Source of silica and its implications for organic matter enrichment in the Upper Ordovician-Lower Silurian black shale in western Hubei Province, China: Insights from geochemical and petrological analysis

Abstract

To improve the understanding of the relationship between silica source and organic matter accumulation, the origin of silica and its implications for organic matter enrichment in the Upper Ordovician-Lower Silurian (O 3 w -S 1 l ) black shale in western Hubei Province in the middle Yangtze area, were investigated through geochemical and petrological analyses. The results show that the O 3 w -S 1 l black shale is mainly composed of five lithofacies with varying graptolite abundance, total organic carbon (TOC), and silica contents. Biogenic silica and terrigenous siliciclastic input constitute the main silica sources in O 3 w -S 1 l black shale and they exhibit an upward inverse variation trend interpreted to be related to sea-level changes. Moreover, with the increase in biogenic silica content or decrease in terrigenous siliciclastic input, TOC values in black shale initially rise and then fall, which is different from originally expected simple linear relationship. We infer that organic matter enrichment and the distribution of silica from different origins are controlled by sea-level changes and variations in terrigenous input in a continental shelf with little hydrothermal influence. An environment with appropriate sea level and terrigenous input should be most favorable for organic matter accumulation, rather than one with excessive high sea level and less terrigenous input.