Developmental characteristics and transport mechanisms of the Upper Triassic submarine fans controlled by narrow shelf in the Taohe Basin, West Qinling Orogenic Belt

The Taohe Basin, located in the West Qinling Orogenic Belt (WQOB), exhibits a series of exposed Upper Triassic submarine fan systems controlled by narrow shelf, which records the last-stage deep-marine deposition along the northeastern margin of the Paleo-Tethys Ocean. Using field and laboratory sedimentological analyses of typical outcrops, we determine the sedimentary assemblages, sedimentary units, and genetic mechanisms of submarine fans in the Upper Triassic Nalu Formation in the Lintan-Zhuoni areas. The results indicate that the Upper Triassic submarine fans in the study area contain various gravity flow deposits, which show four sedimentary assemblages: low- and high-density turbidites, hybrid event beds, hyperpycnites, and reflected turbidites. Presently, most studied outcrops of deep-water deposits exhibit the mid-to-distal sedimentary features of submarine fans, with the dominance of various sedimentary units in the lobes (e.g., lobe channels, axial and off-axis units of lobes, and lobe fringes). The major feeder channels and natural levees, however, are largely absent in the submarine fans. The narrow shelf-controlled submarine fans of the Upper Triassic in the Taohe Basin are developed by multiple transport mechanisms. Specifically, slope instability and sediment reworking via littoral currents on the shelf are subject to indirect sediment supply, whereas hyperpycnal flows act as a direct sediment source. The alternation between both mechanisms is intimately associated with the shelf characteristics and high-frequency sea-level functuations. The narrow-shelf setting is closely related to the semi-closed tectonic framework of the Taohe Basin, while frequent submarine landslides have enhanced the ruggedness of the submarine topography. Influenced by the continuous uplift of the Bailongjiang Uplift, the distal deposits of the Upper Triassic submarine fans in the Taohe Basin are locally transformed by reflected currents, leading to the formation of the partially restricted submarine fan deposits with dual genetic characteristics.