Climatic and Geodynamic Significance of Cenozoic Land Surfaces and Duricrusts of Inland Australia

摘要

Cenozoic duricrusts of interior Australia comprising, in younging order, silcretes, ferricretes, calcretes and gypcretes are products of surficial processes within pedogenic and shallow phreatic systems. They are related to climate and to specific land surfaces and demonstrate a systematic reduction of precipitation from Eocene times to Recent. Early Tertiary columnar silcretes of the Cordillo Surface of interior Australia reflect warm, pluvial conditions at a time, when Australia was still proximal to Antarctica. The inundation of a vast peneplain caused deep leaching of underlying rocks by acid groundwaters in a reducing environment, resulting in kaolinization of feldspars and other silicate minerals. Reduced precipitation in the Oligocene combined with structural differentiation caused reworking of columnar silcrete and the formation of silcrete breccias cemented by ferruginous silica in an oxidizing environment. Seasonally arid conditions leading to fluctuating groundwater levels in Miocene times are indicated by the formation of extensive ferricretes succeeded by cherty dolomites. A further increase of aridity in the Late Tertiary caused widespread formation of gypcretes and chert breccias. The final stage of this development is characterized by precipitation of halite and locally by subhalite formation of sulphides. The development of Australian duricrusts and intervening sediments thus reflects the separation of Australia from Antarctica and northward drift over some 27 degrees of latitude during the last 55 million years, and also a dramatic change of global climatic belts during the course of the enozoic.