Sedimentary paleoenvironment and its control on organic matter enrichment of black shale in Lower Silurian Longmaxi Formation on western margin of Weiyuan area, southern Sichuan Basin

Current research on sedimentary paleoenvironments and the main factors controlling organic matter enrichment in the southern Sichuan Basin primarily focuses on the Luzhou subsag area, while studies on the Weiyuan area near the Leshan-Longnüsi paleo-uplift are relatively scarce. This has led to an unclear understanding of the main controlling factors of organic matter enrichment in the area. Accordingly, through a comparative analysis of the sedimentological and organic geochemical characteristics of the Lower Silurian Longmaxi Formation on the western margin of the Weiyuan area, the main controlling factors and formation models of organic matter enrichment were explored. The results indicated that during the LM1-LM5 sedimentary periods (Rhuddanian), the basin experienced flexural subsidence. Tectonic activity was stable, sea level was at its highest, terrigenous clastic input was low, and the water body was predominantly anoxic. Preservation conditions, terrigenous clastic input, and paleoproductivity collectively resulted in the highest organic matter content in this sedimentary period. In the LM6 sedimentary period (early Aeronian), the basin entered a flexural-migration phase. Tectonic activity intensified, the barrier in the passive continental margin area of northern Chongqing began to open, ocean current activity strengthened, and the bottom-water environment was mainly anoxic to dysoxic. The deterioration of preservation conditions and the increased paleoproductivity were the main factors leading to only minor difference in total organic carbon (TOC) content compared with the LM1-LM5 sedimentary periods. During the LM7 sedimentary period (middle Aeronian), the barrier continued to open, sea level dropped further, bottom-water reducibility became worsened, and ocean current activity continued to intensity. The deterioration of preservation conditions and increased terrigenous clastic input were the main factors causing the decrease in organic matter content. During the LM8 sedimentary period (late Aeronian), ocean current activity reached its peak and paleoproductivity rose to its highest level. However, the bottom-water environment became dysoxic to oxic. The deterioration of preservation conditions was the main factor causing the organic matter content to drop to its lowest level. It is concluded that the continuous decrease in organic matter content within the study area is closely related to the sedimentary paleoenvironment evolution process, which was primarily controlled by paleogeomorphology and sea-level fluctuations.