Depositional processes of Marinoan-age diamictites and cap carbonates in northwestern Tarim, China: Implications for chemical weathering following the Marinoan deglaciation

Abstract

The Marinoan diamictites and the overlying cap carbonates at continental margins bear key information on the paleo-environment evolution during the collapse of Snowball Earth, such as the timespan of intense chemical weathering. Such a sedimentary suite has been recently discovered in the northwestern Tarim Craton of China, but its depositional processes remain controversial. Here, we present stratigraphic and isotope geochemical studies on the diamictites of the Yuermeinak Formation and the overlying cap carbonates of the Sugetbrak Formation in the Aksu region of the northwestern Tarim Craton. Multiple unconformities in the region suggest major tectonic uplifting during the Cryogenian, probably resulting in a mountainous topography and varying dip directions of the overlying cap carbonates. The paleo-elevation of these mountains might have been higher and above sea level. We propose new depositional processes that involved four stages from glacial continental facies to neritic facies and/or alluvial fan systems. The first stage formed the massive diamictites and stratified siltstones with dropstones, recording cycles between glacier retreat and advance. The second stage involved the late transgression at the end of the deglaciation and the formation of calcareous massive diamictites with negative δ13C. The third stage included the onset of cap carbonate deposition and the alternating precipitation of calcareous mudstones and carbonates, reflecting frequent sea-level changes. The fourth stage was related to a widespread marine regression that developed a terrestrial environment and the sedimentation of the sandstones of the Sugetbrak Formation. Furthermore, we suggest that intense chemical weathering on exposed continents after the Marinoan deglaciation likely lasted for only hundreds of thousands of years, releasing ample alkalis into the ocean and facilitating the precipitation of the cap carbonates.