Submarine Canyons, Sediment Storage, and Mass Wasting in the Northern Cascadia Subduction Zone

The northern Cascadia Subduction Zone, offshore of Washington, is characterized by thick sediment accumulation and well-developed submarine canyons that incise the upper slope and outer shelf. Turbidite records from the base of these canyons have been used as a record of paleoseismicity, yet relatively little work has been done to examine the processes of sediment storage, mass wasting, and downslope transport that contribute to turbidity flows in these canyons. New USGS high-resolution sparker multichannel seismic and Chirp subbottom data, combined with detailed multibeam bathymetry collected across this region, provide insight into the morphology, structure, and stratigraphy of the margin.

The outer wedge of the marine forearc here is strongly influenced by Pleistocene glacial sedimentation, where high sediment flux is linked to a weak megathrust interface and development of a broad zone of landward vergent folds on the lower slope. Rapid sedimentation largely infilled these lower slope basins, allowing sediment to backfill many of the canyons and accumulate on the upper slope. The deep, broad paleo-valleys of the Quillayute, Guide, and Willapa Canyons are nearly filled with glacial sediment and now incised by small, shallow channels regrading across a low gradient slope (1-3°). Small-scale mass wasting is pervasive here in the form of canyon sidewall failures, propagating headscarps that fail down to discrete horizons, and open slope gullies that drain canyon interfluves. The steepest upper slope gradients (>10°) are found between Grays and Quinault Canyons, where shallow, listric normal faults along the outer shelf appear to be enlarging the canyon heads and contributing to steep dendritic drainages along a 75km-long swath of the shelf-edge. To the south, Astoria Canyon, a lowstand outlet for the Columbia River, is relatively narrow and steep-walled, confined by lithology and structural uplift along the flanks. These steeper canyon systems appear to have limited sediment storage in the upper reaches, with most deposition on the lower slope and in deep sea fans. Overall, the margin still appears to be re-equilibrating to the massive glacial sediment influx, the lithology of which also likely plays in important role in processes such as fault lubrication and seismic strengthening, further shaping the margin.