Quantitative Measures of the Antiquity and Evolution of Cratonic Surfaces

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9805132StoneErosional planation surfaces are a predominant landform of cratonic regions worldwide, covering millions of square kilometers across all continents. Large areas of these surfaces are mantled by deep weathering profiles that mask bedrock geology and conceal underlying mineral deposits. Their planation can be dated, in many instances, to the early Tertiary and Mesozoic eras, and in some cases, even earlier. The subsequent downwearing and exposure history of these surfaces is generally less well constrained. Preserved planar forms and summit-height concordances, and the survival of ancient deeply-weathered mantles and duricrusts formed in the near-surface environment point to extremely low rates of erosion. In contrast, evidence of active denudation by physical, biological and chemical processes and results of initial cosmogenic isotope studies of rock surface erosion point to downwearing rates of metres per million years, which are difficult to reconcile with their survival. This apparent conflict will be explored by determining erosion rates of selected surfaces over the past 1-5 Ma with cosmogenic isotopes, and comparing them to the erosion rates required to exhume dated Mn oxide assemblages formed by ancient weathering of the same surfaces.Cosmogenic nuclide measurements will be made on resistant and indurated outcrops outcrops (e.g. silcrete, quartz veins within laterite) representative of the broad-scale geomorphic surfaces selected for examination. The time period averaged in these determinations will vary with erosion rate, from 1 Ma if erosion rates exceed 2 m/Ma, to a maximum of 5-6 Ma if erosion rates approach zero. These measurements are aimed at measuring both the magnitude and spatial variability of Quaternary erosion.Weathering ages for the same surfaces will be obtained by 40Ar/34Ar dating of K-bearing manganese oxide minerals precipitated in related deep-weathering profiles. Upper limits to the depth of K-Mn oxide precipitation (from modern anlogues and hydrochemical constraints), combined with the K-Mn oxide ages, will then rpovide upper limits on the average denudation rate since the time of weathering. Argon dating results from the Mt Isa district of Australia indicate that it will be possible to obtain average denudation rates over timescales ranging from 5 Ma to 65 Ma from stepped planation surfaces. Samples for cosmogenic isotope measurement have been collected from these surfaces but not yet analysed.Preliminary work in the Mt Isa area will be extended with studies in the Pilbara Block and Hamersley Basin of Western Australia, and the Carajas region and Quadrilatero Ferrifero on the Brazilian Shield. All three regions show a stepped morphology, with cores of high-standing, dissected plateaus standing above successively lower and younger inset plains. Landscape evolution in these areas can be traced back to the Mesozoic and early tertiary.Comparison of the short-and long-term denudation rates at the four study sites will: (i) Indicate whether the magnitude and spatial variability of Quaternary erosion rates are compatible with the preservation of cratonic planation surfaces. (ii) Reveal whether erosion rates on these surfaces have increased or decreased in the Quaternary, or persisted at rates similar to average values through the Tertiary. Depending on these outcomes, it may be necessary to develop geomorphic evolution models for cratonic terranes that account for planar forms and summit height concordances inherited from ancient planar precursors but lowered significantly and uniformly by subsequent erosion. combined with surface reconstructions, the results will also: (iii) Provide limits on the relative rates of surface downwearing versus scarp retreat, a key ratio in the development of the stepped landscapes characteristic of the areas to be studied.Comparisons between the four geographic areas will reveal the importance of eustatic and tectonic base level changes, local paleoclimate and global-scale forcing factors such as atmospheric CO2 changes and Quaternary climatic deterioration in determining erosional histories. This work will provide a detailed insight into the geomorphic evolution and recent rates of geomorphic activity in low-gradient, low-relief cratonic terranes. The results will be of importance to mineral exploration for ore deposits which occur in these terranes but are commonly concealed by deeply weathered cover. Reconstruction of sediment production and dispersal pathways through time, and the history of episodic weathering and erosion, will provide an interpretive framework for future geochemical prospecting.