High-resolution bathymetry of the axial channels within Monterey and Soquel submarine canyons, offshore Central California

High-resolution multibeam bathymetry and chirp (compressed high-density radar pulse) seismic data acquired from an autonomous underwater vehicle outline in unprecedented detail the shape and near subbottom character of the axial channels within upper Monterey and Soquel Canyons (offshore California, USA). In Monterey Canyon, the bathymetric data span water depths from 100 m to >2100 m and include the confluence with Carmel Canyon at ∼1900 m water depth. The bathymetric data for Soquel Canyon begin close to the canyon head at 100 m water depth and extend down to the intersection with Monterey Canyon. The seafloor within the axis of Monterey Canyon is covered with sediment fill out to 910 m water depth. Below this water depth exposures of underlying strata are common, presumably because of decreasing sediment drape and generally increased erosional resistance of the pre-canyon host strata. The seafloor within the axial channel of upper Soquel Canyon is smooth and contains horizontally layered sediment fill. In contrast, the sediment fill within the incised portions of the axial channel of Monterey Canyon is characterized by distinctive crescent-shaped bedforms down to the limit of the surveys. These differences in morphology and texture correspond with the contrasting cohesive strength of the sediments filling these canyons and the increased propensity for weakly cohesive sands and gravels in Monterey Canyon to fail. Episodic movement of coarse-grained sediments down Monterey Canyon maintains a longitudinal gradient of ∼1.6°. The more cohesive fine-grained sediments in Soquel Canyon stabilize the seafloor and maintain a substantially higher longitudinal gradient (3°-6°) than that measured in Monterey Canyon. The textural and lithologic data, plus previously published observations, indicate that upper Monterey Canyon is currently active, whereas upper Soquel Canyon appears to be inactive as a coarse sediment transport conduit. Episodic seabed sediment failures in active submarine canyons are hypothesized to control the gradient of the axial channel. The propensity for sediment failure in weakly cohesive coarse-grained sediments results in shallower horizontal gradients compared to submarine canyons stabilized by more cohesive fine-grained sediments.