EXTENSIVE TURBIDITE DEPOSITS ON THE PACIFIC PLATE GENERATED BY OUTBURSTS FROM GLACIAL LAKE MISSOULA

As the on-land features related to the catastrophic floods from glacial Lake Missoula became better defined during the last few decades, one of the major unresolved issues has been what happened when the flood discharge reached the ocean at the mouth of the Columbia River. It has now been demonstrated that upon reaching the coast, the flood waters with massive loads of entrained sediment continued flowing across the continental slope as hyperpycnally generated turbidity currents (Zuffa et al., 2000) that flowed as least as far as the Escanaba Trough more than 1000 km distant. For this study, we compiled available 3.5-kHz high-resolution and airgun seismic-reflection data to define the pathway for flood-related turbidity currents from offshore Columbia River to Escanaba Trough. After crossing the Astoria Fan off the Oregon continental margin, the hyperpycnal flows entered the Cascadia Channel. Overflow from Cascadia Channel in the area south on the Blanco Fracture Zone formed the Tufts submarine fan on the Pacific plate. The Tufts Fan is fan is more than 220 km long and 70 km wide; the western and southern boundaries are poorly constrained with existing data. Although there is no distinct fan valley at the head of the fan in the area of overflow from Cascadia Channel, there is a broad convex-upward morphology with sediment waves formed locally on very low slopes. Numerous small channel systems characterize the middle fan, which merges southward into a gently sloping lower fan and ponded basin plain equivalent. The fan consists of turbidite deposits that locally exceed 300 m thickness. The deposits of the Missoula floods are as much as 100 m thick and buried pre-existing relief on the fan. The turbidity currents were >200 m thick and overflowed the crests of small ridges on the flank of Gorda Ridge; these flow-stripped deposits were trapped in local basins to the east of the Tufts Fan. The Tufts Fan and local sediment ponds farther south and in the Escanaba Trough contain a minimum of 600 km³ of flood-related sediment that has been deposited on the Pacific plate. These deposits smooth and fill in topographic relief over an area of at least 20,000 km².