Upwelling-induced organic matter enrichment of the Upper Permian Dalong Formation in the Sichuan Basin, SW China and its paleoenvironmental implications

摘要

The organic-rich siliceous rock was developed worldwide during the Late Permian which contains plenty of information about paleoenvironment. This study reveals the vertical variation in organic matter enrichment of the Upper Permian Dalong Formation in the northern Sichuan Basin, which was related to the expansion of the Paleo-Tethys Ocean induced transgression and upwelling in the northern continental margin of the Upper Yangtze. The Dalong Formation was divided into three units from bottom to top on the basis of lithology and organic matter abundance: the lower calcareous mudstone (Unit A), the middle organic-rich siliceous shales (Unit B), and the upper marlstones (Unit C), with average TOC contents of 4.09%, 6.72%, and 1.20%, respectively. The Unit A was deposited at the early stage of the transgression under seasonal upwelling settings with moderate paleoproductivity evidenced by the moderate-high Cd/Mo ratios and concentration of P, Cu and Zn. The Unit B shows extremely high Cd/Mo ratios and Ni concentration indicating the perennial upwelling and high paleoproductivity induced by the extensive transgression. The Unit C was deposited under similar hydrographic conditions with the Unit A but lower paleoproductivity due to the effect of the latest Permian mass extinction. In addition, the redox conditions varied with the paleoproductivity and positive correlations between P and redox proxies (e.g. V_(EF), N_(EF), Zn_(EF) and Cu_(EF)) were observed (the Unit A: moderate paleoproductivity and suboxic-anoxic conditions, the Unit B: high productivity and anoxic-euxinic conditions, the Unit C: low-moderate productivity and oxic-suboxic conditions), which indicate that nutrients brought to primary producers by the upwelling during the transgression stimulate blooms and accelerate the consumption of oxygen leading to the anoxic-euxinic bottom water conditions. During the transgression, differences in types and strength of the upwelling result in the variation of paleoproductivity and redox conditions, and thus the organic matter abundance.