Geophysical Speculations on the Origin of Stepped Erosion Surfaces

Abstract

Stepped erosion surfaces are a widespread feature in Australia (as in other parts of the world) and suggest a phased lowering of relative sea level without deformation of the land during the later Cenozoic. A hypothesis is put forward that the stepped erosion surfaces are ultimately caused by continental "shift'' induced by deep convection currents. New ocean capacity is created in the "oceanogenic" zone, while a smaller ocean capacity is lost in the orogenic zone, leading to a eustatic drop of sea level. This causes rejuvenation of erosion in the non-orogenic land masses, which tend to rise isostatically. But as the young folded mountains also rise isostatically as compression ceases, they suffer extreme erosion, and the consequent sedimentation in the oceans leads to a eustatic rise of sea level. The isostatic adjustment of the loaded ocean floor and unloaded land, assumed to act with appreciable time lag, leads to a long-continued drop of relative sea level, eustatic insofar as sea level drops with the sinking ocean floor, epeirogenic insofar as the land rises. In older and higher erosion surfaces the epeirogenic component predominates. In lower and younger erosion surfaces the eustatic component may be more significant, but it can never account for more than about 800 ft. of their elevation above present sea level. The adjustment is spasmodic, but other factors may also operate to give the periods of stability of relative sea level which lead to the initiation of erosion surfaces. An erosion surface will continue to work upstream long after it has been once or several times removed from direct control by sea level/base level, and it will indeed do so today if still recognizable, even though it may have been initiated millions of years ago; we must be aware of this "age duplicity." The work particularly of Baulig (1935) and Du Toit (1937) must be thought to have foreshadowed the present hypothesis. Quantitative estimations of the effects of "shift" on relative sea level show results in reasonable agreement with the elevations of actual erosion surfaces, and so give some support to the hypothesis.