Organic matter accumulation mechanisms in the Wufeng-Longmaxi shales in western Hubei Province, China and paleogeographic implications for the uplift of the Hunan-Hubei Submarine high

Abstract

Organic matter accumulation exhibits different distribution patterns in time and space and tends to be controlled by many factors. This study investigated the mechanisms that control the accumulation of organic matter in the Xianfeng area of western Hubei Province, South China through sedimentological and geochemical analyses of wellbore samples. The results showed that the total organic carbon (TOC) content, mineral content, and elemental geochemical data of the Wufeng-Longmaxi Formations varied significantly with depth. Based on the sharp changes in the vertical profiles of the TOC content, Gamma-ray, and numerous geochemical parameters, as well as regional stratigraphy, this study divided the section from the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation into six units (W1, W2, GYQ, L1, L2, and L3). The TOC content was positively correlated with redox proxies (DOPT, VEF, and MoEF) and primary productivity proxies (CuEF and NiEF), and negatively correlated with terrigenous input proxies (Al, K, and Ti). The correlation coefficients suggest that the primary factors affecting organic matter accumulation are redox conditions and paleoproductivity, while terrigenous input is a secondary controlling factor. This study proposed an organic matter accumulation model based on the organic matter characteristics, lithofacies, sedimentary environment, and geological setting of the study area. During the deposition of Unit W1, organic matter was well-preserved owing to the anoxic bottom water environment resulting from strong water mass restriction. During the deposition of Unit W2, the rising sea level led to the development of anoxic-euxinic bottom water conditions, and the joint influence of nutrients from upwelling in the open sea and volcanic eruptions enhanced productivity, resulting in the most intense accumulation of organic matter. Unit GYQ had low TOC content due to low productivity despite anoxic conditions. During the deposition of Unit L1, the uplift of the Hunan-Hubei Submarine High caused the depocenter to shift westward and the sea level in this area to drop, resulting in suboxic bottom water and relatively low productivity in the surface water layer. During the deposition of Unit L2, the regional sea level drop resulted in increased input and dilution of terrigenous clasts, thus leading to low TOC content. During the deposition of Unit L3, continuously enhanced terrigenous input due to climate warming and sea level drop became the major factor controlling the organic matter accumulation. In summary, the main factors controlling the organic matter accumulation differed significantly at different evolutionary stages. Compared with the Jiaoshiba area, the Xianfeng area in western Hubei experienced continuously increased terrigenous input, a less reductive water mass, a significant decrease TOC content, and a westward shift of the depocenter since the early stage of the Longmaxi Formation deposition. These changes may be related to the intense uplift of the Hunan-Hubei Submarine High triggered by the Kwangsian Orogeny. These results provide important implications for the understanding of the accumulation of organic matter in the Wufeng-Longmaxi Formations and paleogeographic changes.