Presenter: Evan Lahr (University of Washington Seattle)

Description:
Sedimentary deposits from submarine canyons reflect the cumulative action of continuous and episodic transport processes, providing an opportunity to interpret the prevalence of past oceanic and geologic phenomena. Although many submarine canyon systems have transitioned to net-depositional sedimentary environments during the Holocene sea level high stand, oceanographic processes remain capable of periodically initiating density driven flows in these less active systems. Astoria Canyon, located along the northern Cascadia Margin, is currently a depositional sedimentary environment, and it exhibits traits common in active canyon systems including a shelf-incised thalweg, a major fluvial supply of sediment, and energetic oceanic processes. Sediment coring surveys were conducted in 2019 and 2021 along the canyon axis to quantify rates of accumulation and to identify structures indicative of episodic deposition and density-driven flows. Core sediment was analyzed to calculate ²¹⁰Pb accumulation rates and to identify depocenters along the canyon floor. These data were used to establish a sediment-accumulation budget for the upper canyon. Coarse layers were evaluated by x-radiography, grain size, x-ray fluorescence, and microscopy to distinguish them from the ubiquitous fine-grained sediments accumulating in the canyon and to constrain plausible source deposits. The steady-state accumulation of fine sediment dominates along the canyon floor, accumulation ranges from negligible to 1 g cm² y^-1, and the greatest depocenter is located at a depth >400 m. These results demonstrate that Astoria Canyon functions primarily as a site of considerable steady-state sediment accumulation, yet episodic sediment transport continues to play a role in canyon dynamics.