Paleogeographic evolution of a rifted cratonic margin: S.W. Australia
Abstract Early rifting in Gondwanaland created the Westralian Aulacogen during the Ordovician and this complex rift valley has accumulated up to 10.5 km of sediment, much of it during the Paleozoic and Mesozoic. The western borders ruptured in the late Jurassic and sea-floor spreading began forming the eastern Indian Ocean. The southern borders suffered, in contrast, downwarping and crustal rupture in Eocene time. Geomorphic, stratigraphic, and paleopedologic studies disclose marginal upward following rupture, leading to deep gorge incision. Crustal relaxations and eustatic sea-level rises led to repeated infillings followed by re-excavations of these gorges. On the plateau the most significant event appears to have been the Permo-Carboniferous glaciation and subsequent to that time there was a complex history of fluvial and lacustrine sedimentation. The mean denudation rate of the West Australian craton has been about 10–20 cm per million years. Four distinctive paleodrainage systems have been mapped in a small sector of the shield: (a) the “Kirup system” is Permian, proglacial, with braided streams, flowing to the northwest as did the continental ice; (b) the “Westralian system” involved deep Jurassic dissection (by 300 m) of the craton's western margin (Darling Range), was westerly trending, mainly following structural control; (c) the “Darling system” of Late Cretaceous to Eocene age was related to high eustatic sea levels and moist and warm climates, the drainage pattern being dendritic within the deep-weathering mantle, the former Eocene shoreline (at about 280 m) of the ria-indented coast being marked by prominent knickpoints and dry valleys; and (d) the “Ravensthorpe system”, also of Eocene age, was directed southwards as a result of the crustal thinning, downwarping and eventual rupture with Antarctica creating the Southern Ocean. Subsequent history has been marked by general desiccation and indurations beginning in the Oligocene and continuing on and off into the Quaternary. At the same time there was progressive cratonic emergence, accompanied by exposure of marine coastal plains and exhumation of the marginal gorges.