Sequence-based lithofacies and paleogeography of Lower Triassic Feixianguan Formation in eastern Sichuan and northern Zhongqing area

Abstract：The Lower Triassic Feixianguan Formation in northeast Sichuan can be divided into two third-order sequences and six system tracts in terms of sedimentary facies and sequence interfaces. Characteristics of sequence stratigraphy of the Feixianguan Formation were analyzed and sequence-palaeogeography maps were compiled. It is indicated that, influenced by regional tectonic uplifting of the Late Permian Changxing Stage in the study area during the development of SQ1 sequence, the product was formed in a regional slow transgression-regression depositional environment. The seawater that had been once deep and open became shallow, the depositional evolution changed from basin facies to open-platform facies which almost formed the platform edge or inner oolitic bank. In the second deposition of the third-order sequence cycles (SQ2), due to arider and hotter climate, sea level falling and water cycle limiting, the sedimentary facies turned into the restricted platform and evaporate platform facies, and the oolitic bank disappeared gradually. Finally, the sedimentary facies was converted to evaporate platform in which gypsum dolomite was developed. The gypsum dolomite served as an important sealant in the Feixianguan Formation, and the brine saved in the same time was helpful to the burial dolomitization in the oolitic bank to form high quality reservoirs in the Feixianguan Formation.