Strange bedfellows: glacial diamictite and cap carbonate from the Marinoan (635 Ma) glaciation in Namibia



The Otavi Group is a Neoproterozoic carbonate-dominated succession up to 4 km thick, which blankets the southern promontory of the Congo craton in northern Namibia. This succession was deposited between 770 and 580 Ma in response to north–south crustal stretching and subsequent thermal subsidence. The main shallow-water platform has a well-defined southern limit, beyond which is a distally tapered foreslope wedge of deep-water carbonate facies. The Ghaub Formation represents the younger of two Cryogenian glaciations of the platform and was deposited during the period of thermal subsidence. A deep negative δ13C excursion, accompanied by increased size, abundance and variety of stromatolites, occurs in the last 10 to 80 m of shallow-water carbonate on the platform beneath the Ghaub glacial erosion surface. The same phenomena are observed before the older (Sturtian) glaciation in other areas, suggesting temporal proximity of the δ13C excursions to glaciation. Growth of ice sheets is manifested by emergence of the platform and development of a falling-stand wedge on the foreslope, composed of upward-coarsening carbonate turbidites and debrites. Rafts of very coarse-grained, well-sorted oolite have no source on the platform. The oolite probably originated at the strandline on the foreslope and was redeposited gravitationally downslope as sea-level fell. The Ghaub Formation is a laterally continuous wedge of carbonate diamictite, limited to the distal foreslope and ca 80 m in average thickness. Tongues of massive to weakly stratified diamictite, representing proglacial rain-out and subglacial tillite, are bounded by thinner, well-bedded units consisting of hypopycnal plume fallout, ice-rafted debris, turbidites and debrites, sorted sands and gravels, and westward-directed contourites. Debris is derived from the falling-stand wedge and the top 80 m of the inner platform. The wedge rests on a laterally continuous erosion surface, presumably cut by ice, and its sedimentary makeup and stratal organization are diagnostic of an ice grounding-line wedge. The subglacial erosion surface cuts a steep-walled trough on the distal foreslope, presumably once occupied by a transverse ice-stream. In the middle of the trough stands a doubly crested moraine composed of amalgamated unstratified diamictites. Terminal deglaciation is recorded by a fining-upward, 10 m thick drape of Fe-rich carbonate debrite and turbidite, loaded with far-travelled ice-rafted debris of all sizes. If deglaciation began with the collapse of ‘sea-glacier’ ice on the tropical ocean, the loss of this buttress could have triggered catastrophic ice-sheet drainage, concomitant with surface ocean ventilation. Complete deglaciation presumably was driven by ice-albedo, ice-elevation and greenhouse-gas feedbacks. The grounding-line wedge and bare upper foreslope and platform are overlain by a transgressive ‘cap dolostone’, which features shallow-water sedimentary structures (sorted peloids, low-angle cross-bedding, tubestone stromatolite and giant wave ripples) from the distal foreslope to the inner platform. The structures and isotopic profiles show that it was deposited diachronously on the time scale of ice-sheet melting globally (kyr), implying very high sedimentation rates. Temperature-dependent isotope fractionation could account for the observed secular and lateral δ13C changes. This interpretation requires sea water pH lower than 7·3 (high pCO2) and warming of at least 45°C in the tropics, consistent with the change in planetary albedo accompanying global deglaciation. The thickness of the highstand part of the cap-carbonate sequence on the platform implies >3 to 5 Myr of tectonic subsidence during the glacial period to create permanent accommodation. The highstand sequence prograded inward from the raised rim of the platform, inherited from karstic and glacial erosion. Sea floor cements, formerly aragonitic, are localized over palaeobathymetric highs (ice-stream moraine, platform rim and inner platform highs). Subsequent aggradation of the platform and coeval foreslope shedding were accompanied by a 0·5 km thick stratigraphic interval in which platform strata are depleted in 13C by up to 2·5‰, compared with coeval foreslope strata; this again suggests temperature-dependent fractionation at low pH. Above this interval, the δ13C gradient reverts to normal (platform strata more enriched in 13C than foreslope equivalents), consistent with CO2 drawdown due to silicate weathering.