Weathering and landscape evolution, Southeast Queensland

Abstract

A combined approach of weathering geochronology, regolith mapping and regolith geochemistry has been applied to unravel the surficial history of the Eastern Australian passive margin. Investigation of 18 weathering profiles, in excess of 100 40 Ar/39 Ar radiogenic dating analyses, two regolith mapping studies, and more than 2500 electron microprobe analyses of supergene (weathering-produced) minerals formed the basis of a comprehensive regional study of the weathering history in southeast Queensland. The spatial distribution and ages of weathering profiles in southeast Queensland shows a strong geomorphological control: deep weathering cover is preserved on the western plateau and the coastal plain, and relatively shallow profiles are present in the regions of active scarp retreat. Weathering profiles analysed on the western side of the Great Escarpment preserve a longer weathering history, spanning from the Middle Miocene to the present. Calculated average erosion rates for this region are in the order of ~1.3 to 1.9 m.Ma-1 for the Late Miocene-present. This is in contrast to weathering profiles on the coastal plain that record a much younger weathering history and higher erosion rates (~4.9 m.Ma-1 for the Pliocene- present). The erosional history proposed for the southeast Queensland margin in this study is consistent with the erosional history obtained for other passive margins based on apatite fission track and cosmogenic isotope studies. The supergene Mn-oxide precipitation record also correlates well with major palaeoclimatic events identified from the sedimentology, palynology and oxygen isotope records for oceanic sediments. Time periods characterised by higher erosion and more arid conditions correspond to an absence of supergene mineral precipitation ages. Conversely, periods identified as reflecting warm/humid conditions are consistent with strong pulses of supergene mineral precipitation. The results support the notion of an episodic distribution of weathering ages and. suggest that supergene mineral precipitation responds to climatic controls. This correlation also suggests that weathering geochronology may provide a numerical and independent method for the study of continental palaeoclimates. Used in tandem with the record preserved in sedimentary basins, weathering geochronology provides necessary direct constraints on continental weathering and landscape evolution.