Acoustic stratigraphy, structure, and history of quaternary deposition in Cascadia Basin

Abstract

Seismic reflection profiles over northern Cascadia Basin, Juan de Fuca Ridge, and Juan de Fuca Abyssal Plain exhibit a thick sedimentary sequence, consisting largerly of turbidites, covering basement. Although most of the section is less than 0·7 × 106 years old, two periods of tectonism and four stratigraphic units can be identified. The two lower units once extended from the continental margin across the basin, covering the north end of Juan de Fuca Ridge and the area west to Explorer Ridge as an abyssal plain. The principal dispersal route at this time, Juan de Fuca Channel, trended roughly north-south on the western side of Cascadia Basin. Block faulting on northern Juan de Fuca Ridge and later, broad upwarping of sediments in the western half of the basin altered this dispersal system, isolating Juan de Fuca Abyssal Plain from subsequent turbidite deposition and shifting the primary thalweg of Cascadia Basin to its present position along Vancouver Valley. Nitinat Fan began to develop during the most recent sedimentary regime, perhaps in response to increased turbidity current activity on the continental margin. The present basin morphology can be attributed largely to topographic control of turbidite dispersal and deposition. The topography, in turn, has been modified by tectonic movements on or near northern Juan de Fuca Ridge.