Modern Depositional Processes within Astoria Canyon, Cascadia Margin

Astoria Canyon incises the continental shelf 22 km offshore the mouth of the Columbia River, a dominant sediment source to the Cascadia margin. Erosive turbidity currents carved the canyon during the Pleistocene low-stand in sea level, but as with most submarine canyons, its thalweg is infilling under modern high-stand conditions. Little is known about the sedimentary processes in northern Cascadia's canyons, but turbidity currents routinely occurred in each of them throughout the Holocene, evidenced by their deep-water turbidite record. This study addresses the modern depositional history and active transport processes in Astoria Canyon through 1) hydrodynamic data from a tripod deployed in the canyon head during May-Aug 2019, 2) a series of 50 cm-long sediment cores collected along the upper canyon and the surrounding shelf rim, and 3) co-located CHIRP profiles. Hydrodynamic data from the canyon head show that maximum current velocities during summer (70 cm/s) are driven by sustained downwelling-associated flows and brief up-canyon internal tidal bores. Supplemental data from long-term buoy records show that surface waves during winter drive significantly higher shear stresses across the seafloor, capable of remobilizing sand and mud not only on the shelf, but also within the canyon head. Sediment core ²¹⁰Pb geochronology indicates steady-state accumulation throughout the shelf and canyon. Integrating these rates with prior studies shows that upper Astoria Canyon exhibits the highest accumulation rates (~1.5 g cm^-2 y^-1) on the WA margin—this is likely a product of mid-shelf mud-belt resuspension and import via the alongshore transport system, and also the Columbia River's seasonal sediment supply to the canyon head during quiescent summer months. Down-core sediment grain size and x-radiography confirm that modern sands are delivered to the canyon thalweg, and bioturbation often overwrites physical structures. The southern shelf edge is more winnowed than the north, consistent with sourcing from the mid-shelf mud belt. The combination of high sedimentation rates and energetic shelf and slope hydrodynamics point to annual transport events within the upper reaches of Astoria Canyon, and perhaps the many other canyons along the Cascadia Margin.