Lithofacies of the Upper Middle Triassic Guanggaishan Formation in Both Sides of the Bailongjiang Uplift, Southwestern Qinling Orogenic Belt: Implications to the Basin Analysis

The lithofacies of the upper Middle Triassic Guanggaishan Formation outcropping along both northern and southern sides of the Bailongjiang uplift (composite anticline), southwestern Qinling orogenic belt were studied in this article. By sedimentation and lithofacies, the Guanggaishan Formation is identified and classified into five types: debris flow, grain flow, turbidity current, traction current, and still-water bedding current, which can be subdivided into more sublithofacies in this paper. Nine cycles of lithofacies sequence have been recognized from the Feixian section, Diebu in northern side of the Bailongjiang uplift, of which a single cycle is composed of gavel debris facies to Bouma sequence, indicating cyclicity and periodicity of tectonism and sedimentation. It is proposed that there are some similarities and discrepancies for lithofacies in two sides of the Bailongjiang uplift. The similarities are well-developed slope debris and turbidity flow facies with little slumping and collapsed facies, and proximal sources comprising limestone. The discrepancies are different texture and cycle number of facies sequence from different sections, different aged strata in source regions. The assemblage of lithofacies indicates that the Guanggaishan Formation is of relatively ramp continental slope facies within remnant sea basin, where the basin had been formed by flexing during continent collision. According to deep sea basins and remnant basin in the Late Paleozoic-Early Middle Triassic for the Taohe basin, north to the Bailongjiang uplift, and to the sharing of the same basin basement for the Ruo'ergai flysch basin, south to the uplift, it is inferred that there could be no shallow platform carbonate facies under the both flysch basins, which means poor prospective for oil-gas resources exploration in the region.