GEOMORPHIC CONSEQUENCES OF TANDEM HARD SNOWBALL EVENTS IN ACTIVE BASIN-RANGE AND PASSIVE-MARGIN TECTONIC REGIMES

Did tropical Cryogenian ice sheets produce small geomorphic changes because of hard snowballs’ weak hydrologic cycle, or large ones because of their longevity (67±5 Myrs)? On the SW corner of Congo craton in NW Namibia, 230 measured sections through pre-, syn- and post-Cryogenian strata in a 20,000 km²area allow erosional and constructional glacigenic landforms to be characterized and mass fluxes averaged in a crustal-stretching regime during early Cryogenian (EC) glaciation (717−661 Ma) and a subsiding Atlantic-type shelf and bathyal slope during late Cryogenian (LC) glaciation (646±5−635 Ma). Both glacial onsets can be identified, where preserved, from chemo- and/or sequence stratigraphy, allowing absolute depths of erosion to be constrained over wide areas for LC, when shelf strata remained horizontal. Average sediment accumulation, based on complete non-zero glacial sections, is 4.0 m Myr⁻¹for EC (n=110) and 3.3−8.8 m Myr⁻¹(n=157) for LC. Active tectonics barely offset a 10% scaling deficit due to EC’s longer duration. Average rates are 10x lower than for post-Cryogenian ice ages of comparable duration, consistent with global data. Because the succession is folded, glacial landforms are exposed in oblique cross-section. EC landforms include a 1.0-km-deep rift-shoulder cirque, a 0.45-km-deep bedrock trough, a 1.6-km-deep subglacial rift lake and ≤0.25-km-high moraines resting on glaciated surfaces with ≤35 m of local relief. During LC, 30-km-wide troughs ≤60 m deep were cut 50−150 km inboard of the carbonate shelf edge, while elsewhere the shelf was eroded ≤30 m in absolute depth. Accumulation mainly occurred on a mid-slope ice-shelf grounding-zone wedge. Built on a glacial erosion surface, the wedge reached 0.6 km in height, accommodated by tectonic subsidence and ice-shelf thinning. Grounding-line migrations, expressed as pro/subglacial cycles, were limited by the forward bedrock slope and stabilization by the wedge itself. So, big in stature, small in sum.