Astronomical forcing paleoclimate and hydrological controls on depositional cyclicity: Genesis of organic‐rich shales in the early Cambrian of the Upper Yangtze area, China
Min Xiong, Lei Chen, Xin Chen, Xiucheng Tan, Jian Cao, Shuaicai Wu, Zuyou Zhang, Chongjie Liao, Kesu Zhong, Gaoxiang WangABSTRACT
The early Cambrian was a crucial stage characterised by rapid biological radiation and dramatic marine environmental changes. The Qiongzhusi Formation organic‐rich shales in the Upper Yangtze area preserved reliable archives of paleoclimate and paleoceanographic evolution. This study integrated cyclostratigraphic, sedimentological and geochemical analyses of 10 drill wells and nine outcrops, spanning from deep depocentre to shallow water shelf. The results reveal that sedimentary cyclicity is governed by astronomically modulated monsoon and hydrological variations forced by ~405 kyr long eccentricity cycles. During eccentricity maxima, intensified monsoonal circulation and weathering increased terrigenous nutrient fluxes. Coupled with active upwelling and hydrothermal activity, increased paleoproductivity, combined with transgression‐induced sulfidic restricted conditions, promoted prominent organic matter (OM) accumulation in Unit 1. Traditional productivity–preservation models cannot fully explain OM variations across Units 2–3. Instead, a unique climate–terrigenous influx–water restriction mechanism dominated in lower Unit 3 deposition, where transgression stratification sustained anoxic conditions and renewed OM accumulation. The rift trough palaeogeography modulates spatiotemporal configuration of upwelling and terrigenous influx, with pulsed hydrothermal activity further regulating sedimentation. This study constructed a tripartite orbitally forced organic‐rich shale genetic model: a high TOC coupling stage (Unit 1), a transitional decreasing OM stage (Unit 2) and a transgressive stage with OM re‐accumulation (Unit 3). These findings reveal that Qiongzhusi shales deposition reflects a dynamic equilibrium between paleoproductivity and preservation. Orbital‐driven climate–ocean interactions and the synergistic effects of hydrothermal‐upwelling systems control the dynamic productivity‐preservation balance governing shale genesis in cratonic rift basins.