Neoproterozoic glacial-rainout intervals: Observations and implications

Stratigraphically discrete glacigenic dropstone intervals have been identified within six separate Neoproterozoic glaciomarine successions: the ca. 720 Ma Chuos Formation (southwestern Congo craton) and Surprise Diamictite (southwestern Laurentia), the ca. 600 Ma Ghaub Formation (Congo craton), Blasskrans Formation (northwestern Kalahari craton) and Wildrose Diamictite (southwestern Laurentia), and the post-595 Ma lower Southern Highland Group (northeast Laurentia). These dropstone intervals are interstratified with hemipelagic, dropstone-free lithologies that record periods devoid of glacial rainout. Such episodic deposition requires, at a most rudimentary level, temporally compatible environmental fluctuations to generate melting of ice. The Ghaub Diamictite is particularly revealing in this regard, given that, according to the snowball Earth hypothesis, it had to be deposited either during glacial maxima (totally frozen seas) or melt back (geologically instantaneous) owing to its low-latitude setting and association with carbonates that record highly depleted delta(13)C values. The consistency of facies features shared by all six glaciomarine successions leads us to speculate that the temporally discrete glacial-rainout events (including diamictites deposited in low latitudes) record a dynamic glacial environment and that such conditions were characteristic of Neoproterozoic glaciations in general. These observations indicate that Neoproterozoic seas were not totally frozen and that the hydrologic cycle was functioning.