Fluvial Quartz Interbeds in the Columbia River Basalts,

Blue Mountains


Adam Johnson

Whitman College, Walla Walla WA 99362



Five mile east of the Wenaha-Tucannon Wilderness border is a deposit of fine grained quartz sand. Its presence is out of the ordinary as it appears to be an interbed within the Saddle Mountain Member flow of Columbia River Basalt Group that dominate the surrounding area. There is no known local source for quartz sand. Preliminary investigations suggest an ancient stream channel. The deposit lies with in a 400’ x 20’ lens that rests atop basalt bedrock with fine clay sediments surrounding it. The lens shape is indicative of a channel deposit. Grain size analysis has shown an upward fining trend depicting a decrease in energy level over the time of deposition. The age of the deposit is between 13 - 6 million years, which may be refined through further stratagraphic analysis. Previous research has yielded fluvial quartz deposits that are interbedded or incorporated into older basalt flows. This research will support previous work that has suggested a continuous fluvial system running from the granites of the Idaho Batholith across eastern Washington through out the deposition of the Columbia River Basalts up through the present.