WIDESPREAD SEDIMENT DEPOSITION IN NE PACIFIC OCEAN FROM CATASTROPHIC FLOODS OF LATEST PLEISTOCENE AGE, NORTHWEST US

While evidence of the effects from latest Pleistocene glacial Lake Missoula and other catastrophic floods are well known in the Scablands of the northwest US, an unresolved issue has been the fate of the flood discharge when it reached the ocean at the mouth of the Columbia River. A pivotal key to this puzzle is provided by distinctive turbidites of equivalent age in Escanaba Trough, southern Gorda Ridge, some 1200 km distant.

To evaluate the connective pathway between the Columbia River and Escanaba Trough, this study compiled available 3.5-kHz high-resolution and airgun seismic-reflection data, long-range side-scan sonar images, and sediment core data between the Blanco Fracture Zone (BFZ) and southern Gorda Ridge (GR).

Upon reaching the ocean via the Columbia River drainage, the sediment carried by the catastrophic floods continued flowing downslope in the ocean as hyperpycnally generated turbidity currents. Because large-scale landslides on the Oregon margin prevented southward flow along the base of the continental slope, these tremendous turbidity currents flowed into Cascadia Channel, through the BFZ, and onto the Pacific Plate.

These data suggest that a nearly 90° right hand bend at the western edge of the BFZ divided the turbidity currents into two separate flows. Portions of the turbidity currents higher than the channel margins continued to travel uninhibited by the bend. These upper, flow-stripped, momentum-driven components moved southward, building the Tufts submarine fan. Most of these flows were trapped by the western flank of the Mendocino Fracture Zone (MFZ) to the south and ponded in basins on the western flank of GR and along the MFZ. The very largest of these flows breached a relatively shallow sill at the western edge of Escanaba Trough and settled into the dead-ended Trough. In contrast, the flows remaining confined in the Cascadia Channel after the bend continued onto the Tufts Abyssal Plain, 1000 km or more to the west.

Understanding these sediment-transport processes along with the known relationships between Wisconsinan and other late Pleistocene deposits in Escanaba Trough suggest that more than 7000 km³ of terrigenous sediment was transported into the NE Pacific by latest Pleistocene catastrophic floods.