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TitleSlipstream: an early Holocene slump and turbidite record from the frontal ridge of the Cascadia accretionary wedge off western Canada and paleoseismic implications
AuthorHamilton, T S; Enkin, R J; Reidel, M; Rogers, G C; Pohlman, J W; Benway, H M
SourceCanadian Journal of Earth Sciences vol. 52, 2015 p. 405-430,
Alt SeriesEarth Sciences Sector, Contribution Series 20130237
PublisherNRC Research Press
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region; British Columbia
AreaSlipstream Slump
Lat/Long WENS-132.0000 -123.0000 52.0000 48.0000
Subjectssedimentology; marine geology; stratigraphy; slope failures; sedimentary wedges; marine sediment cores; piston cores; slumps; slump structures; turbidites; carbon-14 dates; submarine features; facies models; facies analyses; Cascadia Subduction Zone; hemipelagic sediments; foraminifera
Illustrationslocation maps; tables; photographs; core logs; stratigraphic columns; plots; diagrams
ProgramAssessing Earthquake Geohazards, Public Safety Geoscience
AbstractSlipstream Slump, a well-preserved 3 kmwide sedimentary failure from the frontal ridge of the Cascadia accretionary wedge 85 km off Vancouver Island, Canada, was sampled during Canadian Coast Guard Ship (CCGS) John P. Tully cruise 2008007PGC along a transect of five piston cores. Shipboard sediment analysis and physical property logging revealed 12 turbidites interbedded with thick hemipelagic sediments overlying the slumped glacial diamict. Despite the different sedimentary setting, atop the abyssal plain fan, this record is similar in number and age to the sequence of turbidites sampled farther to the south from channel systems along the Cascadia Subduction Zone, with no extra turbidites 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 underlying main slump have a disordered cluster of 14C ages between 12.8 and 14.5 ka BP. 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 from the most foraminifera-rich intervals define a nearly constant hemipelagic sedimentation rate of 0.021 cm/year. The combined age model is defined using only planktonic foraminiferal dates and Bayesian analysis with a Poisson-process sedimentation model. The age model of ongoing hemipelagic sedimentation is strengthened by physical property correlations from Slipstream events to the turbidites for the Barkley Canyon site 40 km south. Additional modelling addressed the possibilities of seabed erosion or loss and basal erosion beneath turbidites. Neither of these approaches achieves a modern seabed age when applying the commonly used regional marine 14C reservoir age of 800 years (marine reservoir correction deltaR = 400 years). Rather, the top of the core appears to be 400 years in the future. A younger marine reservoir age of 400 years (deltaR = 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 requiring no reservoir correction. The high-resolution dating and facies analysis of Slipstream Slump in this isolated slope basin setting demonstrates that this is also a useful type of
sedimentary target for sampling the paleoseismic record in addition to the more studied turbidites from submarine canyon and channel systems. The first 10 turbidites at Slipstream Slump were deposited between 10.8 and 6.6 ka BP, after which the system became sediment starved and only two more turbidites were deposited. The recurrence interval for the inferred frequent early Holocene megathrust earthquakes is 460 ± 140 years, compatible with other estimates of paleoseismic megathrust earthquake occurrence rates along the subduction zone.
Summary(Plain Language Summary, not published)
Study of a marine slide 100 km off Vancouver Island revealed a record of 12 layers caused by megathrust earthquakes. All of the Cascadia earthquake events seen further to the south are recorded here and there are no additional earthquakes present in this local record. The first 10 earthquakes occurred between 10.3 and 6.6 thousand years ago. The recurrence interval for these events is 394 ± 126 years. The intervals and event counts suggest that Cascadia fails all at once along its entire length for the megathrust subduction events.