Characterization of gravity flow deposition of the Lower Triassic Liujiagou Formation in the southwestern Ordos Basin and analysis of its genesis

Through core observation, thin section identification, and X-ray diffraction mineral analysis, the rock types and characteristics, and the genesis were studied. The results show that the core section of Liujiagou Formation develops grayish purple siltstone, rudaceous mixed rock and black mudstone. The siltstone was mainly composed of quartz, rock debris, feldspar, micrite calcite, and matrix, with a matrix-supported fabric, and developed turbidite sedimentary characteristics such as groove casts, graded bedding, flame structures, convolute bedding, and Bouma sequences, identifying it as turbidite. The mixed rock contained a large amount of gravel intraclast and interstitial material, with the latter composed of terrigenous clastic particles, intraclasts, and matrix. It exhibited sedimentary characteristics such as muddy tear-shaped clasts, matrix-supported structure, randomly oriented particles, and abrupt contact with the overlying and underlying mudstones, suggesting it was debris flow deposition. The study suggests that the Mianlue Ocean during the Early Triassic was not completely closed and had a marine transgression impact on the southwestern margin of the Erdos Basin, forming mixed rock strata. Lagoon sediments developed in the Zhengning area, with slopes present, and gravity flow sediments dominated by debris flow-turbidity current were developed at the place of Well C3.