Discovery of Neoproterozoic glaciogenic diamictites and cap carbonate couplet in the Alxa Block, NW China: Evidence from stratigraphic, sedimentologic and geochemical studies

The global-scale glacial events recorded by diamictite and cap carbonate couplets occurred in the late Neoproterozoic and has been recognized on at least 15 paleo-continents. Diamictite and cap carbonate couplets play a pivotal role in establishing regional stratigraphic correlations and understanding the extreme climatic conditions and glacial-interglacial cycles of the Neoproterozoic glaciation. Here we report newly discovered Neoproterozoic glaciogenic diamictite and cap carbonate couplet in the Longshoushan area at the southwestern margin of the Alxa Block, NW China. Based on detailed stratigraphic and sedimentologic studies, we identified massive and stratified diamictites at the bottom of the Hanmushan Group, both with poorly sorted and rounded gravels. The presence of glacial striations and ice-rafted dropstones in stratified diamictites supports a glaciogenic origin. The upward transition from massive diamictites to stratified diamictites indicates the process of glacier retreat. The occurrence of thin-bedded phyllites in the stratified diamictites suggests a short-term deglaciation during the glaciation. The 2- to 2.6-m-thick cap carbonates cover the stratified diamictites and consist of thinly laminated microcrystalline dolomites. The basal cap carbonates contain closely linked sheet cracks, cemented breccias, tepee-like structures and cavities. The cap carbonates show high-resolution ¹³C_PDB chemostratigraphy and negative δ¹³C values (ca. −2.9 to −4.1‰), typical of the Marinoan cap carbonates. Regional sedimentary characteristics and the C-O isotope values suggest that the diamictites and cap carbonate couplet in the Alxa Block likely correspond to the Marinoan glaciation and subsequent deglaciation (ca. 635 Ma), not the previously assumed Ediacaran glaciation. Thus, the diamictite and cap carbonate sequence marks the Cryogenian-Ediacaran boundary in the Alxa Block and provides evidence for further stratigraphic correlation and investigation. This work was financially supported by NSFC projects (grants 42072264, 41730213, 41902229, 41972237) and Hong Kong RGC GRF (17307918).