Paleoenvironmental factors controlling the development of the lacustrine shale interbed in the Jurassic Dongyuemiao Member of the Sichuan Basin, China

Abstract

Dongyuemiao Member shale in the Sichuan Basin, China, is characterized by organic-rich shale intervals with different types of interbeds and accumulation modes. The aim of this study is to elucidate the impact of paleoenvironmental indicators on interbed development. With this aim in mind, we established an interbed classification scheme and quantified the development of different types of interbeds and their frequencies. We categorized the shale interbeds into three types based on interbed type: silt interbeds (SIs), shell fragment interbeds (SFIs), and shell skeleton interbeds (SSIs). The SIs, SFIs, and SSIs are respectively the products of extrabasinal low-density turbidity currents, intrabasinal debris flow, and intrabasinal low-density turbidity currents. We propose that variations in paleoenvironmental conditions primarily influenced the types of interbeds that developed but had minimal impact on the frequency of their development. Models depicting the interbed development within the 1st Submember of Dongyuemiao Member indicate that during the early Dongyuemiao depositional period, under conditions of relatively aridity, weak weathering, high terrigenous input, and strong hydrodynamic activity, SSIs were well developed. In the middle depositional period, as the climate gradually transitioned to more humid conditions, and the weathering intensity and amount of terrestrial input increased, the development of SIs and SFIs significantly increased. During the late depositional period, with a continuous decrease in terrestrial inputs and sedimentation rates, the development of SIs decreased while that of SSIs increased.