四川华蓥山北段上三叠统须家河组沉积环境及聚煤特征

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

华蓥山北段处于四川前陆盆地东部隆后沉积区,笔者根据钻孔岩芯、野外露头、测井等资料,对该区金刚矿区上三叠统须家河组含煤岩系的岩石类型、沉积环境以及聚煤特征进行研究。研究区须家河组从下到上划分为7段,须一、三、五、七段为含煤段,二、四、六段为砂岩段,共识别出砾岩、砂岩、粉砂岩、泥岩和煤等5种岩石类型,区内主要发育河流相和三角洲相,物源主要来自江南古陆。构造作用、沉积环境和古气候等因素影响该区聚煤作用,经历了3次逆冲推覆作用、2次推覆后静止期及最终构造稳定的均衡回弹期,煤层主要形成于推覆后的静止期(须三段和须五段)和均衡回弹期(须七段),四川盆地东部聚煤作用有自西向东、自下而上逐步增强的趋势,华蓥山北段则成为晚三叠世末重要富煤区,可采煤层主要赋存在须五段和须七段,须五段聚煤环境为三角洲平原分流间湾,聚煤作用相对较差,须七段聚煤环境主要为曲流河泛滥盆地,聚煤作用较好。

The northern segment of Huaying Mountain is located in the backbulge area of Sichuan foreland basin duringthe 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 basedon the data from borehole cores,field outcrops and well loggings. The Xujiahe Formation was subdivided into sevenMembers,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,includingconglomerate,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 bytectonic 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 werebest developed in the episodes of tectonic quiescence and isostatic rebound. The coal-accumulation center migratedgradually from west to east and from bottom to top in the Xujiahe Formation. The study area,Jingang Coal Mine,was animportant coal-accumulation zone in the end of the Late Triassic. The mineable coal seams were mainly developed inthe Members 5 and 7,with the coals in Member 7 being relatively thicker. The coals in Member 5 were deposited inthe interdistributary bay of the delta plain environment and the coals in Member 7 were formed in the back basin offluvial environment.