Geochemical characteristics of marine shale in the Wufeng Formation–Longmaxi Formation in the northern Sichuan Basin, South China and its implications for depositional controls on organic matter

Abstract

Marine black shales of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in the Sichuan Basin are considered as excellent source rocks in South China. According to the geochemical characteristics of source rocks, the formation and preservation conditions of organic matter in source rocks are showed. In order to study the aggregation mechanism of organic matter, geochemical proxies were proposed, including redox proxies (V, V/Al, U, U/Al, Mo, and Mo/Al), paleoproductivity proxies (P, P/Ti, Ba, Ba/Al, and SiXS), paleoclimate proxies (CIA), and terrigenous flux proxies (Al, Zr, and Zr/Al). At present, the average content of TOC in Wufeng Formation is 3.63 wt%, which is in the range of 2.59–4.22 wt%. The average content of TOC in Longmaxi Formation is 1.02 wt%, and ranges from 0.45 to 2.07 wt%. The redox proxies show that the Wufeng shale is mainly accumulated under anoxic conditions. Longmaxi shale is mainly accumulated under suboxic-oxic conditions. The P/Ti and excess silicon (SiXS) values indicated that the whole sedimentary sequence had high paleoproductivity, but it was reduced to some extent in the Longmaxi Formation. The reduced Corg/Ptot ratios implies that the decrease of productivity may be caused by the increase of P (phosphorus) burial. The restricted degree of watermass shows that Wufeng shale deposits under strongly restricted watermass, while Longmaxi shale deposits under weakly restricted watermass. During the sedimentary period of the Wufeng Formation, the strongly restricted of the ocean circulation of the northern Sichuan Basin and the effective stratification of water column is beneficial to the preservation of organic matter. During the Longmaxi shale sedimentary period, tectonic movement and global sea level changes controlled the production and preservation of organic matter, affecting terrestrial clastic material flux, redox conditions and nutrient cycling.