Glacial tunnel valleys in the Eastern Goldfields of Western Australia cut below the Late Paleozoic Pilbara ice sheet

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Abstract

Open-cut gold mines near Kalgoorlie in the Eastern Goldfields region of Western Australia, expose a Late Paleozoic (Carboniferous–Permian) network of glacially-eroded valleys filled with tillite and shale. Valleys are incised into Archean and Proterozoic strata of the low relief Western Australian Shield and buried by Tertiary and Quaternary strata. Subsurface data identify a poorly-integrated network of narrow (∼200 m wide), deeply-incised (>80 m) and steep-sided channels that connect broader sub-basins up to 3 km wide. Lowermost infill strata within valleys consist of bouldery diamictites containing abundant striated clasts of far-travelled lithologies and are identified as tillites deposited by a Late Paleozoic ice sheet (Pilbara Ice Sheet) that covered the West Australian Shield. Tillites are blanketed by shales containing Early Permian palynomorphs. The form and sedimentary infill of the buried valley network is comparable to subglacially-cut ‘tunnel valleys’ that formed below the wet-based margins of Pleistocene continental ice sheets and a similar origin is proposed for those at Kalgoorlie. The significance of this finding is that hitherto, little was known of the Late Paleozoic Gondwanan ice sheet that grew on the Western Australian Shield and which contributed substantial volumes of sediment to adjacent sedimentary basins. The preservation of a relict Late Paleozoic glacial topography near Kalgoorlie indicates that post-Permian erosion of the region was minimal and cannot be reconciled with denudation models which propose uniform post-Permian uplift and deep stripping across the shield.

Introduction

The purpose of this paper is to report the discovery of a relict Late Paleozoic glacially-cut topography and associated glacial sedimentary infill exposed in open-cut gold mines near Kalgoorlie and Laverton in the Eastern Goldfields region of Western Australia (Fig. 1). We identify the broader significance of this discovery for constraining the thermal regime of the Late Paleozoic Pilbara Ice Sheet and for the Phanerozoic denudation history of the West Australian Shield (WAS).

WAS (Fig. 1) is a extensive (2 million km2) collage of Archean and Proterozoic terranes that only became a coherent crustal unit in the early Phanerozoic (Myers, 1990, Myers, 1993). It now forms an extensive upland surface of low relief but its Phanerozoic morphotectonic history is still poorly understood (Ollier et al., 1988). Some workers have proposed that Proterozoic peneplanation was followed by long-term tectonic stability and minimal Phanerozoic erosion (e.g. Finkl and Fairbridge, 1979, Sircombe and Freeman, 1999). Others have proposed strong uplift during the Mesozoic and deep denudation (up to 500 m) resulting in the removal of any pre-Cretaceous landforms (van de Graaff, 1981). The divergence of opinion regarding the denudation history is due in part, to a lack of data regarding the geomorphology of the shield. Unfortunately, the present day landsurface is underlain by Tertiary and Quaternary cover strata which obscure the underlying shield bedrock (see Ollier et al., 1988, Glassford and Semeniuk, 1995, Kern and Commander, 1995).

Section snippets

Late Paleozoic glaciation of Western Australia

Western Australia contains an extensive sedimentary record of Late Paleozoic (Late Carboniferous–Early Permian) glaciation preserved in several sedimentary basins that flank WAS (Crowell and Frakes, 1971, Hambrey and Harland, 1981, Eyles et al., 2001). A high latitude Pilbara Ice Sheet appears to have been initiated by uplift of the shield during the mid-Carboniferous Alice Springs Orogeny (c. 360 Ma) when Gondwana collided with Laurasia (Powell and Veevers, 1987, O'Brien and Christie-Blick, 1992

Late Paleozoic glacial tunnel valleys of the Eastern Goldfields region

The network of buried glacial valleys described here occur near Laverton in the Eastern Goldfields region some 100 km west of the nearest outcrops of Permian glacial deposits in the Officer Basin. Additional exposure of the same valley system occurs at Sand King near Kalgoorlie 250 km west of the nearest Officer Basin outcrops (Fig. 1). The Kalgoorlie–Laverton area hosts nickel and gold deposits in highly-deformed northwest–southeast-trending ‘greenstone’ belts. These are dominated by

Age and paleogeographic setting of tunnel valleys and tillites

Regional drilling programs by exploration companies demonstrate that the buried glacially-cut topography in the Laverton area is infilled by diamictites that are blanketed by shale. No outcrops occur but shales recovered by drilling at Thompson Well (Fig. 3) contain an Early Permian palynofloral assemblage (Kriewaldt, 1975). The same stratigraphic succession of tillites and Early Permian ‘postglacial’ shales is regionally widespread in the Officer Basin to the east (Paterson Formation; Jackson

Discussion

Data presented here represent a significant addition to existing knowledge regarding the erosional and depositional record of the Late Paleozoic ice sheet in Western Australia. Thick sedimentary records of Late Paleozoic glaciation occur in the sedimentary basins that lie outboard of the West Australian shield (Crowell and Frakes, 1971; Fig. 1, Fig. 2). In contrast, direct geomorphic evidence of former ice covers on the shield (e.g. striated bedrock, glacially-incised valleys) has hitherto,

Acknowledgements

This work would not have been possible without the logistical support of Metex Resources NL and Gilt Edged Mining NL. They provided access to open-cut mines at Lancefield and Sand King and subsurface drill data. Fieldwork was funded by the Natural Sciences and Engineering Research Council of Canada (Eyles) and the Cooperative Research Centre for Landscape Evolution and Mineral Exploration (de Broekert). We enjoyed useful discussions with Arthur Mory, Phil Playford, Don Glassford, John

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