Deposition of tillites and cap carbonates in NW Tarim, China: Implications for chemical weathering following the Marinoan deglaciation

The formation of Neoproterozoic tillites and overlying cap carbonates is important to understand drastic climatic and paleo-environmental variations during and after the collapse of the Snowball Earth. This rock assemblage has been found at the northwestern margin of the Tarim Craton in China but its depositional processes are still debated. We carried out stratigraphic and carbon isotopic studies on the tillites of the Yuermeinak Formation and overlying cap carbonates in this area. Detailed field observation and stratigraphic comparison demonstrate that these strata formed around the age of Marinoan deglaciation (ca. 635 Ma). Unconformable contacts with bedrocks and the variation of dip directions of the cap carbonates suggest that a mountainous topography developed in northwest Tarim during the Marinoan glaciation. We proposed a four-stage depositional model from glacial to alluvial fan and/or neritic facies systems. The first stage formed the stratified and massive tillites, recording several glacial cycles during the Marinoan deglaciation. The second stage involved the belated transgression at the end of the Marinoan deglaciation, caused the re-cementation of some tillites and the negative δ¹³C of their matrixes. The third stage included the alternating precipitation of calcareous mudstones and cap carbonates after the Marinoan deglaciation, reflecting frequent sea-level changes. The fourth stage was relevant to a terrestrial environment because of a widespread marine regression. Furthermore, we suggest that the timespan of the intense chemical weathering on exposed continents after the Marinoan deglaciation was comparable to the duration of the transgression, lasting for hundreds of thousands of years. This work was financially supported by NSFC projects (grants 42072264, 41730213, 41902229, 41972237) and Hong Kong RGC GRF (17307918).