Marinoan glacial aftermath in South China: Paleo-environmental evolution and organic carbon accumulation in the Doushantuo shales

Abstract

The paleo-environment and organic matter accumulation after the Marinoan “Snowball Earth” event is of great significance to understand the Earth system and petroleum geology during this critical period. However, the present studies in South China are not well constrained due to the lack of continuous sedimentary records. Here, we present an integrated petrological and geochemical study of a unique, continuous, and complete outcrop in the northeastern Sichuan Basin, SW China. Results show that the postglacial Doushantuo shales in the study area can be divided into three intervals (from bottom to top: Unit1–Unit2–Unit3) according to geochemistry evolutionary trend. Multiple redox proxies (VEF, UEF, MoEF, Corg/P, Th/U, and UMo covariation pattern) indicate the bottom redox condition changed from oxic to suboxic, and back to weakly oxic stratigraphically. Babio and Ba/Al paleoproductivity proxies suggest that the primary productivity was middle–high and it was slightly higher in Unit 2. The CIAcorr values of Doushantuo shales range from 72.2 to 86.8 and gradually increase from Unit 1 to Unit 3, indicating relatively intense weathering conditions and global warm and humid paleoclimatic conditions. The Doushantuo shales in our studied outcrop is approximately 100 m in thickness. They are at over mature stage with average random reflectance of 1.86–2.14%, which makes the recovered original organic matter abundance up to ~2.0 wt%. The kerogen of the shales is type II. The accumulation of organic matter was mainly controlled by the preservation environments, with the primary productivity level as a secondary control. These data were further compared with other outcrops reported in previous studies for a comprehensive understanding. The variation of organic carbon contents in the Doushantuo shales hold similar features across the entire South China, with the middle part producing the highest-quality source rocks with conventional and unconventional hydrocarbon geological significance. This suggests that changes in sea level and paleo-weathering after the Marinoan glaciation controlled the enrichment of organic carbon in shales. The enrichment is a large-scale geological event across basins rather than a local one. This might be general and referenced for the postglacial and/or interglacial shales.