Facies, sedimentary environment and sequence stratigraphy of the Surmeh Formation (Jurassic) in Fars region

The Surmeh Formation comprises the Jurassic rocks of the Zagros Mountains and is considered as one of the most important petroleum reservoir units in the Persian Gulf basin. This formation is mainly composed of thick bedded and cliff forming carbonate rocks. The aims of this study are to investigate depositional history and sequence stratigraphy of the Surmeh Formation in the Fars province or eastern Zagros. Field and laboratory examination of the formation revealed the presence of 15 lithofacies related to different evolutionary stages of a carbonate platform. The results of different measured sections indicated that the lower part of the Surmeh Formation is composed of thick-bedded dolomitic facies containing Lithiotis fossils of Toarcian age related to a shallow marine epeiric platform. At the Aalenian tectonic movement along the preexisting basin faults resulted in the formation of a distally steepened ramp and Pars intrashelf basin. Consequently, alternating shale and limestone (calciturbidites) were deposited in the deep marine environment of an outer ramp during Bajocian. During Bathonian, overproduction of carbonates and progradation of platform facies over basinal deposits resulted in reestablishment of the homoclinal ramp that was in existence in the study area until the end of Jurassic. Vertical facies variations and comparison with global sea level curve revealed the existence of two second-order depositional sequences in the Surmeh Formation. The given sequences by themselves include 6 third-order sequences and correlate with lower Zuni I and lower Zuni II super sequences of Sloss (1963).