Sequence-based lithofacies and paleogeography of Lower Triassic Feixianguan Formation in northeastern Sichuan Basin

Based on the study of sequence stratigraphy,Feixianguan Formation is divided into two third-order sequences(SQ1,SQ2) and eight system tracts,of which the lithofacies and paleogeography are analyzed.During the first deposition of the third-order sequence cycles(SQ1),influenced by the regional tectonic uplifting in Changxing stage of Late Permian in northeastern Sichuan,the seawater once deep and wide became shallow,and the depositional evolution changed from basin facies to open-platform facies,of which oolite beach developed well.During the second deposition of the third-order sequence cycles(SQ2),due to arid and hotter climate, fallen sea level,limited water cycle and exposed carbonate platform,the depositional evolution became to restricted platform and evaporated platform facies,and the oolite beach in northeastern Sichuan decreased.Finally,the sedimentary geomorphology was nearly full filled.