Relationship between volcanic activity and enrichment of shale organic matter during the Ordovician-Silurian transition in western Hubei, Southern China

Abstract

It has been disputed whether volcanic activity promotes the enrichment of organic matter in geological periods. In a biostratigraphic framework, this study divided the volcanic eruption periods during the Ordovician-Silurian transition in western Hubei Province based on the longitudinal differentiation of the bentonite development, analyzed the sedimentological and geochemical characteristics of each eruption period, and explored the relationship between the volcanic activity and the organic matter enrichment. The results show that from early to late, the Ordovician-Silurian transition can be divided into 1) a strong eruption period (lower Wufeng) that was mainly dominated by carbonaceous shale deposits (containing both terrigenous silicon and biological silicon, with a medium organic matter content), which corresponded to a stage of strong flexural subsidence; 2) a weak eruption period (Wufeng-Longmaxi transition) that was dominated by organic-rich and biogenic silica-rich siliceous shale deposits, which corresponded to a stage of stable flexural subsidence; 3) a strong eruption period (lower Longmaxi) that was dominated by carbonaceous shale deposits with a medium organic matter content, which involved major changes in the paleogeographic pattern and corresponded to an initial stage of flexural migration wherein the depositional water body became shallow; and 4) a strong eruption period (upper Longmaxi) dominated by organic-poor, terrigenous silica-rich argillaceous shale deposits, which corresponded to a stage of expanded flexural migration wherein the depositional water body became even shallower. The strength of the volcanic activity was closely associated with the differential enrichment of the organic matter. The tectonic activity was stable in the period of weak volcanic activity, which was characterized by low terrigenous inputs, moderate water restriction, and a high paleo-productivity. This was favorable for organic matter enrichment, thereby leading to a high TOC content. During the periods of violent volcanic activity, the Cathaysia Block collided violently with the Yangtze blocks, resulting in more terrigenous input, a higher degree of water restriction, a shallower depositional water body, and a lower paleo-productivity, which had a significant inhibitory effect on the organic matter enrichment. This inhibitory effect was larger than the promotional effect of the volcanic ash on the organic matter enrichment, thereby resulting in a decrease in the TOC content.