Provenance, sedimentary paleoenvironment and organic matter accumulation mechanisms in shales from the Lower Cambrian Qiongzhusi Formation, SW Yangtze Block, China

Sulfur is a critical participant in biogeochemical cycles and in the deposition and preservation of organic carbon, and sulfur compounds and isotope ratios can indicate paleoenvironments. In this study, the major elements, trace and rare earth elements, pyrite morphology, sulfur isotope ratios and detrital zircon U-Pb age dating of shales was performed on shale samples from the Lower Cambrian Qiongzhusi Formation in the Yangtze Block to determine the Early Cambrian marine depositional conditions, the provenance of the shale, and the mechanisms of organic matter (OM) enrichment. The results show that: 1) The source rocks in the sedimentary system were deposited in a continental island arc environment, and originated mainly from the felsic magmatic rocks (such as monzonitic granite) and from tuffaceous rocks in the ancient Kangdian emergent block. 2) Due to the moderate degree of chemical weathering and a generally semi-arid to semi-humid climate, the input of Terrestrial clastics was stable, and the OM dilution effect by Terrestrial clastics was weak; 3) The input of nutrients and the widespread occurrence of hydrothermal activities led to the eutrophication of surface seawater, resulting in high degree of primary productivity; 4) In the restricted sedimentary system, the anoxic-sulfidic bottom water enhanced the enrichment of OM in the Ziyang area, while the bottom water environment was dysoxic with bioturbated sediments and limited preservation of OM in Jingyan area. The lateral heterogeneity of the total organic carbon content in the shale of this formation was greatly affected by changes in OM preservation conditions. These results provide an important basis for a reconstruction of the sedimentary tectonic environment of the Early Cambrian Yangtze Block for a potential shale gas development.