THE TANWAX-OHOP VALLEY FLOOD AND DEBRIS FLOW, AN ICE AGE FLOOD FROM THE CASCADE RANGE INTO THE SOUTHERN PUGET LOWLAND AND LIKELY SOURCE OF SEDIMENTS FOR THE MIMA MOUNDS

Ongoing work in the southern Puget Lowland, in western Washington State, continues to reveal details of a late-glacial outburst flood and debris flow in that region. At the last glacial maximum (~16,950 cal C-14ya), the Puget Lobe ice blocked the Carbon River from flowing into the Puget Lowland at the western boundary of the Cascade Range, impounding glacial lake Carbon. The Puget Lobe began to retreat within 100 years, doing so in several stages marked by clearly delineated kame terraces where the ice margin lay adjacent to the Cascade front and by outwash terraces farther downstream to the west. The first retreat stage released a small portion of lake Carbon (a drop of 15 m), but retreat to the second stage (a further drop of 52 m) released a majority of the remaining volume of the lake. This retreat and outburst flood, between the first and second recessional ice margin positions, resulted in deep incision of the Tertiary and Quaternary age bedrock and fragmental deposits along the Cascade front, particularly the Fox Creek and Ohop Valley channels. This flooding and incision also set the stage for a series of landslides that occurred where these channels were dramatically undercut by the flood. The landslide material was mobilized as a debris flow whose equivalent deposits can be traced more than 100 km flow distance to the west. Today, the composition of the Cascade-Range source-area rock is recognizable within a deposit of variable thickness that ranges from a matrix-rich diamicton to a coarse boulder lag of largely andesitic composition. It is found capping the second recessional outwash terrace at least as far to the west as the Mima Mound region, where the lithology of the mounds can be demonstrated to match the andesitic composition of the debris flow, distinct in lithology from the till and outwash of Puget Lobe deposits. Width of the floodway was as much as 11 km, and incision into the Tanwax and Ohop Valleys was as much as 107 m along a cross section at LaGrande, 32 km downstream of lake Carbon. A branch of the flood traveled down the Skookumchuck River to Tenino and then to the Chehalis River before it rejoined the northern arms of the flood. This research was aided by undergraduate research projects by University of Puget Sound students Zoe Futornick, Matt Hahn, Brittany Parker, and Sondra Tanji; and by Centralia College student Isaac Pope.