. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 243 related to a sea-level lowered to within about 5 fm of the break (Dietz and Menard, 1951). It is clear, from several lines of reasoning, that the sea-level has recently risen eustatically, and that, in general, this recent rise has taken place more rapidly than changes of land level affected by erosion, sedimentation, or diastrophism. However, the shelf break falls at different depths in different areas, ranging from 12 fm to 250 fm. The mean depth of the shelf break is approxi
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 243 related to a sea-level lowered to within about 5 fm of the break (Dietz and Menard, 1951). It is clear, from several lines of reasoning, that the sea-level has recently risen eustatically, and that, in general, this recent rise has taken place more rapidly than changes of land level affected by erosion, sedimentation, or diastrophism. However, the shelf break falls at different depths in different areas, ranging from 12 fm to 250 fm. The mean depth of the shelf break is approximately 73 fm. Heezen et al. (1959) suggested that the lack of agreement in the depth of the shelf break is due to the fact that shelf break does not everywhere represent a single ancient eustatic level. Thus, the most prominent break may locally be a Pliocene or Miocene structural bench, but elsewhere, late Pleistocene or Recent strata may form the shelf break. In some areas the 800 900 100O 1100 1200. 100 200 300 400 East West Fig. 11. Shelf break in the continental slope off New York. The record was made with the Precision Depth Recorder. (After Heezen et al., 1959.) late Pleistocene eustatic low level may truncate the continental slope and there the shelf break may truly represent a late-stage eustatic sea-level. The deeper structure of the continental margin indicates a fundamental structural discontinuity at the base of the continental slope (Fig. 8). It would seem a small extrapolation to attribute a fault origin to the continental slope. Although faulting may have played a part in the earhest history of the Category II provinces, alternate periods of sedimentation and marine planation on the continental shelf, and long-continued erosion by slumps, turbidity currents and deep-sea currents on the continental slope, together with a general sub- sidence of the area, could alone have produced the characteristic form of the continental terrace. d. Submarine canyons Submarine c
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