Cratonic Erosional Unconformities and Peneplains

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Abstract
Using the West Australian craton as a model, but with reference to other shields, a highly generalized planation history of world-wide cratonic regions is traced from Proterozoic to present. Erosional unconformities and associated sedimentary veneers are found to be closely comparable in attitude and altitude, although progressively lower through time. The oldest, still nearly horizontal unconformity is Proterozoic, and subsequent unconformities tend to conform to this original cratonic planation surface. The erosion-sedimentation history has been one of repeated exhumation and reburial, though in places it involved "stacked" veneers. This morphogeodynamic pattern of events we call the cratonic regime. The physiographic regularity of the cratonic regime is interrupted or disturbed in intracratonic basins or upwarps, and along plate margins. Rifted margins involve simple upwarp, deep fluvial dissection, and subsequent relaxation with reburial. Periodic Proterozoic and Phanerozoic glaciations caused further warping and scouring, creating, for example, fjords along the early Gondwana rifts. In the more stable areas the mean rate of lowering of the primary cratonic planation surface has been on the order of 10-20 cm per million years ( ). The progressive reduction of the cratonic surface through time is ascribed to an alternation of etchplanation and pediplanation, leading to a polygenetic surface of low relief. We would like to employ for its description the long-established term peneplain, but without genetic connotation.