Neoproterozoic storm deposits in western Yangtze: Implications for the sea conditions during the middle Sturtian glaciation

The Neoproterozoic snowball Earth hypothesis emerged to explain an inferred planetary wide extreme climate change. However, storm deposits requiring open water conditions in the Huangdongzigou Formation, South China, contradict interpretations of a global Cryogenian hard snowball Earth. These deposits accumulated during the middle Sturtian interval (690–675 Ma) on the basis of U-Pb zircon age data from andesitic tuff and coarse siltstones within the formation. Siltstones and mudstones with abundant storm-generated erosional and sedimentary structures, such as hummocky and swaley cross-stratification occur through the sedimentary succession, which require the action of oscillating ice-free water flow. Chemical weathering indices and resultant mean annual temperature calculations indicate a warm-humid environment during storm events, with the transition to a cold water depositional environment towards the top of the formation, which is possibly related to the next glacial episode.