THE NATURE AND DURATION OF THE STURTIAN GLACIAL EPOCH

Previous geochronological studies have constrained the Sturtian glacial epoch between ca. 717-663 Ma. It has remained uncertain whether strata deposited during this time record one prolonged Snowball Earth glaciation or multiple glaciations. This uncertainty is due in part to limited geochronological constraints but also to disagreement about the depositional setting of clast-poor facies within the glacigenic successions. Here we present new geochronological, stratigraphic, and sedimentological constraints on Sturtian glacial deposits in Mongolia, South China, and throughout Laurentia, and review data from Australia and Namibia. In all of these localities, the Sturtian glacial epoch is characterized by two massive diamictite units that are separated by diamictite-free, graded siliciclastic strata. These intervening clast-poor deposits are commonly interpreted to represent an interglacial period, however, they lack the thick rain-out deposits that would be predicted in a deglaciation scenario in a pro-glacial environment, are notably devoid of clay and primary carbonate, and do not record a transgressive systems tract. Instead, we suggest that many of these clast-poor facies were deposited in a sub-glacial environment below an ice shelf during step-back of the ice-grounding line, similar to Quaternary turbidite-rich facies deposited below the Larson ice shelf of Antarctica. The synchroneity of the relative retreat of the ice-grounding line on multiple continental margins remains uncertain, and may have been due to sublimation and/or tectonic subsidence on newly-rifted margins during a multi-million year glaciation. In this scenario, flow tills of the uppermost Sturtian deposits record the expansion, re-advance, and run-out of wet-based glaciers during deglaciation.