Abstract
Abstract. A series of 23 thin, mostly mud-silt turbidites are found interspersed between larger, well-dated and regionally correlated paleoseismic sandy turbidites that extend along most of the Cascadia margin, northwestern United States. Investigation of the structure, distribution, and sedimentology of these thin mud-silt units supports the interpretation of these units as turbidites originating on the continental slope. Interpretation of mud turbidites is inhibited by bioturbation and lower response to analytical and imaging techniques; nevertheless most of the 23 interpreted beds exhibit most of the characteristics of coarser turbidites. These characteristics include sharp bases, fining upward sequences, darker color, increased gamma and CT density and magnetic susceptibility relative to the hemipelagic background, sparse microfossils, high lithic content, and evidence of transport from marine sources on the continental slope. New core data from sites south of Rogue Apron indicate that sandy and muddy turbidites may be correlated at least 150 km south to Trinidad Plunge Pool for the period ~ 4800 yr BP to present. Many of the mud turbidites initially described at Rogue Apron coarsen southward, becoming sandy turbidites. High-resolution Chirp seismic profiles reveal that turbidite stratigraphy along the base of the southern Cascadia continental slope is continuous, with little variation for at least 240 km along strike. The Chirp data show that turbidites along the Cascadia base of slope are ubiquitous, and likely not sourced solely from submarine canyon mouths, but may also have been delivered to the proximal abyssal plain as sheet flows from the open continental slope and coalescing local sources. Regional stratigraphy reveals that hemipelagic sedimentation rates and total Holocene turbidite thickness and mass are similar at widely separated sites, yet the total thickness of the Holocene section is greater by a factor of two in southern Cascadia. This difference is primarily due to the presence of the 21 mud and two additional sandy turbidites. We conclude that the Cascadia mud turbidites are ubiquitous along southern Cascadia only, with only one likely example of a correlated turbidite limited to the northern margin. Eight onshore sites including three marsh sites and five lakes include potential seismogenic correlatives of the southern Cascadia turbidites. In all, the onshore sites may have recorded > 80% of the events attributed to plate boundary earthquakes offshore during the period 0–6000 yr ago. Slope stability calculations suggest that earthquakes of Mw = 7.0 or greater should generate ground accelerations sufficient to destabilize open slopes and canyon heads with or without excess pore fluid pressure. Estimates of Mw for segmented ruptures are in the range of 7.4–8.7, exceeding the slope stability criteria for typical slopes by at least a factor of ~ four.
Highlights
Hydrology and Earth SystemQuantifying the temporal and sSpactiaiel pnactteernss of great subduction earthquakes through time remains elusive because our instrumental strain observations (GPS, strain meters, and seismology) commonly span one seismic cycle or less.Fanudndthame aepnptallicqaubeilsittiyonoOsf lrocengega-ratdeinrnmgSsslecigpimemenondctaeetlisorne,mcaluinsteurnianng, swered because we rarely have a long enough temporal and Published by Copernicus Publications on behalf of the European Geosciences Union
As foraminifer abundance is much lower in the latest Holocene, samples representing times younger than ∼ 2000 cal yr BP were more difficult to date
Older beds may be correlative, interpretation below that depth is hampered by increasing turbidite frequency, sparse hemipelagic intervals for datable material, and other factors as discussed in Goldfinger et al (2012), we focus on the latter half of the Holocene
Summary
Quantifying the temporal and sSpactiaiel pnactteernss of great subduction earthquakes through time remains elusive because our instrumental strain observations (GPS, strain meters, and seismology) commonly span one seismic cycle or less. A series of 19 Cascadia turbidites has been attributed to great subduction earthquakes based on stratigraphic correlation, radiocarbon evidence, consistency over much of the 1000 km length of the Cascadia margin, and close correspondence with other lines of evidence from paleoseismic sites onshore (Adams, 1990; Goldfinger et al, 2003, 2008, 2012). Analysis of the onshore data and turbidite data as a whole revealed some potential problems with the onshore time series, which were corrected in a collaborative effort with the original authors, and incorporated in the database and results of Goldfinger et al (2012) This effort yielded a stratigraphic record including 41 turbidites at Rogue Apron, at the mouth of Rogue Canyon offshore southern Oregon, and tentative correlations farther south. We add seismic–stratigraphic correlation to the suite of tools we are employing to build on and test the stratigraphic framework for intersite correlation previously constructed using core lithostratigraphy, physical property correlations and radiocarbon data alone (Goldfinger et al, 2012)
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