Geochemistry of marine black shale of Cambrian Qiongzhusi formation, Yangtze plate, southwestern China: implications for provenance and paleoweathering

Abstract

Marine black shale in the Lower Cambrian Qiongzhusi (QZS) Formation in the southwestern Yangtze Plate, SW China, is the primary focus for shale gas development. The formation of organic-rich shale is significantly influenced by the paleoenvironment. Through the analysis of major, trace and rare earth elements (REE), the authors examined the element composition, paleoweathering and provenance of Qiongzhusi black shale (QZS shale). The results revealed that the main components of the Qiongzhusi Formation sample are SiO2, Al2O3 and total Fe2O3 (TFe2O3), with the average values of 64.08 wt%, 15.00 wt% and 5.39 wt%, respectively. Redox-sensitive elements, such as V, Cr, Ni, Zn and U, are richer in QZS shale contrasted with the upper continental crust (UCC). The total concentration of REE (∑REE) of QZS shale is 174.58 ppm on average, which is higher than that of UCC (average 146.37 ppm) and the North American Shale Composite (NASC) (average 173.21 ppm). The ratios of w(SiO2)/w(Al2O3) and w(Al2O3)/w(TiO2), the Zr-TiO2 diagram, the Th/Sc vs Zr/Sc plot, the discriminant function of F1 vs F2 and F3 vs F4, as well as the discrimination diagram of ∑REE vs La/Yb indicated that the main provenances of QZS shale are sedimentary and felsic igneous rocks. The values of a chemical weathering index, the chemical index of alteration (CIA), of the Lower QZS Formation (Stage 1) range from 51.84 to 64.33, indicating a low degree of chemical weathering and a cold and dry climate. The CIA values of the Upper QZS Formation (Stage 2) range from 66.58 to 82.42, suggesting a medium degree of chemical weathering, lilely in a humid climate.