Carbonate Microfacies Analysis on Chihsia Formation(Middle Permian) at Xiakou Area,Western Hubei Province

Xiakou area is located on the north margin of the middle Yangtze platform,where successive middle to upper Permian marine carbonate strata were deposited.By means of carbonate microfacies,this paper conducted analysis on the biotic assemblages,microfacies types and sedimentary environments of the Chihsia Formation in this area.In total,seven types of microfacies were recognized: wackestone with diverse bioclasts,biotic wackestone-packstone with large foraminifers,biotic wackestone-packstone with dasycladacean,wackestone with fragmented bioclasts,biotic packstone with concordant algae and brachiopod shells,biotic packstone with intraclasts,biotic packstone with in-situ corals and algae.The vertical variation of these microfacies indicates that strata of the Chihsia Formation in Xiakou area were mainly deposited in a carbonate ramp environment.The lower part of the formation was deposited in a restricted marine environment of inner ramp,whereas the middle and upper part were formed alternatively in a mud mound environment at the edge of the inner ramp and an open marine environment of the inner ramp.