Yilgarn Craton - Geological Framework

Australia: The Land Where Time Began

Yilgarn Craton - Geological Framework

An extensive exposure of Archaean granite, gneiss and 'greenstone' (basic volcanic rock) comprise the Yilgarn Craton that is located in the southwest of Western Australia (Geological Survey of western Australia, 1975, 1990). The Australian continent is dominated by topographically by high plains and by plateaux that are protectively capped with duricrusts, which are most commonly laterite or bauxite, as it is in the Darling Ranges, though siliceous in some valley fills, as occurs at Buckley's Breakaway, between Norseman and Hyden, and in the Killara district, the northern Yilgarn. Remnants of an old drainage system that has since been dismembered, is at present represented by strings of salinas or salt Lakes, that for prominant features in the landscape (Van de Graaf et al., 1977; Commander, 1989; Kern & Commander, 1993; Clarke, 1994a; Waterhouse et al., 1995). In the Eocene the original channels were initiated and can be compared to the old rivers that were noted earlier in the Mt Lofty Ranges and the Flinders Ranges.

Old and new plateaux

The remnants of the Yilgarn Craton, that are capped by laterite, have been labeled the old Plateau (Jutson, 1914, 1934), in contrast with the high plain of the present above which they stand and which he called the New Plateau, though according to the author¹ this is rather misleading, as it is not entirely bounded by scarps. The latter was formed by dissection of the lateritic duricrust, scarp retreat (recession of the slopes of the valley sides) and the removal of the associated soil, the regolith, that was comprised on kaolinitic clays. The clays were, and continue to be susceptible to erosion, which contrasts markedly with the cohesive and intrinsically fresh granite beneath the regolith. Generally the lower limit of effective weathering, or weathering front, is marked by the New Plateau, which is therefore an etch surface (Jutson, 1914, p. 143; Mabbutt, 1961a, 1961b; see also Falconer, 1911, p. 246).

The Old Plateau and the associated duricrust date from at least the Cretaceous, as the rivers that dissected them deposited strata dated to the Eocene in their lower reaches (Clarke, 1994a, 1994b; Twidale and Bourne, 1998a). It has been suggested the silcrete fills in the valleys may be of the same age, the silica being derived from the weathering that resulted in laterite (Stephens, 1964), though they could be younger, and be comparable to the valley fills from the western Gawler Craton. Corroboration for the Old Plateau being of considerable antiquity is found in the volcanic episode evidenced at Bunbury on the southwestern coast (Playford et al., 1976, p. 195-196). The exposed basalts are from the Early Cretaceous and were extruded at the time of the break away of that section of the Australian continent from Gondwana. The uplift and dissection of the Old Plateau was initiated by this disruption, and the exposure of the New Plateau that resulted. It has been considered that the deep weathering profiles of the Old Plateau date from the Permian-Middle Jurassic (Clarke, 1994b), though later in the Mesozoic and the Early Tertiary they were largely stripped away.

Sections of the New Plateau that are situated adjacent to major channels dating to the Eocene are also of Eocene age, but towards their headwater reaches they become younger, dating to the Miocene, Pliocene (e.g., Salama, 1997; Twidale et al., 1999), as the scarps have been worn back and the new, lower plain has been developed. There surface therefore has an age-range, it is a diachronic surface, being older near the 'mouth' of the river and progressively younger up-valley. The plain was formed recently and is still being extended in the headwater and tributaries, where scarps flank the Old Plateau.

About 60 Ma, at the time when Australia separated from Antarctica, uplift occurred of the present southern margin of the Craton, with the result that the courses of some streams were disturbed and even reversed, resulting in the south coast rivers being short. Other rivers that arise comparatively close to the south coast, such as the Avon and the Lefroy Rivers, flow inland. When the climate changed in the Pleistocene the river systems were modified further. The river systems became dismembered about 2 Ma when aridity set in, though they still flow in the channels beneath the surface (underflow), and locally at the surface occasionally.

Older inselberg forms

The crests of inselbergs such as The Humps, that stand above this land surface that is duricrusted, dating to the Mesozoic, are therefore at least as old, as they existed at the time weathering of the surrounding plane was occurring. Scarp-foot weathering at the hill-plain junction that was long-lived is suggested by pronounced flared slopes. Crestal areas of other inselbergs, such as Boyagin Rock and Jilakin Rock, in the southwest of the Yilgarn Craton, and of Disappointment Rock in the southeast (e.g., Bourne & Twidale, 2002), display similar evidence pointing to great antiquity. All of these stand higher than the adjacent laterite surface, which suggests they are at least of cretaceous age.

There are many well-known inselbergs composed of granite that have been exposed by the stripping of the regolith (weathered rock). Included among these are Hyden Rock on which Wave Rock has developed on the northern slope, one of the best-known Australian landforms (Twidale, 1968b; Twidale& Bourne, 1998a, 1998b, 2001).

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