Supercontinents and glaciation: a perspective from western Gondwana.

Abstract

Precise timing for the formation of the Palaeozoic supercontinent Gondwana has long eluded the geological community. Early hypotheses postulated that amalgamation occurred in the mid-late Neopro¬terozoic via a collision between East and West Gondwana. This idea developed with the identification of discrete collisional events between diverse, relatively small Neoproterozoic continents that amalgamated Gondwana in a piecemeal fashion over 150 million years, with a series of late Ediacaran–Cambrian orogens that represent the final phase of Gondwana amalgamation. A salient feature of the rocks preserving these events in the Neoproterozoic sedimentary record is the preservation of glacial sediments. Significant debate has centred around firstly whether these deposits are in fact glacial and secondly the spatial extent of these glaciations. This thesis addresses these deficits in our knowledge by presenting detailed sedimentology, geochronology, palaeomagnetic results from western Gondwana. The Cryogenian-aged Toekems Sub-basin in the Damara Belt, Namibia comprises a wedge dominantly clastic, glacially influenced sediments. Our field observations and results imply a significant disconformity beneath the Naauwpoort Volcanics and suggest multi-phase rifting during the breakup of south-western Congo Craton from Rodinia. The northern Paraguay Belt in South America developed in response to the collision between the Amazonian Craton, the Rio Apa Block, the São Francisco Craton and the Paranapanema Block. The alleged ‘Brasiliano’ age (~620 Ma) of orogenesis was recently questioned by palaeomagnetic and radioisotopic ages that indicate the closing stages of orogenesis occurred well into the Cambrian that are believed to mark the suture zone of the Clymene Ocean—interpreted amongst the youngest of the Gondwana amalgamation orogens. The post-orogenic São Vicente Granite provides a long sort after minimum age of 518 ± 4 Ma for orogenesis within the belt, constraining the termination of deformation within the northern Paraguay Belt. The Alto Paraguay Group, the youngest stratigraphic unit in the northern Paraguay Belt, contains unequivocal evidence for a glacial influence on sedimentation. ⁴⁰Ar/³⁹Ar detrital muscovite cooling ages from the upper part of the Alto Paraguay Group are as young as 544 ± 7 Ma. When considered with other data presented here, these ages suggest that this package of rocks developed in a mid-Ediacaran glaciation consistent with that expressed in the Gaskiers Formation of Newfoundland, Canada. U/Pb zircon maxi¬mum depositional ages from the top of the Alto Paraguay Group indicate that final sedimentation began no earlier than 527 Ma. The εHf[subscript] signature is consistent with a predominantly Amazonian source until the early- Neoproterozoic at which point the signal becomes significantly more evolved. new palaeomagnetic data from Alto Paraguay Group represent a secondary magnetisation, likely acquired during regional emplacement of Jurassic basalt. This finding is at odds with recent results that have been used to suggest Amazonia was at low latitudes during the Ediacaran, which has implications for the snowball earth hypothesis and the tectonic evolution of the Paraguay Belt. These data, when combined with other evidence discussed here, are consistent with an ocean to the east of the present-day Amazonian Craton that didn’t close until the Cambrian.