Large-scale modification of submarine geomorphic features on the Cascadia accretionary wedge caused by catastrophic flooding events

Abstract

We identify and describe submarine channels, submarine landslides, and three unusual erosional features on the toe of the Cascadia accretionary wedge near Willapa Canyon, offshore Washington, USA. We use new high-resolution multibeam bathymetric data and chirp sub-bottom and multichannel seismic-reflection profiles. Composite data sets were generated from the Cascadia Open-Access Seismic Transects (COAST) cruise and from the site survey cruise for the Cascadia Initiative. This high-resolution data set has illuminated geomorphic features that suggest that this section of the margin underwent largescale erosion event(s) which likely occurred during the latest Pleistocene. Three unusual features imaged superficially resemble slope failures of the landward-vergent frontal thrust ridge but are distinguished from such failures by (1) complete or near-complete incision of the crest of the frontal thrust, anticlinal ridge, and piggyback basin; (2) the lack of semi-coherent blocky landslide debris; (3) asymmetrical incision of feature floors to levels well below the abyssal plain; and (4) connections to the main Willapa Deep-Sea Channel by likely co-genetic but now barely active paleochannels. We conclude that the unusual geomorphic features were likely created by massive turbidity currents created by the Missoula glacial-lake outburst flood events. The floods directed massive sediment volumes through the Willapa Submarine Canyon System, eroding a broad swath of the accretionary wedge and either cutting through or causing slope failure of the frontal thrust. Turbidity- current modeling on a bathymetric reconstruction supports our hypothesis that a large-volume flow like the Missoula floods could have inundated the paleodrainage system and created the unique features we imaged.