Depositional environments and coal-accumulation characteristics of the Upper Triassic Xujiahe Formation in the northern segment of Huaying Mountain,Sichuan Basin

The northern segment of Huaying Mountain is located in the backbulge area of Sichuan foreland basin during the Late Triassic. The Xujiahe Formation of the Upper Triassic is the major coal-bearing strata and the depositional en- vironments and coal-accumulation characteristics of the Xujiahe Formation in the Jingang mine area were studied based on the data from borehole cores,field outcrops and well loggings. The Xujiahe Formation was subdivided into seven Members,named from Member 1 through Member 7 in ascending order,in which Members 1,3,5,and 7 were coal-bearing intervals,while Members 2,4,and 6 were sandstone-rich intervals. Five rock types were identified,including conglomerate,sandstone,siltstone,mudstone,and coal. Fluvial and deltaic facies were recognized in the Xujiahe For- mation,with the Jiangnan Oldland to the east being the major provenance. The coal accumulation was controlled by tectonic activities,depositional environments and paleoclimates. During the Late Triassic,the Sichuan Basin experi- enced three episodes of thrusting,two episodes of tectonic quiescence and one episode of isostatic rebound. Coals were best developed in the episodes of tectonic quiescence and isostatic rebound. The coal-accumulation center migrated gradually from west to east and from bottom to top in the Xujiahe Formation. The study area,Jingang Coal Mine,was an important coal-accumulation zone in the end of the Late Triassic. The mineable coal seams were mainly developed in the Members 5 and 7,with the coals in Member 7 being relatively thicker. The coals in Member 5 were deposited in the interdistributary bay of the delta plain environment and the coals in Member 7 were formed in the back basin of fluvial environment.