四川盆地上三叠统须家河组层序格架下的岩相古地理演化

Paleogeographic Evolution of the Upper Triassic Xujiahe Formation within a Sequence Framework in the Sichuan Basin

摘要: 四川盆地须家河组为一套陆相含煤地层，沉积环境为冲积扇、辫状河、曲流河、三角洲和湖泊。须家河组中发育五个层序界面，包括构造不整合面和河流下切侵蚀面两种类型；四川盆地上三叠统须家河组共划分为四个层序，由下至上为层序I、层序II、层序III和层序IV，分别对应于须家河组一段，须家河组二、三段，须家河组四、五段和须家河组六、七段。须家河组发育的岩相古地理单元主要包括冲积扇、河流、三角洲平原和前缘以及滨浅湖。层序I盆地中汇水不深，但范围较广，盆地中部和北部发育有零星的砂坝沉积；层序II和层序III在盆地西部山前地带发育有冲积扇，盆地中河流和三角洲发育，湖泊的位置向西部迁移；层序IV盆地西北部的构造抬升，地层被剥蚀，盆地中发育多个三角洲朵体，湖泊范围缩小沿平行于盆地西缘方向呈带状分布。聚煤中心主要发育在三角洲平原和三角洲前缘的过渡位置。

Abstract: The Xujiahe formation is a succession of continental, coal-bearing strata that is deposited in alluvial fan, braided and meandering rivers, overbank or shallow lacustrine and deltaic environments. Within the Xujiahe formation, five sequence boundaries are proposed based on cores and outcrops including tectonic unconformities and basinal facies-tract dislocations associated with fluvial rejuvenation and incision. Sequence boundaries define third-order sequences (SQ1, 2, 3 and 4) that correspond to Member 1, Members 2 and 3, Members 4 and 5, and Member 6 and 7. paleogeographic units include alluvial fan, fluvial, delta plain and front, and shallow lacustrine. Shallow lacustrine widely developed during SQ1, and small sand bars deposited in the central and northern basin. Alluvial fans developed in front of the fold-and-thrust belt in western basin in SQ2 and SQ3, while fluvial and deltaic units well developed in the basin, and the lacustrine area shifted to the west. Tectonic uplift occurred in SQ4, and the strata were eroded in the northwestern basin. A number of deltaic lobes developed in the basin, lake area shank and present as a belt parallel to the west margin of the basin. Coal accumulation centers mainly developed in the regions along the boundary of delta plain and delta front.