Lithofacies and glacio-tectonic deformation structures reveal the Snowball Earth ice-sheets dynamics
Geological evidences, including the distribution of Neoproterozoic glacial deposits, cap carbonate and banded iron formation, as well as stable carbon isotope profiles, indicate that the Earth may have been covered by thick ice-sheets at least twice during the Sturtian (~720–660 Ma) and Marinoan (< 654–635 Ma) glaciations. That is the so-called Snowball Earth.
This has inspired a new wave of investigations of Earth's climate system and is now a highly integrative multi-disciplinary field. The developments notwithstanding, detailed examination of Cryogenian rock sequences indicate that the Snowball Earth hypothesis has certain shortcomings and, indeed, certain interpretations of these glacial sequences remain controversial. Debate has focused on the fact that many sections appear to record the advance and retreat of dynamic ice sheets, which contradicts the simple stagnant snowball with its weak hydrological cycle.
Recently, PhD student Chunlin Hu and professor Maoyan Zhu from Nanjing Institute of Geology and Palaeontology, CAS reported findings from four sections of the Sturtian-age Tiesi'ao/Dongshanfeng Formation and related strata on the Yangtze Block in South China, which provides new understanding regarding this issue.
Their work has documented the presence of four lithofacies: massive diamictite, crudely stratified diamictite, stratified to massive pebbly sandstone and dropstone-bearing laminated siltstone and their interpretation is that these rocks were deposited in proximal glacio-marine environments during five successive episodes of glacial advance and retreat (Fig. 1).
They 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. The complete deformation succession of Stages Z1–Z5 (Fig. 2) in this study has never been reported for Cryogenian strata before but whether the newly proposed Crushed Ductile Deformation Model (Fig. 3) is merely a local phenomenon or one that could have broader applications remains to be tested.
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, their 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.
This study has been published on Palaeogeography, Palaeoclimatology, Palaeoecolog and it was supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18000000).
Publication information: Chunlin Hu, Maoyan Zhu*, 2019. Lithofacies and glacio-tectonic deformation structures of the Tiesi'ao/Dongshanfeng Formation on the Yangtze Block, South China: Implications for Sturtian Glaciation dynamics. Palaeogeography, Palaeoclimatology, Palaeoecology. https://doi.org/10.1016/j.palaeo.2019.109481
Schematic diagram showing the dynamic evolution model of the Sturtian Glaciation based on the analyses of the Tiesi'ao/Dongshanfeng Formation
Polished slabs and interpretive diagram of the deformation structures
Schematic diagram showing the (A)
Constructional Deformation Model of Hart et al. (1990), (B) Rigid Bedrock Model
of Eyles et al. (1983) and (C) Crushed Ductile Deformation Model of this study