Earliest Cambrian Carbonate Platform Evolution, Environmental Change, and Organic Matter Accumulation in the Northwestern Yangtze Block, South China

The earliest Cambrian (ca., 538.8–524.8 Ma) was an important period in geological history witnessing significant environmental change, during which organic-rich facies were developed in the Yangtze Platform, South China. However, the contemporaneous paleogeographic and stratigraphic framework within which the environmental change and organic matter accumulation took place remains poorly understood. We investigate this based on facies, sequence stratigraphic, and geochemical analyses of the lowermost Cambrian Maidiping and Zhujiaqing formations in the northwestern Yangtze Block. The results show that the terminal Ediacaran rimmed platform changed into a foredeep carbonate ramp and backbulge basin after the onset of the earliest Cambrian transgression. Across the Ediacaran–Cambrian boundary, the shallow-marine redox condition rapidly transitioned from relative euxinia to an oxygen-rich state. During the late transgression to highstand normal regression, the foredeep carbonate ramp expanded to the cratonic interior, and nutrients brought by intensified continental weathering and upwelling promoted significant phytoplankton proliferation, an increase in oxygen level and primary productivity, and then organic matter enrichment. During the forced regression, the carbonate ramp gradually changed into a rimmed platform. The weakening continental weathering and expanding anoxic area during the forced to lowstand normal regression led to the significant organic carbon burial in the foredeep basin.