depositional environments and sequence stratigraphy of the sorkh shale formation (lower triassic) in the tabas block

the sorkh shale formation (lower triassic) crops out in parts of tabas block. this formation is composed of limestone, dolomite, sandstone, shale and gypsum. its lower contact with the permian rocks and its upper contact with the shotori formation is marked by distinct erosional unconformity. to determine the depositional environments of the sorkh shale formation, stratigraphic sections of godar-e-sorkh (southeast of tabas), godar-e-gachal (southwest of tabas) and islam-abad (southeast of zarand) have been studied. field and laboratory studies have shown that the sorkh shale formatin consists of carbonate (lagoonal to tidal flat environments), silisiclastic (lagoonal, tidal flat and coastal plain environments) and evaporite (playa environment) facies. both carbonate and siliciclastic storm deposites (tempestite) are also recognized. the sorkh shale formation have been formed in a homoclinal clastic-carbonate ramp similar to the peresent day persian gulf. the sorkh shale succession in the tabas block indicates a depositional sequence (3rd_order cycle) bounded by type 1 unconformities. northward increase in carbonate content, northward paleocurrent directions and higher thickness of the lower triassic rocks indicate that the sorkh shale formation was deposited in a failed rift along the kalmard/kuhbanan and nayband faults.