Permo-Carboniferous inheritance in Australian landscapes

Brad Pillans 1

1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia

During the late Carboniferous and Early Permian (~320-280 Ma), Australia was part of the Gondwana Supercontinent, which included Antarctica , India , Africa , New Zealand and South America . Gondwana was situated at mid to high latitudes in the Southern Hemisphere. Evidence for glaciation is widespread and includes glacial tills and striated pavements in all states of Australia , as well as the other Gondwana continents. Although the timing, character and distribution of glacial events and deposits are debated (Jones & Fielding 2004 and references therein), most reconstructions show a large ice cap covering much of southern and western Australia in the Early Permian, with glacio-marine sedimentation in the adjacent Canning Basin. In contrast, in eastern Australia the evidence indicates discrete, short-lived episodes of localized mountain glaciation, with substantial non-glacial intervals in between. Just how much of Australia was covered in ice is unclear, but a large ice sheet, probably several kilometers thick, was likely centred over the Yilgarn Craton in Western Australia . Glacial till and tunnel valleys, dating from this time, are preserved at Lancefield on the eastern margin of the ice sheet, where glacial meltwater drained into the Officer Basin (Eyles & de Broekert 2001). Relict Early Permian landforms including ice-scoured channels, U-shaped valleys, rock drumlins and striated pavements are also common along the northeastern margin of the Pilbara Craton (Playford 2001). However, significant areas of Australia must have been ice free, at least for long periods of time (millions of years) during the major interval (280-320 Ma) of Gondwana glaciation, because paleomagnetic dating of giant (60 m+) weathering profiles in the Tanami and Yilgarn regions, as well as at Northparkes mine in New South Wales (Pillans 2005), indicates widespread deep oxidation of the regolith (Figure 1). K-Ar dating of illitic clays in weathered volcaniclastics within Jenolan Caves, some 200 km ESE of Northparkes, yields a cluster of ages in the range 342-335 Ma (Early Carboniferous), and a zircon fission track age of 345 Ma on one sample is consistent with the K-Ar ages (Osborne et al . 2006), making Jenolan Caves among the oldest currently open cave system in the world. The entry of the volcaniclastic sediments and the morphology of the caves mean that they must have been relatively close to the surface in the Early Carboniferous. Paleokarst features of Early Permian age are also preserved in the northern Canning Basin in Western Australia . In summary, Permo-Carboniferous glacial landforms, weathering profiles and caves at or near the present landsurface, in diverse parts of the Australian continent, suggest a significant Late Paleozoic inheritance in the modern landscape. The survival of ancient landforms and weathering profiles in Australia is usually explained as being a consequence of prolonged tectonic stability and postulated low rates of weathering and erosion. However, while measured rates of long-term (10 5 -10 7 yr timescales) weathering and bedrock erosion in Australia may indeed be low by world standards, they are not low enough to explain the continuous subaerial survival of pre-Cenozoic landforms and weathering profiles. Burial and exhumation must therefore be significant contributing factors in the preservation of ancient features in the Australian landscape, a conclusion that is supported by apatite fission-track thermochronology (e.g. Kohn et al . 2002).

Figure 1. Permo-Carboniferous deep oxidation of Proterozoic rocks exposed in Redback pit (~60 m deep), Tanami gold mine, Northern Territory.

References: Eyles, N. and de Broekert, P., 2001. Glacial tunnel valleys in the Eastern Goldfields of Western Australia cut below the Late Paleozoic Pilbara ice sheet. Palaeogeography, Palaeoclimatology, Palaeoecology, 171: 29-40.

Jones, A.T. and Fielding, C.R., 2004. Sedimentological record of the late Paleozoic glaciation in Queensland , Australia . Geology, 32: 153-156.

Kohn, B.P., Gleadow, A.J.W., Brown, R.W., Gallagher, K., O'Sullivan, P.B. and Foster, D.A., 2002. Shaping the Australian crust over the last 300 million years: insights from fission track thermotectonic imaging and denudation studies of key terranes. Australian Journal of Earth Sciences, 49: 697-717.

Osborne, R.A.L., Zwingmann, H., Pogson, R.E. and Colchester , D.M., 2006. Carboniferous clay deposits from Jenolan Caves , New South Wales : implications for timing of speleogenesis and regional geology. Australian Journal of Earth Sciences, 53: 377-405.

Playford, P.E., 2001. The Permo-Carboniferous glaciation of Gondwana: its legacy in Western Australia . Geological Survey of Western Australia Record, 2001/5: 15-16.

Pillans, B., 2005. Geochronology of the Australian regolith. In: R.R. Anand and P. de Broekert (Editors), Regolith-Landscape Evolution Across Australia . CRC LEME, Perth , pp. 41-61.