GEOMORPHIC AND PALEOCLIMATIC IMPLICATIONS OF AN OXYGEN-ISOTOPE CHRONOLOGY FOR AUSTRALIAN DEEPLY WEATHERED PROFILES

A technique for obtaining age estimates for regolith profiles in Australia, based on the oxygen-isotope composition of the clay mineral assemblage in a profile, is applied to a variety of regolith profiles and kaolinitic sediments from across Australia. Excluding monsoonal regions in the nor-th of the continent, it is possible to distinguish profiles formed in the Late Mesozoic Early Tertiary (deltaO-18 values between +15 and +17.5 parts per thousand) from profiles formed in post-mid-Tertiary times(>+17.5 parts per thousand). In addition it is concluded that there remain widespread remnants of a deep-weathered regolith which developed in pre-Late Mesozoic (Early Cretaceous or Jurassic?) times when Australia was at high latitude. The low deltaO-18 values associated with clays formed in pre-Late Mesozoic times (+10 to +15 parts per thousand) suggest that deep weathering took place in a cool to cold and presumably humid climate, contrary to the traditional belief that deep weathering requires tropical to subtropical temperatures. The formation of deep-weathered profiles at high latitude in a comparatively cold climate may he linked in part to higher past atmospheric CO2 levels.