Depositional Environment as Main Controlling Factor for Low TOC Sediments in the Early Carboniferous Dawuba Formation of the Qiannan Depression

The evolution of the sedimentary environment in the Early Carboniferous Dawuba Formation of the Qiannan Depression significantly controlled the distribution of low-total organic carbon (TOC) sediments. In this study, the core samples were analyzed by thin section microscopy, field emission-scanning electron microscopy, pyrite morphology, X-ray diffraction, and geochemical analysis (TOC, sulfur, organic petrography, and major and trace elements). The formation is vertically divided into two members from bottom to top: Member 1 (average TOC = 1.15%) and Member 2 (average TOC = 0.88%). Depositional environment parameters indicate that Member 1 was in a suboxic-oxic transition environment, with weak detrital influx, and moderate paleoproductivity (more developed algae). Member 2 evolved into a stable oxic environment, with significantly enhanced detrital influx and reduced paleoproductivity. The correlations between multiple geochemical proxies (paleoredox, paleoproductivity, and terrestrial detrital influx) and TOC content indicate that high productivity in Member 1 was the main driver of organic matter accumulation, but the suboxic-oxic environment limited preservation efficiency (1.00% < TOC < 2.00%). Member 2, deposited during sea-level fall, experienced long-term oxic conditions and low productivity due to shallower water. Nevertheless, the partial reduction in the exposure time of organic matter within the oxic water column-driven by rapid detrital accumulation-represents a critical mechanism favoring organic-poor sediments (TOC < 1.00%). In conclusion, the development of low-TOC sediments in the Dawuba Formation reflects a transition from a relatively deep to shallow water column, where the synergistic effects of redox conditions, paleoproductivity, and terrigenous detrital influx controlled the distribution and enrichment of organic matter.