The significance of ice-rafted debris in Sturtian glacial successions

Globally, Sturtian (early Cryogenian) glacial deposits are well expressed, and belong to the oldest Neoproterozoic icehouse Earth event. The evidence for glaciation typically includes the phenomena such as striated pavements, striated clasts in diamictites, and abundant dropstones. More problematic, and potentially more significant, are intercalated deposits that exhibit no apparent evidence of a glacial influence on deposition. These apparently non-glacially-influenced intervals may represent deposition during interglacial periods, or at times when ice sheets transitioned to cold-based ice masses where sediment advection into basins was suppressed. Here, using three case studies from South Australia, northern Namibia, and Death Valley (USA), we show that many IRD-free intervals occur at the top of backstepping successions, where they are best interpreted as glacial minima deposits. In other cases, the volume of IRD in a succession shows less distinct increases and decreases upsection. Rhythmic intercalation of IRD-bearing and IRD-free intervals with glaciomarine turbidites can also be observed. These latter examples may be interpreted to record variations in debris content of ice margins, switch on/switch off of ice streams, or simply dynamic oscillation of a hinterland ice margin.