SLIPSTREAM: AN EARLY HOLOCENE SLUMP AND TURBIDITE RECORD FROM THE FRONTAL RIDGE OF THE CASCADIA ACCRETIONARY WEDGE OFF WESTERN CANADA AND PALEOSEISMIC IMPLICATIONS

Slipstream Slump, a well-preserved 3 km wide sedimentary failure from the frontal ridge of the Cascadia accretionary wedge 85 km off Vancouver Island, Canada, was sampled during CCGS Tully cruise 2008007PGC along a transect of 5 piston cores. Shipboard sediment analysis and physical property logging revealed 12 turbidites interbedded with hemipelagic sediments overlying the slumped glacial diamict. This record is similar to the sequence of turbidites seen farther to the south along the Cascadia Subduction Zone, with no extra ones present in this local record. Given the regional physiographic and tectonic setting, megathrust earthquake shaking is the most likely trigger for both the initial slumping and subsequent turbidity currents, with sediments sourced exclusively from the exposed slump face of the frontal ridge. Planktonic foraminifera picked from the resedimented diamict of the main slump have ¹⁴C ages between 12.8 and 14.5 ka. For the post-slump stratigraphy, an event-free depth scale is defined by removing the turbidite sediment intervals and using the hemipelagic sediments. Nine ¹⁴C dates define a nearly constant hemipelagic sedimentation rate of 0.021 cm/yr. The age model is strengthened by physical properties correlations to the turbidites for the Barkley Canyon site 40 km south. The combined age model is defined using only planktonic foraminiferal dates and Bayesian analysis with a Poisson-process sedimentation model. The commonly used regional marine reservoir age of 800 years (ΔR=400 years) gives the top of the core an age 400 years in the future. A younger marine reservoir age of 400 years (ΔR=0 years) brings the top to the present and produces better correlations with the nearby Effingham Inlet paleo-earthquake chronology based only on terrestrial carbon. The high resolution dating and facies analysis of Slipstream Slump in this slope basin setting demonstrates that this is a useful type of target for sampling the paleoseismic record. The first 10 turbidites were deposited between 10.8 and 6.6 ka, after which the system became sediment-starved and only 2 more turbidites were deposited. The recurrence interval for the inferred Early Holocene megathrust earthquakes is 460 ± 140 years, compatible with other estimates of paleoseismic megathrust earthquake occurrence rates along the subduction zone.