Geochemistry, Paleoenvironment, and Mechanism of Organic‐Matter Enrichment in the Lower Silurian Longmaxi Formation Shale in the Sichuan Basin, China

To investigate the mechanism of the organic-matter enrichment in the Lower Longmaxi Formation shale, the geochemistry and total organic carbon (TOC) of the Longmaxi Formation black shales in the Jiaoshiba, Zhaotong, and Weiyuan areas of the Sichuan Basin were analyzed. Paleoproductivity proxy parameters (Ba-bio, Si-ex, and Ni/Al), clastic influx proxies (TiO2 and Ti/Al), redox indices (V/Cr, Ni/Co, V/(V+Ni), and U/Th), and hydrothermal indicators (Fe, Mn, and Y concentrations; Fe/Ti ratio and a Ni-Zn-Co diagram) were employed to decipher the paleoenvironment of the Lower Longmaxi Formation shales. TiO2 and Ti/Al indicated low terrigenous detrital influx in all three areas. However, Ba-bio, Si-ex, and Ni/Al indicated high productivity in the Jiaoshiba area. V/Cr, Ni/Co, and U/Th indicated higher oxygen content with larger fluctuations in the Zhaotong and Weiyuan areas. Fe, Mn, and Y concentrations and the Fe/Ti ratio implied greater active hydrothermal activity in the Weiyuan area. These heterogeneities were considered to be closely related to the paleoenvironment and paleogeography, and the large basement faults that developed during the Chuanzhong paleo-uplift could have provided vents for deep-hydrothermal-fluid upwelling. The redox indices (V/Cr, Ni/Co, and U/Th) and a paleoproductivity proxy (Ni/Al) displayed a significant correlation with the TOC, suggesting that both excellent preservation conditions and high paleoproductivity were the controlling factors for the enrichment of organic matter in the Longmaxi Formation shale. There was no obvious correlation between the clastic influx proxy (Ti/Al) and the TOC due to the extremely low supply of terrigenous debris. The hydrothermal indicator (Fe/Ti) was negatively correlated with the TOC in the Weiyuan area, indicating that hydrothermal activity may have played a negative role in the accumulation of organic matter. This study suggests that the enrichment of organic matter in the Longmaxi Formation marine shale varied according to the paleogeography and sedimentary environment.