Early Cretaceous climatic influences on organic matter enrichment in lacustrine shales: Insights from the Liupanshan Basin of northwestern China

Abstract

Organic-rich petroleum source rocks are widespread in Early Cretaceous strata, but the related paleoclimatic conditions are still uncertain. Aptian-Albian lacustrine sediments from the Liwaxia and Madongshan formations in the Liupanshan Basin of northwestern China provide vital material to reveal correlations between the Early Cretaceous paleoclimate and the formation of organic-rich lacustrine shales. Outcrop samples were analyzed for total organic carbon (TOC), C/N ratio, CaCO3 content, lipid biomarkers, and compound-specific n-alkane carbon isotopes. Results show that organic matter (OM) of the Liwaxia Formation was mainly sourced from lacustrine plankton and deposited in a weakly stratified and suboxic water column under a hot and arid climate. Organic matter in the lower part of the Madongshan Formation was derived from a mixture of aquatic organisms and higher plants under a warm and humid climate; plant carbon sources were widely available, and the water column was strongly saline, stratified, and anoxic. Elevated temperatures and a less humid climate accompanied the deposition of the upper part of the Madongshan Formation. Higher plants were the primary OM source, and the carbon source became limited. The water column was hypersaline and stratified, and the bottom water was less anoxic. Abrupt changes in the OM inputs and depositional environments documented in the Madongshan Formation were ascribed to global climatic variations. A warm and humid climate during the Aptian-Albian transition enhances continental nutrient inputs and promotes the formation of reduced water conditions, which directly controls the formation of the coeval organic-rich shales in lacustrine basins in China.