TECTONIC AND SEQUENCE-BASED LITHOFACIES PALEOGEOGRAPHIC FEATURES OF LATE TRIASSIC IN WESTERN SICHUAN FORELAND BASIN

The Late Triassic in the Western Sichuan Foreland Basin is a key to understand the evolutionary history of this basin.Using bore hole,outcrop,and seismostratigraphic data,the authors studied the tectonic sequence of the Upper Triassic in the Western Sichuan Foreland Basin.The Upper Triassic was divided into three tectonic sequences.Taking the tectono-sequence system tract as the mapping unit,it also systematically maped the lithofacies paleogeographic chart and discussed the evolutionary features.The results indicated that Tectonic Sequence 1(TS1) includes Ma'antang Formation and Xiaotangzi Formation,and it evolve from episode Ⅰ of Indo-Sinian Movement.There were developed neritic and marine-continental transition environment in the Western Sichuan Foreland Basin.TS2 includes the Second and Third members of the Xujiahe Formation.The Western Sichuan Foreland Basin started forming and developed marine-continental transition environment in TS2.TS3 includes the Fourth and Fifth members of the Xujiahe Formation.Because of influence by the Anxian Movement,the Longmenshan Thrust Belt was uplifted and formed mountains,which changed the Western Sichuan Foreland Basin into a terrestrial sedimentary environment.The most important factor controlling the basin formation and evolution of the Late Triassic in the Western Sichuan Foreland Basin is tectonic activity.