Sedimentary environment evolution and organic matter enrichment mechanisms of the cambrian Qiongzhusi Formation in the southwestern Yangtze Block

The Lower Cambrian black shale in the southwestern margin of the Yangtze Block not only records the early life explosion and paleoclimate fluctuation events, but also contains abundant shale gas resources. However, due to the influence of tectonic-sedimentary differentiation, the sedimentary paleoenvironment and organic matter enrichment mechanism in this area are not completely clear, which restricts the effective exploration and development of shale gas. Based on regional drilling data and analysis of major, trace, and rare earth elements from profile samples across different facies belts, this study determines variations in marine depositional conditions and organic matter (OM) enrichment mechanisms during the Early Cambrian. The results show that: (1) In the study area, the depositional setting changes from a continental margin in the west to a continental island arc toward the east. The detritus was derived mainly from the Kangdian Oldland. The paleoclimate shifted from dry and cold to warmer and more humid conditions, with moderate chemical weathering. During the deposition of the first Member of Qiongzhusi Formation(Q1), Hydrothermal activity was intense in the eastern area and decreased gradually to the west. (2) The depositional environment of the Q1 on the southwestern margin of the Yangtze Block evolved from restricted and anoxic in the west to weakly-restricted and anoxic in the east. During the subsequent deposition of the Second member of Qiongzhusi Formation (Q2), a marine regression transformed the basin. This shift resulted in predominantly oxic conditions, more open marine circulation, and a relative increase in terrigenous sediment supply. Dysoxic conditions persisted only locally within the trough valley. (3) Primary productivity was higher in Q1 than in Q2, with the deep-water trough and hydrothermal zones providing ample nutrients for OM enrichment. (4) In Q1, OM enrichment in the western provenance-influenced area was redox-controlled (preservation model), while in the eastern upwelling zone, increased nutrient input led to productivity-dominated control (productivity model). High-quality source rocks in the trough were co-controlled by high productivity and anoxic conditions: transgression brought nutrient input and formed anoxic bottom water, jointly promoting OM enrichment. In Q2, the preservation model dominated. These findings provide a scientific basis for selecting favorable targets for shale gas exploration.