DURABLE TERRESTRIAL GEOLOGY AND CATCHMENT SLOPE PROMOTE SUBMARINE CANYON HEADWALL INCISION

Submarine canyons typically originate at amphitheater-like escarpments (headwalls) cut into the continental shelf. Despite their status as gateways between the terrestrial and marine realms, the processes responsible for headwall incision and connectivity to terrestrial sediment sources remains poorly understood, in part due to the difficulty of making direct process observations. In contrast, recent field and laboratory studies demonstrate that the flux and caliber of transported bedload are significant controls on bedrock incision by rivers. Could submarine incision be similarly influenced by coarse sediment flux?

Along the west coast of the contiguous U.S., twenty ‘active’) headwalls (< 10 meters depth) intercept littoral sediment and route it into submarine canyons. In contrast, over 700 ‘inactive’ headwalls appear to have been active during the last glaciation maximum (LGM), but have since been abandoned at the shelf edge during Holocene transgression. Canyon headwalls are notably absent offshore of much of central Oregon and parts of central California.

To better understand the boundary conditions required for submarine canyon headwall incision, we conducted a comprehensive assessment of the geology and geometry of 4855 terrestrial catchments >0.1 km² upgradient of both ‘active’ and ‘inactive’ submarine canyon headwalls (n=811) along the west coast of the contiguous U.S. and the Channel Islands of California. We also measured these parameters where headwalls are absent. We used an inverse distance weighted extrapolation to predict the relative characteristics (lithology, slope, rainfall) of fluvial sources to these ‘inactive’ headwalls during the LGM. Our analysis reveals strong correspondence between headwall occurrence and the integrated area of durable bedrock (plutonic and metamorphic rocks) draining to the coast, and an absence of headwalls where less durable bedrock (sedimentary and volcanic rocks) is drained. This finding is consistent with recent observations of higher bedload flux in rivers that drain durable lithologies of the Klamath Mountains versus those draining the less resistant Oregon Coast Ranges. Headwalls also occur offshore of small catchments that drain durable lithologies, where active tectonics have steepened terrestrial catchments and narrowed the shelf.