Origin of the shelf break off Southeast Australia

Abstract

Abstract Evidence from seismic reflection profiles across the southeast Australian continental margin indicates that in some areas eustatism has not influenced the shelf‐break zone. Where this is the case, insufficient sediment has accumulated along the rifted continental margin since the opening of the Tasman Sea to compensate for subsidence and to raise the sea floor to within reach of wave‐base erosive influences during low sea levels. In some instances there is no evidence of outbuilding, and the sediments are draped over basement structures; thus the change of slope at the shelf break is the surface expression of basement relief and changes in the depth of the shelf break are related to changes in depth of basement. Erosion during times of low sea level has played no part in the development of the shelf break and therefore it is not necessary to invoke neotectonism to explain variations in its depth. Other seismic sections reveal clear evidence of outbuilding of the wedge of sediments which accords with the generally accepted concept of continental shelf development. It is suggested that moulding of the major morphological elements of the outer shelf and development of the shelf break may have occurred as early as the late Miocene. The buried basement surface is commonly smooth at the crest of the continental slope, and irregular under the shelf. The smooth surface may be a marine abrasion platform formed during the initial transgression across the continental margin after rifting and opening of the Tasman Sea in the Late Cretaceous. The basal sediments resting on irregular basement under the outer shelf are probably continental in origin.