|
June/July 2001 |
|
Abstracts of Talks |
||||||||||||||||
|
Fig.1. It has long been known that large areas of the Gondwana Supercontinent, comprising Australia, Antarctica, Africa, India, and South America, were subject to repeated glaciations during the Late Carboniferous and Early Permian (about 280 to 320 million years ago), but there has been no agreement on the form and magnitude of those glaciations. In Western Australia, the best evidence of glaciation is displayed in strata and landforms of Early Permian age, but some authorities have claimed that this does not point to the existence of major ice sheets in the area at that time (e.g. Meyerhoff and Teichert, 1971; Eyles, 1993; Baillie et al., 1994; Dickins, 1996). That view is now challenged by new evidence, mainly derived from preserved Early Permian glacial landforms, which indicate that a succession of major wet-based ice sheets, several kilometres thick, extended across all or most of the Australian continent and other parts of Gondwana during the Early Permian (Playford, 2001, Fig. 1). The ice, moving away from the south pole, typically travelled north-northwest over Western Australia and South Australia and north-northeast over Victoria and Tasmania, as shown by striations in glaciated pavements formed below the ice (Playford, 2001, Fig. 2).
Fig.2. In the northern Canning Basin, the uppermost surface of the Devonian limestone ranges is subhorizontal, essentially marking the level of the Early Permian unconformity. The unconformity surface, where freshly exposed, is a striated glacial pavement, indicating movement of the ice to the north and north-northwest, directly towards the mountainous area of the Kimberley Block, which at that time was probably entirely covered by ice. Extensive karst features and glacial valleys were formed in limestones in the subglacial environment and are now exposed where sandstones and shales of the overlying Grant Group have been removed by Cainozoic erosion. The exhumed Early Permian geomorphic features consist of extensive cave systems, canyons, dry valleys, tunnels, and dolines. Notable examples include Mimbi Caves, Windjana Gorge, Menyous Gap, and Tunnel Creek. Early Permian glacial landforms are commonly preserved in Precambrian rocks along the northeastern margin of the Pilbara Craton (Forman, 1960; Noldart and Wyatt, 1962; Hickman and Clarke, 1994; Hickman and Bagas, 1998; Williams, 1999; Williams and Trendall, 1998a,b, Playford, 2001). They comprise ice-scoured channels, U-shaped valleys, rock drumlins, and polished striated pavements. Conspicuous examples include the valley of Shay Gap, a drumlin field near Carawine Pool, and a major linear channel north of Pearana Rockhole. These features of the modern landscape were originally formed below the base of a thick continental ice sheet during the final culmination of the glaciation. The Permo-Carboniferous ice age was one of the most momentous events in the geological evolution of Western Australia, with far-reaching geomorphological effects that are still evident in modern landscapes. Relict glacial landforms are widespread and show that extensive areas of the Great Plateau (of Jutson, 1950), covering the Yilgarn Craton and parts of the Pilbara Craton and Kimberley Block, were planed down below the ice, forming a level surface above which a few remaining hills and mountains projected. During the Mesozoic and Early Tertiary this glaciated surface was dissected by a network of rivers. Remnants of those river systems are preserved today as shallow palaeochannels crossing the otherwise level surface of the Great Plateau, a surface that basically is inherited from the Permo-Carboniferous glaciation of this part of Gondwana. References
|
