LARGE-SCALE FORESET, BOULDER-GRAVEL BEDS IN THE LOWER SPOKANE RIVER VALLEY, NORTHEASTERN WASHINGTON

Large-scale foreset beds of sub-rounded boulder-rich gravels are exposed in the northeast portion of the Channeled Scablands. The large (at least 10 meters thick), high-angle, foreset beds are exposed in the lower Spokane River Valley along the difficult-to-access shores of Lake Roosevelt (the large reservoir impounded by Grand Coulee Dam).

Clasts in the large-scale foresets range up to 2 meters in diameter although most are pebble-to-cobble-sized. The largest clasts may have been ice-rafted to the site. Lithology of the gravels is predominantly granitic rocks and basalt. The thick beds indicate deposition due to flow expansion during an outburst flood from Glacial Lake Missoula to the east. The giant foresets are similar to the expansion bar deposits exposed near the confluence of the Palouse and Snake Rivers and to the sediment deposited by expansion flows of modern Icelandic jokulhaups.

Massive, light-colored beds of silt/clay fines lie above the giant foreset beds; these lacustrine sediments formed within Glacial Lake Columbia/Spokane. Beds of dark sand (predominantly basalt fragments and biotite grains) occur interspersed within the lacustrine beds. Soft-sediment deformation structures are ubiquitous, affecting both the light-colored fines and the dark sands. Both types of beds are laterally continuous up to 75 meters. At one locality, rip-up clasts of light fines are suspended in the dark sands, evidence of substantial shear stresses and erosion on the bottom of Glacial Lake Columbia/Spokane. Similar dark sands and light fines are also exposed in a ravine 100 meters above Lake Roosevelt, indicating a substantial minimum thickness of the sediments. We believe the silt/clay beds represent deposition in Glacial Lake Columbia/Spokane, and the dark sands resulted from deltaic deposits of the Spokane River drainage feeding into the lake during the Late-Wisconsin.

The age of the giant foreset beds must be at least 12,700 years B.P. (the last Lake Missoula outburst) and might be pre-Late Wisconsin. The lacustrine fines and the sandy deltaic deposits are younger and are constrained by scour-filling volcanic tephra from Mazama's climactic eruption of 6850 years B.P.