Abstract

Observations made from a submersible operating within the sinuous central segment of La Jolla submarine canyon have provided spectacular examples of erosion at two outcrops below a depth of 300 m. One outcrop displays extensive destruction through bioerosion of siltstone members. Burrowing and grazing activity of galatheid crabs has led to slumping and spalling where cavities cluster, intersect, or parallel the face of this outcrop.

A second site on the outside wall of a right-angle bend in the canyon axis revealed an undercut extending 23 m beneath the vertical canyon wall. A well-established encrusting fauna reaching to the canyon floor indicates that abrasive high-velocity currents have not occurred at this site for at least 6 months. The depth of this feature requires that the process responsible for such erosion take place on the sea floor. Sediment samples from the fan and fan valley below this part of the canyon reveal coarse deposits lacking features typical of turbidites, yet indicative of high-velocity submarine currents. It is concluded that the undercut is a relict erosional feature remaining from a previous period (Pleistocene) when runoff producing the latest rapid rise in sea level must have generated “submarine floods” charged with coarse detritus.

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