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Glaciogenic and associated strata of the Otavi carbonate platform and foreslope, northern Namibia: evidence for large base-level and glacioeustatic changes

By
Paul F. Hoffman
Paul F. Hoffman
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USASchool of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 2Y2, Canada (e-mail: hoffman@eps.harvard.edu)
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Published:
January 01, 2011

Abstract

Two discrete, mappable, glaciogenic formations occur within the Otavi Group, a 3±1-km-thick carbonate-dominated platform of late Neoproterozoic age, developed on the SW promontory of the Congo craton in northern Namibia and exposed in bordering late Ediacaran fold belts. Each is overlain abruptly by an expanded postglacial carbonate sequence, the younger of which begins with a globally-correlative transgressive dolopelarenite. The older Chuos glaciation (<746 Ma) occurred during a time of north-south crustal stretching. Debris derived from upturned older rocks collected in structural depressions. The younger Ghaub glaciation (635 Ma) occurred, after stretching ceased, on a thermally-subsiding marine platform and its distally-tapered foreslope. A continuous ice grounding-zone wedge (GZW) occurs on the distal foreslope, while the upper foreslope and outer platform are devoid of glacial debris and only small pockets of lodgement facies exist on the inner platform. Debris in the GZW is derived from a distinctive falling-stand wedge that is unique to the foreslope and from immediately older strata mined preferentially from the inner platform. The GZW rests on a smooth surface that includes a transverse steep-walled trough presumably cut by an ice-stream, within which is a towering doubly-crested moraine composed of composite, massive, carbonate diamictite. The surface suggests that the ice-sheet was grounded on the distal foreslope, implying a large fall in base level at a glacial maximum that predates the GZW. The glacial record ends with Fe-stained beds, rich in ice-rafted debris, that are notably absent from the moraine, upper foreslope and platform, which were apparently above sea-level at that time.

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Contents

Geological Society, London, Memoirs

The Geological Record of Neoproterozoic Glaciations

Emmanuelle Arnaud
Emmanuelle Arnaud
University of Guelph, Canada
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Galen P. Halverson
Galen P. Halverson
McGill University, Canada
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Graham Shields-Zhou
Graham Shields-Zhou
University College London, UK
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Geological Society of London
Volume
36
ISBN electronic:
9781862394117
Publication date:
January 01, 2011

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