Backscatter Image of Santa Monica Margin

[Backscatter of Los Angeles Margin]

 Backscatter data were mapped at:

  • 2-m grid spacing from the 20 to 40 m isobath
  • 4-m grid spacing between the 40 and 100 m isobath
  • 8-m grid spacing >100 m depths

The almost featureless low backscatter of most of the continental shelfis interrupted by very high backscatter on the Santa Monica Plateau. Thehigh backscatter areas represent outcrops of Pliocene sediment first mappedby Vedder (1). The plateau presently lies at less than 100-m water depths;consequently, during the Late Pliocene and Quaternary eustatic lower sealevels the plateau was exposed and sediment was stripped from the surface.Vestiges of these same outcrops (shown by high backscatter) can be seenon the west and south sides of the Palos Verde inner shelf. The remainderof the shelf appears uniformly low in backscatter intensity.

A previously unidentified scissor fault trends across the lower PalosVerdes slope and a parallel fault at the base of the slope has capturedthe channel of Redondo Canyon and deflected it to the southeast along thebase of slope for more than 4 km before it makes its way onto the SantaMonica Basin proper.

The effects of the Los Angeles River extending across the continentalshelf during each eustatic low sea level can be seen by the large sedimentwedge that has buried the continental margin. Compare the steep and deeplyincised continental slope off Malibu with the gently sloping and gulliedslope directly west of Los Angeles. The Los Angeles River trended directlyto this area until 1824 when it was permanently diverted toward Long Beachby a large flood.

The shaded-relief image of the land surrounding Santa Monica Bay wasconstructed using USGS 7.5 minute DEMs (Digital Elevation Models).

(1) Vedder, J.G., L.A. Beyer, Junger, Arne, G.W. Moore, A.E. Roberts, J.C.Taylor, H.C. Wagner. 1974. Preliminary report on the geology of the continentalborderland of southern California: U.S. Geological Survey MiscellaneousField Studies Map MF-624, p34, 9 sheets.

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