Physiography of the Exmouth and Scott plateaus, Western Australia, andadjacent northeast Wharton Basin

The continental margin of Western Australia is a rifted or 'Atlantic'- type margin, with a complex physiography. The margin comprises a shelf, an upper and lower continental slope, marginal plateaux, a continental rise, and rise or lower slope foothills. Notches or terraces on the shelf reflect pre-Holocene deposition of prograded sediment, whose seaward limit was determined by variations in relative sea level, wave energy and sediment size and volume. The upper continental slope has 4 physiographic forms: convex, due to sediment outbuilding (progradation) over a subsiding marginal plateau; scarped, due to erosion of convex slopes; stepped, due to deposition at the base of a scarped slope; and smooth, due to progradation of an upper slope in the absence of a marginal plateau. Lying at the same level as the upper/lower slope boundary are 2 extensive marginal plateaux: Exmouth and Scott. They represent continental crust which subsided after continental rupture by sea-floor spreading. Differential subsidence, probably along faults, gave rise to the various physiographic features of the plateaux. The deep lower continental slope is broken into straight northeasterly-trending segments, that parallel the Upper Jurassic/Lower Cretaceous rift axis, and northwesterly trending segments that parallel the transform direction. The trends of the slope foothills are subparallel to the rift direction. The 4 abyssal plains of the region (Perth, Cuvier, Gascoyne and Argo) indicate a long history of subsidence and sedimentation on Upper Jurassic/Lower Cretaceous oceanic crust.