Geochemical characteristics and organic matter enrichment mechanism of black shale in the Upper Triassic Xujiahe Formation in the Sichuan basin: Implications for paleoweathering, provenance and tectonic setting

The first, third and fifth members (henceforth referred to as T3x1, T3x3 and T3x5, respectively) of the Upper Triassic coal-bearing Xujiahe Formation black shale are among the most significant hydrocarbon source rocks in the Sichuan basin. Here, we present geochemical data for the Upper Triassic black shales from core from Well LD-1 to determine their paleoenvironmental conditions, paleoweathering, provenance transitions and tectonic setting. The V/(V + Ni) vs. U/Th, V/Cr vs. U/Th and Ni/Co vs. U/Th bivariate plots and the TFe-TOC-TS (total Fe-total organic carbon-total Sulphur) ternary diagram indicate that the synsedimentary redox regime of almost all the shale samples was oxidizing. Ba/Al, Sr/Al and P/Ti data combined with quantitative biogenic Ba data indicate that moderate-high primary paleoproductivity levels prevailed during deposition. The Sr/Cu ratio and C-value combined with the sedimentary features are indicative of warm-humid climate conditions. The Ti/Al deposition rate proxy and decompacted sedimentation rate are positively correlated with the total organic carbon (TOC) content. According to our multiproxy approach, black shale development in the Xujiahe Formation was mainly controlled by the primary productivity level, foreland basin setting, high tectonic subsidence and sedimentation rate and exhibited a limited correlation with water column redox conditions. Through the compilation and calculation of various weathering indices for the Xujiahe black shale, we suggest that caution must be taken when inferring paleoclimate characteristics based on the chemical index of alteration (CIA) because of the influence of multiple nonweathering factors. Provenance-sensitive elemental ratios (Th/Sc vs. Zr/Sc and Co/Th vs. La/Sc) indicate that the clastic contribution to Xujiahe black shales was of a predominantly felsic character. The black shales exhibit a transition from T3x1 to T3x3 to T3x5, whereby T3x1 was primarily sourced from the Proterozoic to early Paleozoic strata of the Qinling orogeny, T3x3 was primarily sourced from the Neoproterozoic complex in the Longmen Shan, and T3x5 was primarily sourced from the Songpan-Ganzi flysch strata, which contributed recycled material because of folding and strong southeastward thrusting.