Abstract:

In this study, depositional environment and sequence stratigraphy of the Khoshyeilagh Formation has been investigated in three stratigraphic sections including Pelmis, Robat-e Qarehbil, and Kuhe Ozon in the northeast of Iran. The thickness of the Khoshyeilagh Formation in these sections are 182, 260, and 463 m, respectively. Based on the facies analysis of the Khoshyeilagh Formation, 3 clastic facies, 11 carbonate facies, and 1 hybrid facies were identified. The vertical and lateral changes of the Khoshyeilagh Formation show a shoreface marine depositional environment that begins with alluvial sediments and continues by marine sedimentation. According to these changes, a homoclinal ramp model is proposed for the Khoshyeilagh Formation. This ramp model shows a relatively gentle slope that continues to the basin and is similar to the present-day carbonate ramps, like the southern coast of the Persian Gulf and Shark Bay in the Australian carbonate ramp. This depositional model consists of five subenvironmental units including shoreface, tidal flat, lagoon, shoal, and open marine environments. The lithofacies analyses and their interpretations represent a decrease of depth at the end of Frasnian which is followed by an intensive sea level drop like the other global examples at this time. After a period of time and during Famennian the sea level rised and the open marine facies (bioclast spicule wackestone-packstone) were deposited. With a regard to the identified biozone, Late Devonian age (Late Frasnain-Early Famennian) is obvious for the Khoshyeilagh Formation. The sequence stratigraphic analysis and depositional model evaluations led to the identification of two third-order complete sequences, which are limited by discontinuities as a result of sea level changes at the base and top of the sequence, and an incomplete sedimentary sequence at the end of the studied section. The transgressive systems tract (TST) in this sequence is mainly composed of the shoal and open marine facies, the maximum flooding surface (MFS) is identified by the bioclast wackestone facies, and the highstand systems tract (HST) mainly consists of the lagoon, tidal flat, and shoreface facies.

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