Facies distribution, paleoenvironment and sequence stratigraphy model of the Oligo- Miocene Asmari Formation (Fars Province, south of Iran)


Abstract: The Oligocene (Rupelian-Chattian) to Miocene (Aquitanian)-aged Asmari Formation is considered one of the most significant petroliferous reservoirs in the Middle East. The Oligocene-Miocene Asmari Formation represents diverse facies patterns in different parts of Zagros, so analysis of its facies in different provinces of the Zagros Basin is necessary. In this study, the sedimentary depositional sequences of Asmari Formation in the Firuzabad (south of Shiraz) and Bastak (west of Bandar-Abbas) sections in the Zagros Basin (Fras Platform) have been analyzed for distinguishing facies characteristics, interpreting the paleoenvironment and determining dominant depositional sequences. Based on field investigations and laboratory studies, 13 microfacies have been determined; these are organized in 3 facies associations containing outer ramp (open marine), mid ramp, and inner ramp (shoal, lagoon, and tidal flat). Sedimentary characteristics, arrangement, and comparison of horizontal and vertical facies indicated that the successions of the Asmari Formation sediments were deposited in a homoclinal ramp-like platform with a gentle slope and the deeper part of the basin located in the Firuzabad section. Analysis of detailed facies frequency showed that the outer ramp is the most frequent facies association in the Firuzabad and the inner ramp (lagoon subenvironment) is the most frequent facies association in the Bastak. Sea level variations during Oligo-Miocene time resulted in creation of two third-order depositional sequences in both sections. The depositional sequences consist of transgressive systems tract and highstand systems tract. In all the identified sequences, transgressive systems tract (TST) demonstrate retrogradational stacking pattern with deepening-upward trend while highstand systems tract (HST) indicate progradational stacking pattern with shallowing-upward trend.

Keywords: Oligocene-Miocene, Asmari Formation, paleoenvironment, sequence stratigraphy, Zagros Basin

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