STRATIGRAPHY AND SEDIMENTOLOGY

The sections in our study are located on Bakhshi Mountain (92 km southwest of the town of Tabas), Madbeiki Mountain (12 km of northwest of the Bakhshi Section), Gachal Mountain (approximately 100 km southwest of Tabas), Rahdar mountain (65 km west of Tabas) and Halvan mountain (80 km northwest of Tabas) in the Kalmard area, Iran (Figure 1).

The type section of the Khan Formation was established by Aghanabati (1977) as being on Bakhshi Mountain, 8.5 km southeast of Kalmard Karavansaray. Five stratigraphic sections were measured and sampled, and more than 546 samples were collected for both geochemical and paleontological studies (Figure 2). Because the successions discussed here show minor differences from one another, their lithological features will be explained in general. The Khan Formation overlies the limestone of the Gachal Formation where Mississippian Archaesphaera, Endothyra and Earlandia were reported (Gorgij 2002). It mostly consists of red to brown cyclic sequences that start with gravel to cobble conglomerate or with very coarse to coarse sandstone. The size of the clasts decreases upward, and the topmost portion of each cycle are shallow water thick- to medium-bedded packstone to grainstone. The distinguishing feature of this formation is the siliciclastic material that encompasses many of the units. The lithological content of the Khan Formation suggests its deposition near a terrigenous source (Arefifard 2006).

There are several cyclic sequences present in the Khan Formation. Each starts with gravel to cobble conglomerate or with very coarse to coarse sandstone (Figure 2). The size of clastic grains decreases upward. This part of the cycle shows the transgressive phase. With increasing water depth, carbonate sediments that consist mainly of shallow water thin to medium-bedded packstone to grainstone with a few horizons of mudstone, are deposited. The fossil content of this limestone is commonly dominated by small foraminifers, fusulinids, corals, bryozoans, crinoid stems and brachiopods. It is also quite thin, suggesting a short duration stabilization within the depositional basin. Medium-bedded fusulinid-bearing limestone of the Khan Formation was found in a second sedimentary cycle in the Madbeiki, Gachal and Rahdar sections. The first cycle was present in the Halvan section (Figure 3). There was no packstone or grainstone at the Bakhshi section. Dolomite and dolomitic limestone form part of the carbonate sediments of each cycle in some sections of the Khan Formation. For example, in the Halvan section the carbonate part of the third cycle is defined by the predominance of grey medium-bedded dolomite. Petrographic studies of these dolomites demonstrate that they are secondary in origin. The topmost portion of each cycle terminates in red colored sandstone, showing a possible regressive phase.

Through the analysis of five stratigraphic sections it becomes apparent that the number of sedimentary cycles in the Khan Formation varies from section to section. The Madbeiki, Rahdar and Gachal sections possess two cycles, and Bakhshi and Halvan sections show four and three cycles, respectively. Additionally, the regressive, siliciclastic sediments of each cycle are a result of basin uplift and consequent erosion. Therefore, transgressive siliciclastic deposits of the cycle directly overlay carbonate sediments. The lower boundary of the Khan Formation is placed at a disconformity, which is recognized by channeling immediately above the Mississippian limestone of the Gachal Formation. It is overlain by Lower Triassic yellow vermiculite limestone of the Sorkh Shale Formation. The disconformity between the Khan and Sorkh Shale Formations is marked by a bauxite horizon.

The early Artinskian is marked by a regression in the Kalmard region. Continental conditions in the area remained until the early Triassic. As such, uplifting did not give way to subsidence until the end of Permian, as evidenced by the Lower Triassic Sorkh Shale Formation that unconformably overlies the late Sakmarian-early Artinskian Khan Formation. Facies analysis, petrographic studies and lithological features of the Khan Formation suggest that it was deposited in a shoreline environment with mixed siliciclastic-carbonate sediments.