Shelf Break Off Research Articles

Origin of the shelf break off Southeast Australia

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.

Shallow Structures: Shelf Edge of Continental Margin Between Cape Hatteras and Cape Fear, North Carolina: GEOLOGICAL NOTES

Two types of structural framework are predominant on shallow seismic profiles recorded at the continental shelf edge off North Carolina between Cape Hatteras and Cape Fear. At least six prominent progradational sedimentary units, which are separated by disconformable surfaces, indicate that the continental shelf edge seaward of Cape Fear has been formed mainly by outbuilding. Uniformly thick parallel beds predominant north of Cape Fear are the result of relatively uniform upbuilding-outbuilding processes. Within this parallel bedded structure, a distinct erosional surface can be traced from the outer shelf to the upper continental slope, where it is truncated by another buried erosional surface. Surface topographic features near the shelf break off North Carolina have been formed largely by erosional processes, although locally organic accretion may have occurred. Small terraces and ledges that are exposed or buried were eroded into the outer shelf and upper slope during pauses in the fluctuating sea levels of the Pleistocene and Holocene Epochs. Notches on the upper continental slope appear to have two origins: (1) erosional, i.e., the notch was carved by the Gulf Stream, and (2) erosional or nondepositional, i.e., the notch was formed between the exposed back slope of a terrace and the sedimentary wedge occurring on the terrace platform.

Bathymetry of the Gibbs Hill area, Bermuda

Precision depth recordings were made South of Gibbs Hill, Bermuda, along a number of closely spaced sounding tracks. The accuracy of transit navigation and the sounding coverage were considered sufficient to justify correcting the soundings for both sound velocity and slope. The corrected soundings show that a steep sided submarine spur extends south-south-east from the shelf break off Gibbs Hill. The spur has a topographic relief of 300 fm and is about 3 miles long and 1 2 mile wide. At the 1000 fm curve, the feature broadens into a platform that extends another 12 miles seaward to the 2000 fm curve. True slopes along the steep portion of the spur are as high as 50° and the bottom echoes appear to be from a number of highlights causing “crescent” on the record. Where a definite break in slope occurs, the apparent bottom shown on the bathygram is that of a diffraction pattern from an edge and all points on this crescent migrate to a point, which is the true position of the break in slope.