Microfacies and sedimentary environment of the Mishan Formation in the Shahdadi Area, the southeast of the Zagros Basin (N Bandarabbas)

The Zagros structural Zone is an important structural zone in the Middle East because of its valuable and enormous hydrocarbon reservoirs. In this zone, the Fars Group is a key stratigraphic unit; which consists of valuable reservoir and cap rocks. The Mishan Formation is the middle member of the Fars Group, was deposited in the Zagros Basin during the Early Miocene to Pliocene. The facies changes of the Mishan Formation in the Zagros basin is considerable and interesting for researchers. One of the unique lithofacies of this formation is the Guri Member, which is out cropped in the Hormozgan province in many of anticlines. In the Hormozgan province, the Mishan Formation is poorly studied because of harsh climatic condition and accessibility, therefore, there are few published literatures have been carried out about it (Heidari et al., 2014a, b; Rashidi et al., 2014a, b).To consider the microfacies and reconstructing the depositional environment of this formation in the southeast of the Zagros Basin, a stratigraphic section of this formation in the Shahdadi Area has been chosen and sampled. The studied section locates at 27° 29ˊ 1.5 ̋ N & 56° 43ˊ 30  ̋ E near the Shahdadi area, 105 km north of the Bandar Abbas. The main surrounded unites include Cenozoic strata of the Zagros basin. The studied section overlays the Razak Formation and underline by the Aghajari Formation unconformably.

The total thickness of the studied section is 446 m, 78 loos and 236 hard samples have been collected from the Mishan Formation strata. For microfacies analyses, thin sections prepared from hard samples. The loos samples have disaggregated by sodium sulfate solution and frizzing method and Sodium Hexa Meta Phosphate solution for micropaleontological and grain size analyzes. The microfacies have named after Dunham (1962) and Carozzi (1989). The faunal content and grain size of the marly strata compared with Geel (2000), BouDagher-Fadel (2008).

Based on the lithostratigraphic analyzes, tow lithofacies, include limestone lithofacies and marl lithofacies have been distinguished. The limestone lithofacies involves three rock unites and includes nine microfacies (L1-L9) base on the carbonate components and textural characteristics. These microfacies have been compared with the Wilson (1975) and Flugel (2010) standard microfacies. This comparison shows that the identified microfacies have deposited in intra tidal, restricted lagoon, sand shoal, path reef, non-restricted lagoon and open marine facies belts. The marl lithofacies, based on the faunal content and grain size analyzes, includes deposits of the restricted lagoon, non-restricted lagoon and open marine environments and involves tow rock unites. The charactristics of identified facies belts, such as lateral extend, lack of the onchoids, pizzoids, aggregates and turbidities have suggest a homoclinal carbonate ramp as the sedimentary model in the studied area for the Mishan Formation. This platform can be divided to distinct facies associations include inner ramp, middle ramp and outer ramp.The inner ramp facies association includes intra tidal, restricted lagoon and sand shoal facies belts; the middle ramp facies association includes non-restricted lagoon and patch reef facies belts and the outer ramp facies association includes open marine facies belt. Deposits of the non-restricted lagoon, restricted lagoon and open marine facies belts are the most common deposits in the studied section respectively, which are deposited in deepening upward cycles.

The field, microfacies and micropaleontological studies show that the Mishan Formation in the Shahdadi area, the north of Bandarabbas, overlays the Razak Formation’s clastic strata unfonformably. This formation underlays by the Aghajari formation unconformably and the effects of erosional activities are observed at the top of the Mishan formation. The Mishan Formation in the studied area consists of three limestone and tow marl rock unites and considerably includes the Guri Member. The microfacies analyses of the marly and limestone strata suggest a hemoclinal carbonate ramp as the sedimentary model for the Mishan Formation in this area. The suggested ramp includes inner ramp, middle ramp and outer ramp facies assemblages and intra tidal, restricted lagoon, sand shoal, non restricted lagoon, patch reef and open marine facies belts.