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

Abstract

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 d 13 C 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 d 13 C 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 coarsegrained, 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, icerafted debris, turbidites and debrites, sorted sands and gravels, and westwarddirected 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 finingupward, 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 1

The Late Cryogenian Warm Interval, NE Svalbard: Chemostratigraphy and genesis

“…Both early and late Cryogenian glaciations, referred to as the Sturtian and Marinoan (based on locations in South Australia), respectively, appear to be globally distributed (Li et al, 2013). In some regions, the history of one or both glaciations is locally or regionally complex, with distinct glacial retreat intervals recognized in South Australia (Williams et al, 2008, Le Heron et al, 2011Rose et al, 2013), Namibia (Hoffman, 2011;Le Heron et al, 2013), Scotland (Spencer, 1971;Arnaud and Fairchild, 2011) and Oman (Leather et al, 2002;Rieu et al, 2007a). However, semi-continuous Cryogenian successions display a clear stratigraphic motif in which two glacial units bound an unambiguously non-glacial interval, representing a mid-Cryogenian interlude of unknown duration (~5-27 My; Rooney et al, 2014).…”

“…The most comprehensive studies have been performed on thick carbonate successions in South Australia (e.g. McKirdy et al, 2001;Giddings and Wallace 2009a, b;Rose et al, 2012;Hood & Wallace, 2014, 2015Wallace et al, 2015) and Namibia (Halverson et al, 2002(Halverson et al, , 2005Hoffman and Schrag, 2002;Hurtgen et al, 2002;Hoffman and Halverson, 2008;Hoffman, 2011), and mixed carbonate-siliciclastic sediments of NW Canada (Hofmann et al, 1990;Narbonne and Aitken, 1995;Day et al, 2004).…”

Comparative Stratigraphic and Geochronological Evolution of the Northern Damara Supergroup in Namibia and the Katanga Supergroup in the Lufilian Arc of Central Africa

“…Its clast suite is dominated by carbonate fragments derived from the underlying Otavi rocks. However, its cap carbonate succession, the Maieberg Formation, is laterally extensive and again a single-cycle marker unit of marllimestone rhythmite up to 700 m thick in places (Hoffman and Halverson 2008;Hoffman 2011). A positively weathering, tan-coloured cap dolostone, the Keilberg Member (Hoffmann and Prave 1996), is a characteristic and widespread marker horizon at the base of the formation.…”

“…Cyclical deposition typifies most units of the Otavi Group in the western part of the Northern Platform (Hoffman and Halverson 2008). The Ombombo Subgroup (Hoffmann and Prave 1996), which is divided into the Beesvlakte, Devede and Okakuyu Formations (Tables 1, 2), coarsens upwards but fines northwards in concert with a northward deepening of western parts of the Northern Platform 'lagoon' (Hoffman and Halverson 2008;Hoffman 2011). The contact of the Beesvlakte Formation with underlying rocks of the Nosib Group appears to be an abrupt flooding surface (Hoffman and Halverson 2008).…”

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

“…These cap carbonates have been of special interest, understandably, as they may provide significant hints about the link between the earliest diversification of animals and the most severe glaciation in Earth's history. Shields (2005) proposed that the Marinoan cap carbonates were primarily deposited in low-salinity meltwater lid referred to as "Glacial Lake Harland" (Hoffman, 2011) and later in mixed seawater with contribution from deep snowball meltwater. However, Ohno et al (2008) speculated that the cap carbonates formed in surface seawater mixed with a large quantity of deglacial meltwater from continental crusts, based on the high 87 Sr/ 86 Sr and low 88 Sr/ 86 Sr ratios from the Doushantuo cap carbonates of South China.…”