Sedimentary facies constitutional features and their response to tectonic control of the Upper Triassic strata in the Jingmen-Dangyang basin

Abstract：The Upper Triassic Jiuligang Formation and Wanglongtan Formation constitute a classic clastic sedimentary succession deposited in the foredeep belt of the Jingmen-Dangyang basin. Eight lithofacies, six constitutional elements and five depositional systems can be recognized from the outcrop data. On the basis of sequence boundaries composed mainly of subaerial unconformities, the upper Triassic strata of the Jingmen-Dangyang basin can be subdivided into four sedimentary sequences (TS1, TS2, TS3 and TS4), each one of which was formed during the loading and unloading of orogenic cycles. The Jiuligang Formation is dominated by coal-bearing sedimentary cycles of lakes and swamps, reflecting the condition of relatively inadequate sediment supply and high accommodation rates and an underfilled basin. In contrast, fluvial systems dominate the Wanglongtan Formation characterized by thick-bedded sandstone sedimentary cycles, suggesting adequate sediment supply and low accommodation rates and an overfilled basin during the early-middle deposition period of Wanglongtan Formation. Accommodation rates and sequence stacking are mainly controlled by subsidence rates induced by adjacent orogenic belt. Under the warm and humid climate conditions, depositional systems transformed from low energy lakes and swamps to high energy rivers, and each upward fining sequence indicates terrain gradient and sedimentary media energy gradually decreased in an orogenic cycle.