Long-Term Landscape Evolution: Early Miocene and Modern Rivers in Southern New South Wales, Australia

Abstract

Basaltic lavas that flowed into river valleys draining westward from the uplands of southern New South Wales have preserved the Early Miocene landscape to a remarkable degree. The reconstruction of this landscape shows that the major topographic features of the highest part of the Australian continent had taken on essentially their present form by the Mid-Tertiary, and that some of these features were formed during the Mesozoic. Plateau surfaces have been lowered by only about 2 to 5 m/Ma, and most major streams have incised by only about 5 to 18 m/Ma, with a maximum incision of 30 m/Ma, since the Early Miocene. Despite the passage of 20 Ma, stream gradients have not waned, nor have major breaks in profiles been eliminated. On the contrary the modern stream profiles are strikingly similar to their Early Miocene counterparts, suggesting that the evolution of the profiles has been determined mainly by variations in stream power, rather than by the headward retreat of nickpoints through the drainage system. These findings, together with those of earlier research in the uplands of New South Wales, imply a rate and pattern of Cenozoic denudation very much at odds with widely accepted models derived from areas, like the Appalachians, that have no chronological and stratigraphic datum comparable to the widespread lavas of southeastern Australia.