Controls on organic matter accumulation in marine mudstones from the Lower Permian Zhanjin Formation of the Qiangtang Basin (Tibet), eastern Tethys

Abstract

Organic-rich mudstones in the Lower Permian Zhanjin Formation may be primary source rocks in the Qiangtang Basin. However, the factors controlling organic matter accumulation remain unclear. Based on total organic carbon (TOC), geochemical, and mineral data, the paleoclimate, redox conditions, paleosalinity, hydrothermal activity, and primary productivity of the Zhanjin Formation mudstones (well QZ-5) were studied to discuss the organic matter accumulation mechanism. The Zhanjin Formation mudstones at 688-675 m and 672–622.8 m have higher TOC contents (avg. 1.52%) than those (avg. 0.94%) at 674-673 m and 662–659.5 m. Medium chemical index of alteration (CIA) values of samples at 688-675 m and 672-668 m indicate a warm and humid climate, whereas those at 674-673 m and 666–659.5 m indicate rapid cooling due to low CIA values. High Sr/Ba and S/TOC ratios indicate a high-salinity water column during the Zhanjin Formation mudstones deposition. Fluctuating UEF and MoEF values and low to medium Th/U, V/Cr, and Corg/P ratios suggest that these studied mudstones formed in an oxic to dysoxic environment. Relatively low Al/(Al + Fe + Mn) ratios, positive Eu anomalies, and Fe–Mn-(Cu + Co + Ni) × 10 and Co–Zn–Ni ternary diagrams indicate that these mudstones were influenced by atypical (weak) hydrothermal activity associated with relatively high femic terrigenous detrital influx. Relatively high P/Al ratios reveal that the level of primary productivity in the surficial water column was relatively high, but high Ni/Al and Cu/Al ratios may be influenced by paleoclimate and hydrothermal activity. Good correlations among the TOC concentrations and paleoclimate, detrital influx, and hydrothermal activity indicators combined with poor correlations between TOC and primary productivity, paleosalinity and redox proxies demonstrate that organic matter accumulation in the Zhanjin Formation mudstones was mainly controlled by detrital influx (adsorption of clay minerals) and hydrothermal activity, but not by primary productivity levels and preservation conditions such as paleosalinity and redox conditions.