Tracing the Neoproterozoic Marinoan Snowball Earth thaw from deep sea to coastal waters

Abstract

The deglaciation of the Neoproterozoic Snowball Earth occurred over a few thousands to several tens of thousands of years. This short interval was characterized by a rapid increase in global sea level, resulting in an abrupt deposition of the post-glacial cap carbonates overlying the glacial deposits. However, the transition from glacial diamictites to cap carbonates in different depositional environments displayed considerable variability, indicating a more complex deglacial process than previously thought. To elucidate the end of the Snowball Earth, a comprehensive facies analysis was performed on the Cryogenian Nantuo Formation (the Marinoan glacial deposits) in the Nanhua Basin, South China. The analysis revealed that the Nantuo Formation encompasses a spectrum of facies associations, including subglacial, proximal glaciomarine, distal glaciomarine, and non-glacial marine facies, with obvious differences observed in the deglacial sequences between deep and shallow marine environments. In the deep-water environments, which was the deglacial deposits underwent a transition from massive diamictite to turbidite sequences, succeeded by the deposition of fine-grained sandstone to siltstones, and finally capped by the deposition of cap carbonates. In contrast, the shallow-water environments showed a progression from sheared diamictites to massive diamictite, which then evolved into cap carbonates. These observed differences highlight the spatial heterogeneity inherent in the deglaciation process, with the deep-water facies deglaciating before the shallow-water facies, implying a sequential retreat of the ice sheet towards the terrestrial domain. The findings support the hypothesis that the Neoproterozoic Snowball Earth deglaciation was a progressive event, beginning offshore and gradually extending onshore, which induced changes in depositional patterns and ocean alkalinity.