Abstract

Early carboniferous deposits of the Kalmard block, with various characteristics in different outcrops, are recognized by Gachal Formation. Generally speaking, this formation comprises of four different members (A, B, C and D), consisting of carbonate and evaporative rocks. Gachal Formation is composed chiefly of 55 meters sandstone and limestone interbedded with shale. According to the lithological and microscopic studies, Gachal Formation is deposited in beach, semi-restricted and open lagoon, shoal and open marine environments. Vertical changes of microfacies and the curve of its depth changes account for the high thickness of the facies of shoal and bar sub-environments and the low thickness of the facies of lagoon and open marine sub-environment as well. Gachal Formation rocks in Rahdar section are deposited in a low-angle homoclinal ramp located in southern Paleothysis Ocean. The carbonate-sandstone sequence in Gachal Formation in Rahdar section is composed of a depositional sequence separated from each other by type 1 sequence boundaries. Deposits of this formation are separated from lower deposits by type 2 sequence boundary and from Khan Group by unconformity. The sedimentary sequence identified in this formation points to the age of late Visean, conforming to Kaskaskia IV. The erosional boundary between Gachal and Khan Formations is relatively compatible with drop in sea level at Late Kaskaskia global scale.

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