Slipstream: an early Holocene slump and turbidite record from the frontal ridge of the Cascadia accretionary wedge off western Canada and paleoseismic implications

Abstract

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 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 ΔR = 400 years). Rather, the top of the core appears to be 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 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.

Assessment of submarine slope failures off Vancouver Island, British Columbia

“…Data from the 2008 expedition (Haacke et al, 2008) and analysis and age-dating of sediments (Hamilton et al, 2015) have revealed that the slope-failures are all likely older than ~8,000 years. Sediment cores were taken at failures C, D, and E, as well as F during a subsequent expedition (Riedel and Conway, 2015).…”

“…Sediment cores were taken at failures C, D, and E, as well as F during a subsequent expedition (Riedel and Conway, 2015). Modern sediments draping the original failure-mass include turbidites from more recent megathrust earthquakes (Hamilton et al, 2015) but sediment thicknesses were insufficient to drape the original structures. Therefore, the apparent "fresh" look from the multibeam data is misleading.…”

“…13a), which has been termed 'Slipstream' by Hamilton et al (2015). The ridge rises ~500 m above the abyssal plain (Table 1).…”

“…The sedimentation pattern during glacial and interglacial periods is different and the interface between these two types of sediments can easily be seen in sediment cores (Hamilton et al, 2015). At the end of glaciation cycles, sedimentation was typically much higher and sediments from those deglacial deposits are often fine-grained (typically gray in color), silt or silty mud (Hamilton et al, 2015).…”

“…The purpose of the expedition was to characterize the sediments hosting gas hydrate deposits and to assess the role of gas hydrates in altering sediment properties and possibly affecting the nature of submarine slope failures potentially triggered by megathrust earthquakes and their tsunami potential (later further assessed by Scholz, 2013). The most striking outcome of the cores taken at the frontal ridges was a detailed record of turbidite events allowing the reconstruction of past megathrust earthquakes off Vancouver Island (Hamilton et al, 2015).…”