Depositional Environment and Organic Matter Accumulation Mechanism along the Southwestern Marginal Yangtze Platform (South China) during the Middle Devonian: Implications for Low Content of Organic Matter

Abstract

To determine the sedimentary environment of the black shales in the Middle Devonian Huohong Formation in the southwestern marginal Yangtze Platform and analyze the controlling factors of organic matter accumulation, this study selects the black shales from the southwestern marginal Yangtze Platform and conducts comprehensive analyses including total organic carbon (TOC), major and trace element. The TOC range from 0.26% to 1.27%, averaging 0.53%, indicating a low organic matter content. Redox-sensitive and paleowater depth indicator elements (Mo < 5 ppm, Co < 40 ppm, Cu < 90 ppm, Ba < 1000 ppm) suggest that relatively shallow depositional water depths. Redox indicators (U/Th < 0.75, V/Cr < 2.0, Ni/Co < 5) suggest that the oxic sedimentary environment. The Mo/TOC < 4.5 reveals strong water restriction, while paleosalinity indicators (S/TOC > 0.5) reveals a saline-brackish water environment. The Lower Ti/Al (0.007–0.012) implies limited terrigenous clastic input, and the CIA values (46.40–72.85) indicate a paleoclimatic shift from warm-humid to cold-arid conditions in the provenance area. The paleoproductivity indicators P/Ti < 0.34 and biogenic Ba (Ba_bio) < 200 ppm indicates low paleoproductivity. The (La/Yb)_N far from 1 reflects a relatively low sedimentation rate. The Al–Fe–Mn ternary diagram and Al/(Al + Fe + Mn) > 0.4 and (Fe + Mn)/Ti < 15 confirm the absence of significant hydrothermal activity during deposition. The low organic matter content is mainly controlled by two factors: the oxic environment is unfavorable for organic preservation, and insufficient nutrient supply resulted in low paleoproductivity, limiting organic input. Additionally, slow sedimentation enhanced organic consumption in sediments. This study established an organic matter accumulation model for the Middle Devonian Huohong Formation shales and concludes that the depositional conditions were unfavorable for organic enrichment.