Sources and Triggers of Seabed Remobilization Within Canyon Head Environments on the Cascadia Margin

A significant volume of sediment is transported across the shelf on active margins, and once emplaced in a range of slope environments, has the potential of being remobilized by processes such as extreme surface-gravity wave events, internal wave motions, and potentially earthquake shaking. Instrumentation was placed in the head of Astoria Canyon, which incises the Cascadia Margin offshore of the Columbia River, to evaluate flow velocity, bed stress and bed sediment response over a 4-mo period capturing the transition from high to low seasonal discharge. Suspended sediment concentrations of up to ~100 mg/L, sediment trap samples, and fine unconsolidated bed sediment evidence active sediment delivery to the head of the canyon. Within the canyon head, observed surface gravity waves of ~4-5 m were capable of resuspending sandy mud sediment from the bed, and long-term buoy records show that extreme wave conditions of > 9 m occur ~1% of the time on the margin. Intermediate nepheloid layers at shelf depth observed within the canyon indicate capture of sediment from the alongshelf transport system, including sand. Brief, but highly energetic, internal bores were seen progressing up the canyon followed by periods of enhanced sediment in suspension, with concentrations that have the potential to form sediment gravity flows and move fine sediment deeper into the canyon. These observations provide initial constraints on the dynamical stages of delivery to and formation of sediment gravity flows across the margin, and are key to improving our understanding of the triggering, magnitudes, and resulting deposits of rapid downslope transport fluxes.