28th DeBeers Alex. Du Toit Memorial Lecture, 2004. On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record

The snowball earth hypothesis is a unified theory accounting for the global distribution of Cryogenian (roughly 720 to 635 Ma) glacial and glacial marine deposits, their global synchroneity demonstrated by chemostratigraphy, and their close association with thick carbonate strata and sedimentary iron deposits (banded iron formation) in certain areas. It postulates that on two separate occasions, around 710 and 640 Ma, the ocean froze over from pole to pole for long periods ( i.e., millions of years). The postulate has been widely criticized as being incompatible with the glacial sedimentary record indicating the former existence of fast-moving wet-base ice and open proglacial waters. The younger Cryogenian glaciation in northern Namibia presents an excellent opportunity to investigate the sedimentary record. The area was then a vast shallow-water carbonate platform situated in the tropics or subtropics. The platform had a sharply-defined southern edge, beyond which a stratigraphically tapered foreslope wedge descended into deep waters of the northern Damara extended terrain. The platform and foreslope were undergoing broad regional subsidence with no local structural deformation at the time of the younger glaciation. The Fransfontein Ridge (a present physiographic feature) is a simple homocline exposing a continuous 60-km-long section of the foreslope wedge. The western two-thirds of the ridge parallels the paleoslope contours 6 to10 km south of the foreslope-platform break, where the seafloor would have been 550 to 800 m below the top of the platform based on modern analogs. The eastern third of the ridge angles up the foreslope to the platform edge. The Ghaub Formation is a sharply-bounded wedge of glacial marine strata that blankets the lower foreslope and tapers up-slope to a pinch-out 6±1 km south of the platform edge. It comprises a stack of marine till tongues (massive and poorly-stratified diamictites including debris flows, grounding-line proximal ‘rain-out’ and ice-contact diamictite) with subordinate stratified proglacial sediments variably rich in ice-rafted debris. The sediment is almost exclusively derived from the then top 60 m of the underlying carbonate (limestone and dolostone) platform and foreslope. Although most sediment transport indicators in the proglacial and subglacial strata are southerly-directed ( i.e., down-slope), rare starved ripple trains indicate traction currents flowing westward parallel to the paleoslope contours. These contour currents may imply open waters on the Damara seaway south of the platform. The Ghaub Formation rests on a continuous erosion surface that carves out of the underlying strata a broad steep-walled trough, 0.1 km deep and ~18 km wide, assuming its axis is oriented roughly transverse to paleoslope contours. The glacial deposits average 80 m in thickness outside the trough, are attenuated on the sides of the trough, and form a steep moraine-like ridge in the trough axis. The ridge rises 600 m above the floor of the trough and is only 7.5 km wide at its base. It is composed of massive carbonate diamictite lacking proglacial strata. The ridge is interpreted as a transverse medial moraine formed near the mouth of a relatively narrow paleo-ice stream that eroded the trough. Ice streams are corridors of fast-flowing wet-base ice within an ice sheet and are thought to be responsible for up to 90% of the total ice drainage from the present Antarctic and Greenland ice sheets. Climate modeling suggests that annual mean surface temperatures on tropical continents with frozen oceans in the Cryogenian would have been similar to present Antarctica. Therefore, the existence of Cryogenian ice streams is not surprising and suggests that fast-flowing wet-base ice is not incompatible with a frozen ocean. Directly below the trough is a 20 km-wide by 500 m-thick carbonate grainstone prism representing a complex of submarine channels and levees, showing that the area of the trough was a major submarine drainage system long before the glaciation began. It was localized by a tectonic subsidence anomaly going back to the time of the older glaciation, when the platform was undergoing active crustal stretching. The moraine ridge was a prominent topographic feature at the end of the younger glaciation. The post-glacial cap dolostone was attenuated by winnowing on its flanks, but spectacular crystal fans of sea-floor aragonite (pseudomorphosed) are developed exclusively over the moraine in limestone directly above the cap dolostone. The basal part of the Ghaub Formation outside the ice-stream trough contains a mappable ‘drift’ of terrigenous siltstone. The most likely source of this detritus is a regionally extensive sheet of fine-grained terrigenous sediment that is erosionally cut-out by the ice-stream trough. Accordingly, the entire Ghaub Formation outside and inside the trough must be younger than the cutting of the trough. If the trough was cut by an ice stream at a glacial maximum (or maxima), then the Ghaub Formation is entirely recessional. Sedimentary features ( e.g., ice-rafted debris, contour current indicators) within the Ghaub Formation do not represent conditions at the glacial maximum and do not constrain the extent of ocean ice cover at that time. If Cryogenian glacial deposits are recessional in other areas, issues still in dispute such as the maximum extent of marine ice cover will only be resolved with new geochemical data, for example iridium concentrations (cosmogenic dust proxy), boron isotopes (seawater pH proxy), and osmium isotopes (weathering proxy).