Upper Triassic sedimentary facies and sedimentary evolution in the northeastern Sichuan foreland basin

The present paper deals with the sedimentary sequences and sedimentary facies types in the Upper Triassic Xujiahe Formation in the northeastern Sichuan foreland basin in terms of field outcrops and well data. Eight types of sedimentary facies are discriminated, including alluvial fan, braided stream, meandering stream, fan delta, braided delta, meandering delta, lake and marine delta facies. The palaeogeography went through the evolution from the marine facies to the transitional facies and continental facies, suggesting the basin-mountain coupling and sedimentary responses in the northeastern Sichuan foreland basin during the Late Triassic. During the deposition of the first member of the Xujiahe Formation (i. e., the Xiaotangzian), the bulk of the study area was uplifted and developed into the continent under the influence of the Indosinian movement. The marine delta deposits were developed only in Guangyuan and Jiange. During the deposition of the second member of the Xujiahe Formation, the highly overthrusting in the Micangshan-Dabashan tectonic zone resulted in the gradual subsidence of the northeastern Sichuan foreland basin. The braided delta deposits were organized in the frontal part of the Micangshan-Dabashan zone. The shallow lake deposits appeared in the southwestern part of the basin. During the deposition of the third member of the Xujiahe Formation, the gradually decrease of tectonism in the MicangshanDabashan zone led to the decrease of sediment supply. The braided delta deposits occurred in the frontal part of the Micangshan-Dabashan zone. The shallow lake deposits prograded from the marginal part to the central part of the basin. Till the deposition of the fourth to sixth members of the Xujiahe Formation, the highly overthrusting and tectonic uplifting in the Micangshan-Dabashan zone permitted the rapid subsidence of the northeastern Sichuan foreland basin, and the sudden increase of abundant sediment supply. From the the marginal part to the central part of the basin, there occurred the alluvial fan, fan delta, braided delta, meandering stream, meandering delta and shallow lake deposits.