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 und er lain 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 ma y be conformable. Tectonic uplift resulting in sub-aerial erosion may have been driven by a change in Pacific and Juan de Fuca plate motion. A structu re contour map of the deformed unconformity and correlated seaward reflecto r from seismic reflection data clearly outlines deformation into major sync lines and uplifted submarine banks. Regional margin-parallel 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 are high, Rapid uplift and possible eustatic sea-level fall resulted in the for mation of the late Miocene unconformity. Basin subsidence and outer are hig h uplift effectively trapped sediments within the basin, which resulted in a relatively star,ed abyssal floor and narrower Pliocene accretionary wedge , particularly during sea-level highstands. During the Pleistocene, the out er are 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.