Origin and development of Cascadia deep-sea channel

Abstract

Cascadia channel formed initially in the north-south oriented topography on the eastern flank of the actively spreading Juan de Fuca ridge. The late Cenozoic volcanic basement was first covered by transparent pelagic and hemipelagic sediments, which were then overlain by horizontally deposited turbidites. Throughout much of its early history Cascadia channel probably existed only as a low-relief leveed transportational-depositional feature. With the onset of late Pleistocene glaciation and the related lowering of sea level, large volumes of sand and gravel from the continent deposited on the upper slope or the outer shelf led to the initiation of erosive turbidity currents, which converted the lower and middle portions of Cascadia channel into an erosional feature. This activity, combined with a change in base level and/or channel gradient, led to the initiation of downcutting. During this period of channel erosion the plain on either side of the channel was apparently being built up by turbidity current flows proceeding southward along the western part of Cascadia abyssal plain and west from Astoria fan. During Holocene time, turbidity currents derived from Columbia River sediments continued to flow the length of Cascadia channel and through the Blanco fracture zone. Within the past 6600 years a downdropped basin in the fracture zone has been ponding the Holocene flows. The upper channel remains a depositional feature, and turbidity currents continue to overflow the channel onto levees. Along portions of the middle channel, perhaps owing to increased gradients and backcutting, the flows remain entirely channelized and are either erosional or nondepositional. Along the deeply incised lower channel, Holocene flows have deposited thick layers of sediment on the channel floor, and no overflow has occurred.