Tectonics of the Neogene Cascadia forearc basin: Investigations of a deformed late Miocene unconformity
The continental shelf and upper slope of the Oregon Cascadia margin are underlain by an elongate late Cenozoic forearc basin, correlative to the Eel River basin of northern California. Basin stratigraphy includes a regional late Miocene unconformity that may coincide with a worldwide hiatus ca. 7.5–6 Ma (NH6). The unconformity is angular and probably subaerially eroded on the inner and middle shelf, whereas the seaward correlative disconformity may have been produced by submarine erosion; alternatively, this horizon may be conformable. Tectonic uplift resulting in subaerial erosion may have been driven by a change in Pacific and Juan de Fuca plate motion. A structure contour map of the deformed unconformity and correlated seaward reflector from seismic reflection data clearly outlines deformation into major synclines and uplifted submarine banks. Regional marginparallel variations in uplift rates of the shelf unconformity show agreement with coastal geodetic rates. The shelf basin is bounded to the west by a north-south–trending outer arc high. Rapid uplift and possible eustatic sea-level fall resulted in the formation of the late Miocene unconformity. Basin subsidence and outer arc high uplift effectively trapped sediments within the basin, which resulted in a relatively starved abyssal floor and narrower Pliocene accretionary wedge, particularly during sealevel highstands. During the Pleistocene, the outer arc high was breached, possibly contributing to Astoria Canyon incision, the primary downslope conduit of Columbia River sediments. This event may have caused a change in sediment provenance on the abyssal plain ca. 1.3–1.4 Ma.