Early glacial erosion and deposition during end-Cryogenian (Marinoan) glaciation along the southern Otavi carbonate paleoplatform margin, Namibia

Thursday, 18 December 2014
Tony Prave, University of St Andrews, St Andrews, KY16, United Kingdom, Karl Heinz Hoffmann, Geological Survey of Namibia, Windhoek, Namibia, Daniel James Condon, NERC Isotope Geosciences Laboratory, Keyworth, United Kingdom and Simon Tapster, British Geological Survey, NERC Isotope Geoscience Facilities, Nottinghamshire, United Kingdom
Cryogenian glacial deposits are characterized by large variations in thicknesses and lithofacies that reflect a range of depositional settings, from terrestrial and shallow marine to slope and basinal. Sedimentary studies have shown that these were formed by highly dynamic ice sheets; however, in the absence of reliable depositional age constraints on the timing on glacial erosion and sedimentation, these have been unable to resolve if they represent high rates of sedimentation at the end of long-lived (many Myr) glacial episodes with complete sea-ice covered oceans, or if they are the product of essentially continuous sedimentation (with possible multiple advance-retreat cycles) during less extreme glaciations of much shorter duration due to open water conditions and attendant lower albedo.

Here we present a precise CA-ID-TIMS U-Pb zircon age of 639.3 Ma from one of three thin (0.5-15.0 cm) laminated air-fall ash beds interlayered with proximal glaciomarine deposits (2.5 m dropstone-bearing interval between thick massive subglacial diamictite) of the Ghaub Formation in northern Namibia. Combined with a revised age of 635.5 Ma from an ash bed within basinal strata of the Ghaub Formation it constrains the duration of end-Cryogenian (Marinoan) glacial deposition along the Otavi paleoplatform margin to at least four Myr. We also document stratigraphic evidence for deep (several hundred m) subglacial erosion along the shelf-slope break and relate this to erosive wet-based ice sheet activity at a glacial maximum many million years prior to glacial termination. In addition, we interpret the set of three thin, ash beds as distal air-fall deposits and regard their presence within ice-rafted debris as indicating transient open water conditions near the shelf-slope break at the time of deposition. The implications of the ages are of some significance for models attempting to better understand the circumstances and conditions of Cryogenian glaciations.