Lithofacies and glacio-tectonic deformation structures of the Tiesi'ao/Dongshanfeng Formation on the Yangtze Block, South China: Implications for Sturtian Glaciation dynamics

摘要

The Cryogenian Period (similar to 720-635 Ma) witnessed the most extensive and prolonged glaciation events in Earth history. Vast ice sheets of at least two global glaciations, the Sturtian (similar to 720-660 Ma) and Marinoan (< 654-635 Ma), advanced to sea level at low latitudes, a scenario that is at the heart of the general Snowball Earth hypothesis. This hypothesis is supported by results of recent work in geochronology and geochemistry, but certain parts of the glacial sedimentary records remained debated regarding ice sheet dynamics. Here we report findings from four sections of the Sturtian-age Tiesi'ao/Dongshanfeng Formation and related strata on the Yangtze Block in South China. Our work has documented the presence of four lithofacies: massive diamictite, crudely stratified diamictite, stratified to massive pebbly sandstone and dropstone-bearing laminated siltstone and our interpretation is that these rocks were deposited in proximal glacio-marine environments during five successive episodes of glacial advance and retreat. We have also identified an interval of deformation structures near the base of the Tiesi'ao/Dongshanfeng Formation that is characterized by upward increasing strain intensity, indicating that these structures were caused by overriding ice sheets. All the observations collectively suggest that the Sturtian ice sheets in South China were warm-based, rapidly moving, and sensitive to changes in climate. Further, our data suggest that the ice grounding line reached the slope area, implying a major drop in global sea level, enormous ice sheet thickness, or both during the Sturtian Glaciation.