Abstract

We examine the influence of bedrock strength properties, imparted by jointing and fracturing, on the retreat of erosional escarpments across passive continental margins. Two types of escarpment are identified, and focus is placed on the gorge head–knickpoint form of escarpment, where upland streams drain across the escarpment. Along the Macleay River system of eastern Australia, major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth, despite differences in bedrock lithology and order-of-magnitude differences in drainage areas upstream of the gorge heads, implying an average escarpment retreat rate of 2 km/m.y. since the margin formed 85 m.y. ago. Field observations from Apsley River gorge, a Macleay River tributary, indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls. Where dominant jointing dips into hillslopes, failure occurs by block and column toppling. Conversely, where jointing dips out of hillslopes, mass wasting occurs by joint-parallel rock sliding, allowing slopes to remain steep. As material is shed from the gorge sidewalls, the toppling slopes develop lower overall gradients, the gorge becomes asymmetric in cross section, and the channel remains nearer to the steeper sidewall affected by joint-parallel sliding. Coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint–gorge head and to facilitate mass wasting from the gorge sidewalls. Peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus, to incise the stream channel, and to erode the toes of the sidewalls. If these conditions on fluvial processes are met, then hillslope processes, including rockslope failure mechanisms, are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin.

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