Where Did They Go and How Big Were They? Latest Pleistocene-Age Catastrophic Floods and Their Deposition in the NE Pacific Ocean

J. A. Reid¹ and W. R. Normark^2
1U.S. Geological Survey, Santa Cruz, CA, [email protected]
²U.S. Geological Survey, Menlo Park (deceased)

While onshore evidence of latest Pleistocene-age glacial Lake Missoula flooding effects is well known in the Scablands of the Pacific Northwest, the fate of these catastrophic floods beyond the Columbia River mouth was largely undefined. Initial results gathered from Escanaba Trough (ET) in the southern Gorda Ridge (GR) in 1970 (DSDP core 35) and 1996 (ODP site survey and core 1037) linked the Missoula Floods with distinctive turbidites of equivalent age in ET, some 1200 km distant.

In 2003, we evaluated the connective pathway between the Columbia River and ET using seismic-reflection data, long-range side-scan sonar images, and sediment core data between the Blanco Fracture Zone (BFZ) and GR.

Upon entering the ocean, sediment in the humongous floodwaters continued moving downslope as hyperpycnally generated turbidity currents. Large-scale landslides on the Oregon margin prevented the flows’ southward movement along the base of the continental slope, driving these tremendous flows into Cascadia Channel, through BFZ, and onto the Pacific Plate.

The geophysical and sedimentological data indicate that a nearly 90° right hand bend at BFZ’s southern edge divided the largest turbidity currents into two separate flows: those portions above the channel margins continued uninhibited by the bend, moving southward and building the Tufts submarine fan. Most of these flows ponded in basins on GR’s western flank and along the Mendocino Fracture Zone to the south. The very largest flows breached a shallow sill at ET’s western edge and settled into the dead-ended Trough. In contrast, the flows remaining confined within the Cascadia Channel continued onto the Tufts Abyssal Plain, 1000 km or more to the west.

Understanding these sediment-transport processes along with the known relationships between other late Pleistocene deposits in ET indicate that more than 7000 km3 of terrigenous sediment were transported into the NE Pacific by latest Pleistocene catastrophic floods.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009