Late Cenozoic sedimentary forms on the northwest Australian continental shelf

Abstract

Little terrigenous detritus is supplied to the broad northwest Australian continental shelf which, in common with many other shelves, is a zone of winnowing and transport rather than deposition. Shallow seismic reflexion profiles across the shelf and upper continental slope reveal apparent gentle warping of the Late Cenozoic sediments and at least three erosional surfaces above a disconformity identified with the important regional Late Miocene hiatus. The Late Miocene erosion surface is believed to be relatively flat-lying over quite wide areas and to crop out on the upper continental slope at 240 m water depth south of the Rowley Shoals. The Rowley Shoals themselves stand on a gently sloping terrace which is believed to be an expression of this same surface downwarped to about 400 m. The outer edge of the shelf south of the Rowley Shoals supports a long low ridge apparently consisting of fine-grained sediment and Globigerina sand. Seismic evidence indicates that this ridge is a constructional sedimentary body unrelated to submerged land forms or to offshore bars. It rests on a flat-lying channelled surface of erosion and marks a zone of maximum accumulation of pelagic material and fine sediment winnowed from the shelf. Such sediment normally is deposited in much deeper water on the continental slope. Other plano-convex lenses of sediment probably occur deeper in the Late Ceinozoic sedimentary sequence on the continental shelf and might be misinterpreted as structural deformation.