Main factors controlling the sedimentation of high-quality shale in the Wufeng–Longmaxi Fm, Upper Yangtze region

Abstract

In this paper, the shale of Upper Ordovician Wufeng Fm–Lower Silurian Longmaxi Fm was taken as an example to reveal the distribution patterns and the main sedimentation controlling factors of high-quality shale in the Upper Yangtze region. This study was made from the aspects of plate movement, fluctuation of sea level, palaeo-productivity, deposition rate and paleogeographic environment, based on the field outcrop sections and drilling data of the southern Sichuan Basin, together with geochemical element testing and biostratigraphical analysis results. The following findings were obtained. First, the collision and joint of Yangtze Plate and its periphery plates and the intra-plate deformation were gentle in the early stage, strong in the late stage, gentle in northwest and strong in southeast, so the sedimentation center in the Upper Yangtze region migrated to the northwest and the closure of sea area in the southern Sichuan Basin changed from weak in early stages to strong in late stages. Second, at the turning period from Ordovician to Silurian, sea levels presented the cycle change of deep–shallow–deep–shallow. Due to the combination of high sea levels and a stable ocean basin in early stages, an extensive anoxic tectonic sedimentary space favorable for organic matters preservation was formed in the sea floor. Third, due to the effect of tectonic movements and sea closure, palaeo-productivity of sea areas in the southern Sichuan Basin presented a trend of being high in the early stage and low in the late stage, and the deposition rate was also low in the early stage and high in the late stage. And fourth, extensive deposition and distribution of shale rich in organic matters and silica was mainly controlled by a stable sea basin with a low subsidence rate, a relatively high sea level, semi-closed waters and low deposition rates. To sum up, the high-quality shale in the Upper Yangtze region is characterized by multiphase superimposition, lateral extension and northwestward renewal of sedimentary ages, and it is mainly deposited in Wufeng–Rhuddanian age and secondarily in Aeronian age. The main exploration layers are Wufeng–Rhuddanian in southern–eastern Sichuan Depression, Rhuddanian–Aeronian in Weiyuan area, and Wufeng–Telychian in central–northern Sichuan Basin. Keywords: Upper Yangtze region, Late Ordovician–Early Silurian, High-quality shale, Bentonite, Sea level, Palaeo-productivity, Deposition rate, Main exploration layers, Southern Sichuan Basin