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

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