Sedimentary characteristics of samples collected from some submarine canyons

Abstract

Oriented rectangular cores of 20.3 × 30.5 cm and 45.7 cm high have been collected in a number of submarine canyons off southern California (U.S.A.) and off the southern tip of Baja California (Mexico) for a detailed study of their sedimentary structures. By applying several methods, mainly X-ray radiography, to vertical slices of the sediment samples, an abundance of information was obtained. The clayey shelf sediments in southern California reveal few primary sedimentary structures, which are only slumped and reworked by burrowing organisms. Sediments from the axis of the submarine canyons are mainly coarse and contain parallel lamination, current-ripple lamination, and sometimes small-scale graded bedding; slump phenomena and locally some reworking by animals are less important structures. A short distance outside the canyon axis the samples are much finer in grain size; they show fewer primary structures and more secondary ones. Sometimes the whole sample is completely disturbed by burrowing organisms, as is the case in a sample collected from a canyon terrace. One sample was collected from a gully on the apron of the La Jolla Fan Valley. This is similar to the facies model of ancient turbidites with regard to the type and succession of the sedimentary structures present. A box core from a small basin in the middle of the Gulf of California was found to be entirely reworked by organisms.