dokhtar Limestone formations (above). All stratigraphic evidence points to an Early Callovian age of the formation. The Sikhor Formation is restricted to the southern and central Shotori Mountains and consists of two members: The Kuh-e-Neygu Member (new) is composed of fluvialdeltaic conglomerates, sandstones, and siltstones grading into marly silt of the Baghamshah Formation. The overlying Majd Member (new) is characterised by mixed siliciclastic-carbonate sediments that record the interfingering of carbonate ramp sediments with fluvialdeltaic sands and silts. Evidence of erosional truncation of the underlying Baghamshah Formation and confinement of the siliciclastic sediment to a comparatively narrow, NNW-SSE elongated strip suggest that the formation had its origin in the asymmetric uplift of a westdipping tilted fault block in the southern Shotori Mountains that shed its sediment predominantly in a northern and eastern direction. After erosional levelling, the former uplifted areas were overgrown by the highly productive Esfandiar Carbonate Platform. The Sikhor Formation thus is evidence of an extensional tectonic pulse in the early Callovian and underlines that this area of the Tabas Block was a tectonically highly unstable area during most of the Jurassic.