Sedimentology, depositional environment and sequence stratigraphy of the Asmari Formation (Oligocene-Lower Miocene), Gachsaran Area; SW Iran

Abstract

This study investigates the depositional environment and sequence stratigraphy of the Asmari Formation (Oligocene-Miocene) in Gachsaran Area. The formation is carbonate sequence, which is laid down in the southern side edge Neotethys Ocean (Zagros area). The Asmari Formation represents sedimentation on a carbonate ramp. Seven major microfacies and three subfacies are recognized which include: MF-1) planktonic foraminifer wackestone-packstone, MF-2) nummulitic-bioclastic-corallinacean wackestone-packstone, MF-3) bioclastic grainstone, MF-4) ooid-grainstone, MF-5) bioclastic-miliolid-borelisid, MF-6) miliolid-intraclast-bioclast and MF-7) carbonate mudstones (laminated mudstone, fossiliferous mudstone and algal mudstones). This carbonate succession consists of four, thick shallowing-upward sequences. Based on shallowing and deepening trend of depositional facies and changes in the cycle stacking pattern four third-order depositional sequences were identified. With the concern of 7 microfacies kinds, five carbonate sedimentary environments are represented. These environments from basin to land: outer slope to basin and slope sited in outer ramp; shoal and shelf lagoon located between middle ramp-internal ramp; and restricted lagoon and low intertidal too supratidal set up in the internal ramp. The formation indicates a large-scale trend of decreasing accommodation upward. The lower Asmari layers was laid down on open-marine conditions and represented by a laterally equivalent shaley faices, assigned to that generally the Pabdeth formation and communicated prograding side edge to the upward gradient attitudes, while the center to the upper parts of the formation were deposited in lower energy settings with more frequent evidence of exposure and evaporitic conditions, reflecting ramp settings established behind the prograding margin. The top side of the sequence is obviously marked by an erosional surface, which is overlaid by thick tidal flats. The surface erosional shows intense staining and leaching out suggesting diagenetic alteration during the sub-aerial exposure. With petrographic methods five kinds of dolomite are differentiated in this formation: 1- dolomicrite 2- dolomicrosparite 3- sucrosic dolomite 4-cloudy center dolomite and 5-saddle dolomite.