Abstract

Acoustic-reflection profiles showing the thickness of marine sediments and details of the structure at shallow depth within underlying bedrock make available new quantitative data for sedimentological and structural study of submerged continental margins.

Acoustic profiles across narrow continental shelves and upper basin slopes off southern California show barren wave-cut rocky shelves. In other localities they show wave-cut rock surfaces covered with up to 60 feet of “Recent sediment”. More complex areas have what is believed to be Pleistocene alluvium covering a strong reflecting surface, probably older bedrock, with Recent sediments superimposed. A majority of the southern California basin slopes are believed to be dip slopes of adjacent landward structures shown on the recorded profiles.

Recent sediments range widely in thickness and are generally lenticular with greatest accumulation on the central part or inner part of the shelf. These deposits mark our modern drowned shelves as areas of real or potential deposition; the body of California shelves, however, is made up of ancient rocks and structures modified by wave planation in lowered Pleistocene seas. The present depth and profile of California shelves are vestiges of the Pleistocene which have been slightly or locally modified.

Shear-strength tests of sediment from special continental-slope cores taken off southern California indicate that these steep slopes are at least locally stable and that considerably greater thicknesses of sediment could accumulate on them. Acoustic penetration of these slope deposits shows they are relatively thin compared to nearby shelf deposits. These relatively thin but structurally stable slope deposits indicate that much of the detritus introduced to the continental shelf in this area may be carried diagonally across the shelf and trapped by canyons which act as arteries of distribution to the deep sea.

Sediment cover and the structure at shallow depths of the continental shelf and upper slope together with shear-strength tests of slope deposits off California indicate that most theories of the origin of the continental terrace may be too generalized.

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