The cratonic surfaces of Australia represent one of the most stable landscapes on Earth. Features formed close to the land surface in early- and mid-Tertiary times, such as deepweathering profiles, silcrete and palaeochannel fills, are widely preserved indicating extremely low average erosion rates. We are investigating whether Quaternary erosion rates on these surfaces are compatible with their long-term preservation, or have increased to levels above the long-term average. The latter scenario, which might reflect Quaternary climate changes, has been invoked to explain preservation of ancient Australian landscape features. To address this question, we are combining 40Ar/39Ar dating of supergene Mn oxides and sulfates, to determine sub-surface weathering ages, with cosmogenic isotope studies, to measure erosion rates of the overlying duricrust surfaces over the past few million years.
Initial results covering three morphological and climatic provinces in the Mt Isa region of northern Australia show a roughly inverse correlation between erosion rates and weathering ages. The highest erosion rates, of 2-4 m/Ma, occur in the area of highest present-day rainfall, where deep-weathering ages are 3-6 Ma. Landscape surfaces in the more arid regions of the study area give much older weathering ages (up to 50 Ma) and lower erosion rates, on the order of 1 m/Ma or less. Thus far, the comparison between short-term erosion rates and long-term preservation of weathering profiles does not call for significant increases in Quaternary erosion. A more extensive data set will be presented at the meeting.
*Supported by National Science Foundation grant EAR-9805132.