Paleoenvironment and Organic Matter Accumulation of the Upper Ordovician-Lower Silurian, in Upper Yangtze Region, South China: Constraints from Multiple Geochemical Proxies

Zisang Huang,Xingzhi Wang,Jingwei Cui,Xiyan Yang,Rukai Zhu,Yiqing Zhu,Wenrui Shi

doi:10.3390/en13040858

Abstract

The origin of the organic-rich shale in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation is complex and controversial. This paper reports the geochemical data of Wufeng-Longmaxi Formations in the Upper Yangtze region to restore the paleoenvironment and explore the accumulation mechanism of organic matter. The total organic carbon (TOC) content of the Wufeng Formation was relatively high, with an average of 2.86%. The Lower Longmaxi Formation showed the highest TOC content, with an average of 3.99%, and the upper part was a continuously low value with an average of 1.22%. The paleoproductivity proxies (Babio, Cu/Al, Ni/Al, Siexcess) showed that in the Katian and Rhuddanian-Aeronian Stages, the Upper Yangtze Sea had high primary productivity, indicating that organic matter accumulation was more affected by terrigenous influx and redox conditions. Al, Zr, and Zr/Al indicated that terrigenous influx was relatively high in the Kaitian-Hirnantian Stages, it was at a constant low in the Rhuddanian Stage, and increased again in the Aeronian Stage. The correlations between redox-sensitive trace elements (Mo, U, V) and TOC indicate that the organic-rich shale of the Wufeng Formation was deposited in the anoxic–euxinic environment. In the Longmaxi Formation, organic-rich shales formed in a more hypoxic environment, and overlying organic-lean shales formed in a suboxic environment. Therefore, the anoxic–euxinic conditions of the Late Ordovician Yangtze Sea were the main reason for the organic matter accumulation, but the high terrigenous influx caused by regression and/or structural controls diluted the organic matter to some extent. For the Early Silurian, a complete transgression–regression cycle changed terrigenous influx and redox conditions, resulting in significant differences in organic matter accumulation.

Highlights

In specific geological periods and sedimentary environments, the Earth has formed organic-rich shales with strong heterogeneity [1]
Environmental evolution has a great influence on the geochemical elements of shale [11,12], which provides a feasible method for reconstructing the paleoenvironment and exploring the accumulation mechanism of organic matter
This was because W201 was close to the Chuanzhong uplift and was in a relatively shallow position of the water mass, which was vulnerable to terrigenous materials and the turbulence of seasonal water mass, resulting in an unstable variation trend of redox-sensitive elements (RSTEs) concentration

Summary

Introduction

In specific geological periods and sedimentary environments, the Earth has formed organic-rich shales with strong heterogeneity (mineral composition, geochemical elements, organic content, etc.) [1]. Previous studies have shown that factors affecting organic matter accumulation may include sea level rising, anoxic events, plankton proliferation, and seawater stratification [4,5,6]. Recent studies have shown that the correlations of MoEF -UEF and redox-sensitive trace elements-total organic carbon can further reflect the restriction degree and redox conditions [11,13]. Elements such as Ba, Cu, and Ni are closely related to the flourish degree of plankton in the ocean, and can be used to indicate the strength of primary productivity [13,15,16]

Results

Discussion

Conclusion