Geochemistry of the Lower Silurian black shales from the Upper Yangtze Platform, South China: Implications for paleoclimate, provenance, and tectonic setting

Organic-rich shales of the Lower Silurian Longmaxi Formation have produced commercial amounts of natural gas. The nature of the paleoclimate and tectonic setting of the provenance region from which these deposits were derived remains the topic of debate. This paper describes the results of a multi-faceted geochemical investigation of 46 samples of the Longmaxi Formation of the Upper Yangtze Platform. Chemical Index of Alteration (CIA) values of the analyzed samples indicate that parent rock experienced relatively weak to medium chemical weathering prior to deposition. Th/Sc, and Zr/Sc ratios suggest that the sediment shed from the provenance area experienced three kinds of different recycling and sorting trend. Variations of Index of Compositional Variability (ICV) and K2O/Al2O3 ratios suggest that the sediment source shifted over paleogeographic locations. The Al2O3(CaO*+Na2O)-K2O triangular diagram, Th/Sc and Th/Yb ratios, and Al2O3-TiO2, TiO2-Zr, and La/Th-Hf bivariate diagrams suggest that the sediment was primarily derived from felsic and intermediate igneous rocks. High- and low-silica multidimensional diagrams and discriminant function-based multidimensional diagrams indicate an active tectonic setting for the sediments in the south and east of the Upper Yangtze Platform and a passive continental margin setting for the sediments in the north part. Also they illustrate that the sediments in the central and west part were sourced in both an active tectonic setting and a passive continental margin. Variation of multiple geochemical proxies provide new insight into temporal and spatial variations of the provenance, climate, and tectonic setting from which sediment of the Longmaxi Formation were derived.

Automated summary

This study conducted a multidimensional geochemical investigation of the Longmaxi Formation and found that the samples experienced relatively weak to medium chemical weathering, the sediment shed from the provenance area had different recycling and sorting trends, the sediment was primarily derived from igneous rocks, and the tectonic setting varied across different geographic locations.