Sediment Transport and Gravity Flow Routing in Highstand Submarine Canyons: Multiscale Evidence from Sediment Stratigraphy and Water Column Observations

Abstract

Submarine canyons are the sites of the largest individual sediment transport events on Earth—sediment gravity flows—which over time build submarine fans, which are among the largest sedimentary deposits on the planet. While canyons are known to exert globally significant control on sediment transport, modern sediment transport events remain poorly understood due to challenging data collection and unpredictable event timing. Today, most canyons are partially decoupled from the coastal processes that are known to trigger flows, and this has led to the view of a majority of highstand submarine canyons as sedimentologically inactive, with implications for the interpretation of stratigraphy accumulating on continental margins. This thesis presents four complementary studies that integrate sediment stratigraphy and nearbed hydrodynamic observations to investigate event-driven sediment transport in seismically active, highstand-detached submarine canyons. The first study examines the seasonal processes that initiate sediment transport through a highstand-detached submarine canyon head on the northern Cascadia margin. Using nearbed hydrodynamic observations collected over a three-month deployment, this study evaluates how surface waves and river plume dynamics resuspend and transport sediment under modern oceanographic conditions. Results reveal distinct seasonal transport regimes, with summer transport dominated by persistent canyon-aligned currents and internal waves, and more energetic winter conditions estimated to generate bed stresses sufficient for widespread sediment resuspension. These observations demonstrate that sediment transport remains active within the canyon despite separation from littoral and fluvial processes and that the relative importance of transport mechanisms varies seasonally. The second study analyzes sediment cores from a canyon head to evaluate whether nonseismic turbidity currents are preserved in modern canyon stratigraphy. Turbidite deposits are confirmed within the canyon, and multiple metrics of geochemical and sedimentological properties reveal modern, nonseismic turbidite deposits. These turbidites are ~15 cm thick, graded, and internally laminated, with low excess 210Pb activity and depositional ages that align with major Columbia River floods. Their organic matter composition is distinctly terrestrial and recent (δ¹³C = −26‰, C:N = 18, and elevated lignin concentrations), indicating a fluvial source. These results provide the first direct evidence of modern nonseismic turbidite deposition in northern Cascadia and demonstrate that highstand canyon stratigraphy preserves flood-driven event beds. They further show that turbidite composition reflects sediment source and that such deposits contribute significantly to sediment and carbon accumulation within canyon systems. The third study expands this framework by comparing flood- and earthquake-generated deposits within the same canyon system. Astoria Canyon, which has experienced both major floods and the 1700 CE Cascadia megathrust earthquake, provides a natural laboratory for linking event deposits to independently constrained triggers, with particularly well-preserved stratigraphy in an oxygen minimum zone. Sediment core geochronology, geochemical composition, and density structure reveal two distinct modes of gravity-flow deposition: terrestrial-rich turbidites generated by river floods and heterogeneous, marine-sourced mass transport deposits associated with seismic events. A diagnostic compositional model can distinguish between these origins at multiple core sites along the canyon axis, demonstrating that organic carbon signatures can provide a robust indicator of deposit source, whereas internal density structure varies over short spatial scales and is not consistently diagnostic. These findings revise assumptions of highstand canyon sediment routing by showing that nonseismic sediment gravity flow triggers remain active, coseismic deposits can accumulate on the upper slope, and deposit structure is not reliably correlatable over short distances. The fourth study addresses the process-based controls on sediment transport through direct observation. Although highstand canyons are often overlooked as active sediment transport systems, their bathymetry strongly modifies ocean circulation and enhances the exchange of water, nutrients, and sediment across continental margins. Using year-long moored observations in two adjacent canyons with contrasting morphology, this study evaluates whether sediment transport is synchronous under similar oceanic forcing and the parameters needed to form sediment gravity flows based on the canyon head dynamics. Results show that bottom boundary layer currents are persistently downslope and that sediment resuspension occurs under a range of conditions, with smaller events during summer and larger events associated with storm waves and increased river discharge. Despite similar regional forcing, sediment transport between the two canyons is largely asynchronous, indicating that canyon morphology exerts a primary control on sediment routing. Together, these studies demonstrate that highstand submarine canyons are not dormant but instead host complex, episodic sediment transport regimes driven by both seismic and oceanic processes. These findings improve upon the conventional interpretations of highstand dormancy and seismoturbidite records, and more broadly, this dissertation shows that highstand submarine canyon channels remain partially active and the dynamics within canyon heads impose a complex filter on downslope transport.