Modern Depositional Processes within Astoria Canyon, Cascadia Margin

Astoria Canyon incises the central Cascadia Margin, 22 km offshore of the dominant sediment source along the margin, the Columbia River. Sediment-gravity flows have occurred in the canyon during the last ~7 ka, with some turbidity currents reaching the deepwater Astoria Fan. The modern supply of sediment to Astoria Canyon and the processes that act to transport, remobilize, and deposit sediment within the canyon are poorly understood. A series of ~60 cm-long sediment cores were collected along the upper canyon thalweg and the surrounding shelf rim, along with co-located CHIRP sub-bottom profiles and hydrodynamic data from a tripod deployed in the canyon head during May–Aug 2019. Together, these data characterize the modern depositional environment both within the upper canyon and on the adjacent shelf. Grain-size analyses and 210Pb geochronology of the cores indicate that modern sands have been delivered to the canyon thalweg, suggesting off-shelf transport processes are active under highstand conditions and could provide sediment to the Holocene turbidites observed down canyon. Modern sediment characteristics and depositional styles are distinct between regions along the northern and southern canyon rim, and CHIRP profiles aid in outlining the spatial extent of these facies. Regional variability in bed sediments from short cores demonstrates a heterogeneous remobilization potential along the canyon rim and implies directional import to the canyon head from the north. Once in the canyon head, tripod data show that local hydrodynamics are sufficiently energetic to remobilize bed sediment. This combination of active sedimentation and transport processes within the canyon head help to define the range of environmental phenomena capable of importing and remobilizing sediments within Astoria canyon, and perhaps other canyons along the Cascadia Margin.