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Paleomagnetic investigation of the basal Maieberg Formation (Namibia) cap carbonate sequence (635 Ma): Implications for Snowball Earth postglacial dynamics
Crustal eduction and slab-failure magmatism in an Orosirian (2.05–1.80 Ga) postcollisional cratonic foredeep: geochronology of Seton volcanics and Compton laccoliths, Tu Cho (Great Slave Lake), NWT, Canada
Glacial erosion on a snowball Earth: testing for bias in flux balance, geographic setting, and tectonic regime
A template for an improved rock-based subdivision of the pre-Cryogenian timescale
Cusp tectonics: an Ediacaran megakarst landscape and bidirectional mass slides in a Pan-African syntaxis (NW Namibia)
Abstract Lithospheric cusps occur where arcs are joined end to end. Where a subducting plate moves directly into a cusp, the slab experiences lateral constriction due to the cusp geometry. Buckled slabs of Cenozoic age occur at cusps (also known as ‘syntaxes’) in the Arabian, Indian, Pacific, Juan de Fuca and other plates. Here I report an Ediacaran example from the cusp of the Congo Craton where Pan-African collision zones meet at a right angle in NW Namibia. The craton was blanketed by syn- and post-rift Neoproterozoic marine carbonate, disconformably overlain by collision-related foredeep clastics. The disconformity has little stratigraphic relief in a 900 km-long fold belt rimming the craton, except within 60 km of the cusp apex where foredeep deposits bury a megakarst landscape floored by exhumed crystalline basement. Forebulge uplift, estimated from palaeokarst relief, was ≥1.85 km. This far exceeds characteristic forebulge heights of c. 0.5 km and matches the deepest part of the Grand Canyon of Arizona (USA). Coeval with megakarst development, map-scale mass slides moved coherently westwards and southwards towards the advancing accretionary prisms. Rapid burial by foredeep clastics preserved the megakarst palaeosurface and associated mass slides; folding them brought protection from complete destructive resurfacing for eons.
TRUE GLOBAL WARMING INFERRED FROM ALPINE RECESSIONAL MORAINES BY SCOTTISH PHYSICIST JOHN LESLIE IN 1796
Sedimentary depocenters on Snowball Earth: Case studies from the Sturtian Chuos Formation in northern Namibia
Early extensional detachments in a contractional orogen: coherent, map-scale, submarine slides (mass transport complexes) on the outer slope of an Ediacaran collisional foredeep, eastern Kaoko belt, Namibia
Tuzo Wilson and the acceptance of pre-Mesozoic continental drift
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
Birthdate for the Coronation paleocean: age of initial rifting in Wopmay orogen, Canada This article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.
Abstract Neoproterozoic glacial records have been discovered on 23 palaeocontinents, their rate of discovery changing little since 1871. Yet, half of all the resulting publications appeared since 2000. The history of research before 1998 is described in five stages defined by publication spikes; subsequent work is not covered because historical perspective is lacking. In stage 1 (1871–1907), ‘Cambrian’ (now Neoproterozoic) glaciation was recognized successively in Scotland, Australia, India, Norway, Svalbard and China. Criteria for recognition included faceted and striated pebbles in matrix-supported conglomerates resting on ice-worn bedrock pavements. In stage 2 (1908–1940), Neoproterozoic glaciation was shown to have been widespread in Africa, Asia and the Americas. Major textbooks summarized these findings, but the rejection of continental drift (to account for late Palaeozoic glacial dynamics) put a chill on research. In stage 3 (1942–1964), the occurrence of glacial deposits within carbonate successions, as well as nascent palaeomagnetic observations, suggested that Neoproterozoic glaciers reached sea-level at low palaeolatitudes, but the belated recognition of sediment gravity flowage caused glacial interpretations to be prematurely abandoned in key areas. In stage 4 (1965–1981), the extent of Neoproterozoic glaciation was rethought in light of plate tectonics. Distinctive chemical sediments (iron±manganese formations and cap carbonates) were identified. In basic climate models, modest lowering of solar luminosity resulted in global glaciation due to ice-albedo feedback, and deglaciation due to greenhouse forcing resulted from silicate-weathering feedback in the carbon cycle. Neoproterozoic glacial geologists were blind to these ideas. In stage 5 (1982–1997), reliable palaeomagnetic data combined with glacial marine sedimentation models confirmed that Neoproterozoic ice sheets reached sea level close to the palaeoequator.
Chemical sediments associated with Neoproterozoic glaciation: iron formation, cap carbonate, barite and phosphorite
Abstract Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were precipitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern seawater, with a weaker hydrothermal signature than Archaean–Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the meltdown of ice-sheets was rapid, cap dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap dolostones, either as digitate seafloor cement associated with Fe-dolomite at the top of the cap dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like breccias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phosphorite formed during a Neoproterozoic deglaciation.
Glaciogenic and associated strata of the Otavi carbonate platform and foreslope, northern Namibia: evidence for large base-level and glacioeustatic changes
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.