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.