The depositional mechanism of organic-rich siliceous shales in Upper Yangtze area: Response to the Kwangsian Orogeny in South China

Abstract

The organic-rich siliceous shale in Upper Ordovician is the major source rock for shale gas in the South China, while the deposition mechanism of these rocks is still under debated. Here we present systematic geochemical and petrographic characteristics of the Upper Ordovician sediments to evaluate the redox conditions, paleo-productivity, terrestrial input, and to investigate the depositional mechanisms of these organic rich siliceous shales in the Wufeng Formation. In shallow water areas, the Linxiang Formation is dominantly characterized by grey thick Limestone. The Upper Daduhe Formation is composed of calcareous mudstone intercalated with argillaceous limestone, while the Lower Daduhe Formation consists of limestone interbedded with mudstone. At deep water area, the Linxiang Formation is dominated by limestone and muddy limestone. The lower and upper Wufeng Formation are mainly characterized by grey argillaceous shale and organic-rich siliceous shale respectively. It is suggested that the lithological and environmental changes in the Yangtze area were the response to the development of the Kwangsian Orogeny. During the late Ordovician, the Yangtze basement constantly descended and the sea level rose relatively due to the collision between the Yangtze and Cathaysia blocks. Thus, the oxygen concentration decreased with the rising sea level according to the UEF values and Ce/Ce* ratios, which resulted in anoxic water condition and promoted the preservation and enrichment of organic matter in marine sediments. Meanwhile, uplifts (e.g., Jiangnan - Xuefeng, Qianzhong and Chuanzhong) constantly expanded around the Sichuan Basin, leading to high terrigenous input that is reflected in geochemical and Nd isotopic compositions. Additionally, primary productivity gradually increased, because the Si/Al ratios exhibit an increasing trend in both shallow and deep water areas, and it may be related to the high clastic influx which provided adequate nutrients for siliceous organisms. Overall, anoxic water condition and high biogenic Si influx caused by Kwangsian Orogeny are considered as the most important factors controlling the organic-rich siliceous shale in the Wufeng Formation.