An ice grounding-line wedge from the Ghaub glaciation (635 Ma) on the distal foreslope of the Otavi carbonate platform, Namibia, and its bearing on the snowball Earth hypothesis

Abstract

Our detailed examination of the Ghaub Formation (possibly 635 Ma) on the distal foreslope of the Otavi carbonate platform is part of a regional study of the Congo paleocontinental margin in northwestern Namibia. Detrital carbonates of the Ghaub Formation disconformably overlie the Franni-aus Member of the Ombaatjie Formation, a coarsening-upward stack of carbonate turbidites and oolite-clast debris-flow breccias interpreted to be a glacioeustatic falling-stand wedge. Within the main Ghaub Formation, carbonate diamictites are interleaved with mesoscale, laminated to cross-laminated (climbing rippled) grainstones and mudstones, and conglomeratic carbonates. Amalgamation of diamictite units is observed where interleaved facies (grainstones/mudstones) are laterally discontinuous due to reactivation of erosion, followed by renewed deposition. The diamictite package is progradational overall and 80 m thick on average. It is overlain by the 5–15-m-thick Bethanis Member, which is unique in its lateral continuity, composite fining-upward trend, and distinctive interbedding of turbidite grainstones, argillaceous siltstones, climbing-rippled mudstones, and meter-scale stromatolite dropstones. Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting. We infer a glacial maximum represented by the sub-Ghaub disconformity, followed by the main Ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude, terminated by the Bethanis episode of unusually widespread iceberg calving and slope instability. The Bethanis Member is overlain conformably by the Keilberg Member of the Maieberg Formation. Reconstruction of the foreslope places the Ghaub grounding-line wedge >1.3 km vertically below the rim of the platform, implying an enormous base-level change upon deglaciation, when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment. The magnitude of base-level change supports the pan-glacial hypothesis that dynamic (thick) ice sheets existed simultaneously on virtually all continents. The snowball hypothesis that the oceans were also covered by glacial ice (sea-glacier) provides a simple explanation for the main Ghaub-to-Bethanis transition—terminal deglaciation was triggered by collapse of the sea-glacier.