Multiple geological events controlling rock types development: A case study of Wufeng-Longmaxi formation in southern Sichuan Basin, China

Abstract

During Late Ordovician to Early Silurian, global geological events such as volcanic activity, glaciation, and mass extinction affected sedimentary environments and organic matter (OM) accumulation, and controlled the development of organic-rich shales. Taking the Wufeng-Longmaxi Formation (WF-L Fm) shales in southern Sichuan Basin as the research object, based on the core, thin section and scanning electron microscopy (SEM) observations, combined with X-ray fluorescence (XRF) two-dimensional element distribution maps, X-ray diffraction (XRD), trace elements, carbon isotope composition and total organic carbon (TOC) data, the relationship between geological events and sedimentary environments, OM accumulation as well as rock types development are analyzed, and the sedimentary environments response to geological events is elaborated. Eight rock types have been identified, and the minerals occurrence model has been characterized. By using Cu content, Mo content, δ13C and TOC to measure paleoproductivity, Sr content and Sr/Cu to judge paleoclimate, Sr/Ba to measure paleosalinity, and U/Th, V/Cr, V/Ni, and Ni/Co to analyze redox conditions, the sedimentary environments of the WF-L Fm shales can be furtherly divided into 7 intervals. In general, the deep water hypoxic-anoxic environments from Interval Ⅰ to Interval Ⅴ is conducive to the formation of organic-rich shales. The organic-rich shales formed in the anoxic environments from Interval Ⅰ to Interval Ⅴ are related to the fact that volcanic activities not only release a large amount of nutrients, promote biological prosperity and improve productivity, but also make the water in a sulfide anoxic environment, which is conducive to the preservation of OM. The rapid rise and fall of sea level caused by the Hirnantian glaciation harmed the living environments of organisms, led to the mass extinctions and provided material sources for OM accumulation, resulting in high TOC in the rock types of Interval IV. Irregular gravity flow can enhance terrigenous input in deep water for a short time and dilute OM, which is one of the reasons for the low TOC in the rock types of Interval Ⅵ to Interval VII. Upwelling current enhances nutrient flux in surface water and improves primary productivity of water, thus affecting the development of WF-L Fm organic-rich shales. Based on the aforementioned interpretations, the sedimentary model of the southern Sichuan Basin was established.