Geochemical characteristics in the Longmaxi Formation (Early Silurian) of South China: Implications for organic matter accumulation

Abstract

The organic-rich Early Silurian sediments (Longmaxi Formation) on the Yangtze platform are considered to be one of the main source rocks in South China. In order to investigate the mechanism of organic accumulation, multiple geochemical proxies, including redox parameters (S/C and trace-element ratios), productivity indices (P and Ba contents) and clastic influx indicator (Ti/Al ratios), are presented here from the Hehua section, Yichang, Hubei province. The Longmaxi sediments have high TOC contents (3.00–10.85 ％, avg. 5.63 ％), low Pyrite sulfur contents (0.42–1.14 ％, avg. 0.59 ％), high P (0.02–0.44 ％, avg. 0.12 ％) and Ba contents (576.65–2542.43 ppm, avg. 1592.17 ppm). Calculated S/C, Th/U, Ni/Co, V/Cr and V/(V + Ni) ratios are 0.05–0.24 (avg. 0.12), 0.23–9.86 (avg. 2.30), 2.65–46.24 (avg. 15.46), 0.27–14.42 (avg. 3.75) and 0.36–0.96 (avg. 0.75), respectively. S/C and trace–element ratios, together with pyrite morphology, suggest dysoxic/anoxic conditions prevailed during deposition of the Longmaxi intervals, which was mainly caused by a worldwide marine transgression after the melting of Gondwana glaciation. Productivity proxies indicate that the organic-rich Longmaxi sediments were deposited in moderate to high primary productivity water columns. The consistent Ti/Al ratios suggest a rather homogeneous nature for the detrital supply. Geochemical data provide a good constraint on the accumulation of organic matter and relevant oceanic environment. Anoxic conditions, together with relatively high productivity, were likely responsible for the accumulation of organic-rich sediments during the Longmaxi interval.