Depositional environment and evolution of the upper Ediacaran to lower Cambrian transition black rock series in Northern Guizhou Province, China

Abstract

The black rock series in northern Guizhou within the Yangtze plate is a benchmark deposit in South China and spans the Neoproterozoic to early Palaeozoic transition period. This period was associated with many major global geological events and the Cambrian Explosion, as well as the formation of a metal-rich layer containing Ni, Mo, platinum group element (PGE) and shale gas resources. However, the evolution of the sedimentary characteristics and palaeoenvironmental conditions of this black rock series remain unclear, and the corresponding enrichment mechanism and organic matter model must be established. Our research revealed that the black rock series formed mainly in a continental or oceanic island arc environment and was influenced by hydrothermal activity to varying degrees. Sediment was supplied from terrestrial sources and supplemented with marine hydrothermal material. An anoxic environment occurred in the research area, with a moderately stratified water column. Hence, deposition occurred in a transitional redox environment with relatively smooth circulation. Moreover, the degree of water restriction in northern Guizhou fluctuated between intense and moderate. The pyrite particle size distribution suggests that the formation environment transitioned from sulfidic to anoxic to hypoxic, with early sulfidic-to-suboxic transition and a gradual oxidizing trend from the bottom up. From the lower dolomite to upper black shale, the overall water exchange power decreased, and the salinity index suddenly decreased, indicating that the sedimentary process was impacted by a rapid increase in the relative sea level and deepening of the sedimentary environment. The chemical index of alteration (CIA) and chemical index of weathering (CIW) demonstrate that dolomite and phosphorite formed in dry and hot climates, while black shale and siliceous rocks formed in warm and wet climates. This study revealed the process of organic matter deposition under reducing, anoxic, and sulfidic conditions, clarified the palaeoenvironment and depositional processes, and provided an enrichment model for organic matter in this black rock series.