Soil production on a retreating escarpment in southeastern Australia

The functional dependence of bedrock conversion to soil on the overlying soil depth (the soil production function) has been widely recognized as essential to understanding landscape evolution, but was quantified only recently. Here we report soil production rates for the first time at the base of a retreating escarpment, on the soil-mantled hilly slopes in the upper Bega Valley, southeastern Australia. Concentrations of 10 Be and 26 Al in bedrock from the base of the soil column show that soil production rates decline exponentially with increasing soil depth. These data define a soil production function with a maximum soil production rate of 53 m/m.y. under no soil mantle and a minimum of 7 m/m.y. under 100 cm of soil, thus constraining landscape evolution rates subsequent to escarpment retreat. The form of this function is supported by an inverse linear relationship between topographic curvature and soil depth that also suggests that simple creep does not adequately characterize the hillslope processes. Spatial variation of soil production shows a landscape out of dynamic equilibrium, possibly in response to the propagation of the escarpment through the field area within the past few million years. In addition, we present a method that tests the assumption of locally constant soil depth and lowering rates using concentrations of 10 Be and 26 Al on the surfaces of emergent tors.

[1]  William E. Dietrich,et al.  Quantification of soil production and downslope creep rates from cosmogenic 10Be accumulations on a hillslope profile , 1993 .

[2]  J. Weissel,et al.  The kinematics and pattern of escarpment retreat across the rifted continental margin of SE Australia , 1996 .

[3]  A. Costin Carbon-14 Dates from the Snowy Mountains area, Southeastern Australia, and their Interpretation , 1972, Quaternary Research.

[4]  J. Braun,et al.  Controls on post‐mid‐Cretaceous landscape evolution in the southeastern highlands of Australia: Insights from numerical surface process models , 1999 .

[5]  D. Lal,et al.  Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models , 1991 .

[6]  William E. Dietrich,et al.  Cosmogenic nuclides, topography, and the spatial variation of soil depth , 1999 .

[7]  I. Prosser,et al.  Holocene valley aggradation and gully erosion in headwater catchments, south-eastern highlands of Australia , 1994 .

[8]  K. Nishiizumi,et al.  Cosmic ray production rates of 10Be and 26Al in quartz from glacially polished rocks , 1989 .

[9]  I. Mcdougall,et al.  Potassium‐argon ages on the Cainozoic volcanic rocks of New South Wales , 1974 .

[10]  J. Nott,et al.  Long‐term drainage evolution in the shoalhaven catchment, southeast highlands, Australia , 1992 .

[11]  W. Dietrich,et al.  The soil production function and landscape equilibrium , 1997, Nature.

[12]  C. D. Oilier The Great Escarpment of eastern Australia: Tectonic and geomorphic significance , 1982 .

[13]  P. O’Sullivan,et al.  Multiple postorogenic denudation events: An example from the eastern Lachlan fold belt, Australia , 1996 .

[14]  H. Kooi,et al.  Escarpment evolution on high‐elevation rifted margins: Insights derived from a surface processes model that combines diffusion, advection, and reaction , 1994 .

[15]  J. Weissel,et al.  Influence of rock strength properties on escarpment retreat across passive continental margins , 1997 .

[16]  Jeffrey A. Dunne,et al.  Scaling factors for the rates of production of cosmogenic nuclides for geometric shielding and attenuation at depth on sloped surfaces , 1999 .

[17]  R. W. Galloway Late Quaternary Climates in Australia , 1965, The Journal of Geology.

[18]  P. Bishop Southeast Australian late Mesozoic and Cenozoic denudation rates: A test for late Tertiary increases in continental denudation , 1985 .

[19]  G. Tucker,et al.  Erosional dynamics, flexural isostasy, and long-lived escarpments: A numerical modeling study , 1994 .

[20]  R. Young The Tempo of Geomorphological Change: Evidence from Southeastern Australia , 1983, The Journal of Geology.