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Journal of Sedimentary Research

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Provenance, Stratigraphic Architecture, and Hydrogeologic Influence of Turbidites on the Mid-Ocean Ridge Flank of Northwestern Cascadia Basin, Pacific Ocean

¹ Department of Geological Sciences, 101 Geology Building, University of Missouri, Columbia, Missouri 65211, U.S.A.; UnderwoodM{at}missouri.edu
² Department of Geological Sciences, 101 Geology Building, University of Missouri, Columbia, Missouri 65211, U.S.A.; present address: Missouri Department of Natural Resources, Jefferson City, Missouri 65109, U.S.A.
³ Earth Sciences Department, University of California, Santa Cruz, California 95064, U.S.A.
⁴ Pacific Geosciences Centre, Geological Survey of Canada, Box 6000, Sidney, British Columbia V8L 4B2, Canada
⁵ Earth Sciences Department, University of California, Santa Cruz, California 95064, U.S.A.; present address: Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185, U.S.A.
⁶ Department of Geosciences, P.O. Box 33 04 40, University of Bremen, 28334 Bremen, Germany
⁷ Earth Sciences Department, University of California, Santa Cruz, California 95064, U.S.A.; present address: Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, U.S.A.

The northwestern edge of Cascadia Basin (North Pacific Ocean) is unusual because late Pliocene to Holocene turbidites lap onto juvenile oceanic crust of the Juan de Fuca Ridge. Subsidence of the ridge flank combines with irregular westward progradation of the turbidite facies to create a stratigraphic section that coarsens and thickens upward. The sand provenance is mixed. Individual turbidity currents have funneled into the area through several shelf–slope and abyssal-floor conduits, including Vancouver Valley, Juan de Fuca Channel, Barkley Canyon, and Nitinat Canyon. Local bathymetric blockage, deflection and reflection of flow paths by basement relief, remobilization by intrabasinal submarine slides and debris flows, episodic channel switching, and sporadic overbank flooding have combined to produce erratic recurrence intervals for the turbidity currents. Only the tallest basement highs have remained isolated from turbidite deposition during the last 500,000 years. Spatial and temporal differences in sediment accumulation are important because they modulate the three-dimensional responses of compaction and consolidation. Those changes in physical properties govern where and when hydraulic communication with the underlying basement shuts down. The basal hemipelagic layer of Cascadia Basin transforms to an effective hydrologic seal (seepage rates < 1 mm/yr) once the sediment–basalt interface is buried by 100–150 m of strata. Rapid accumulation of turbidites, therefore, accelerates the hydrogeologic conversion of igneous basement from open to sealed.