Mechanisms of organic matter accumulation in marine clear water conditions: The Upper Permian Dalong Formation, Sichuan Basin, China

Abstract

The Late Permian to Early Triassic marked a pivotal phase in paleoenvironmental and tectonic shifts. The Kaijiang-Liangping intracratonic sag, a tectonic geomorphology formed by Emei taphrogenesis in the Sichuan Basin, is situated within a clear water carbonate platform. Under these conditions, the black shales of the Dalong Formation in the sag have ultrahigh organic matter content. However, the mechanism by which these conditions control the accumulation of such organic matter remains unclear. Petrological and geochemical analyses of well DY-1H revealed four distinct units within the Dalong Formation: A, B, C, and D, with average total organic carbon contents of 3.00%, 9.59%, 4.57%, and 0.27%, respectively. The kerogen maceral, carbon isotope, and pyrolysis results show that the organic matter mainly comprises Type Ⅱ₂ kerogen. Benthic plants growing in clear water may be the primary source of this kerogen. Shallow water, suboxic conditions, strong volcanism, and high productivity characterize Unit A. Unit B features restricted ocean circulation, anoxic conditions, weak upwelling, moderate volcanism, and high productivity. Unit C is characterized by anoxic conditions, strong upwelling, weak volcanism, and moderate productivity. Oxic conditions and low productivity define Unit D. These findings challenge traditional models that struggle to explain the accumulation of ultrahigh organic matter in Unit A under suboxic conditions, Unit C under moderate productivity, and Unit B with abnormally high organic matter content. The flourishing of benthic plants, a considerable source of Type II₂ kerogen that resists decomposition and favors preservation, is the dominant factor controlling the ultrahigh organic matter accumulation of black shales in Units A, B, and C under clear water conditions. Oxidized bottom waters and decreased benthic plant growth were crucial to the sharp decline in organic matter.