Clastic Dikes in Lake Roosevelt Bluffs - Northeastern WA

Clastic dikes filled with coarse sand, pebbles, and up to golf ball-sized cobbles intrude shoreline bluffs along Lake Roosevelt in the Upper Columbia River valley. Material that fills the dikes was derived from above in gray, sandy outwash. The dikes show no vertical sheeting or silt skin walls - features common in sheeted dikes in slackwater basins of south-central Washington. I counted 14 dikes along a 1.5 km traverse of the beach south of Hunters Bay Campground, located a few miles north of the Spokane-Columbia confluence. The wedge-shaped dikes descend 10-20m through the bluff, are truncated at their tops by the modern soil. The widest dike exceeds 50cm. All taper downward and pinch out near shoreline level. No preferred orientation was noted. They are widely separated from one another and do not appear to form polygonal networks. They appear to be isolated features associated with toppling of the unsupported bluff (lateral spreads). The dikes are passively filled fractures.

Dark gray sandy fills contrast with the light colored silty-clayey host sediment - alternating layers of soupy lake bottom muds and silty sluggish-river/weak bottom current beds. The dikes intrude through the entire section, therefor post-date Glacial Lake Columbia. They are equivalent in age to or or slightly younger than the overlying outwash gravel. The Cordilleran Ice Sheet had retreated north of here when the dikes formed.

The site is located just beyond the limit of influence of the Missoula floods, just north of the farthest backflooding up the Columbia River (dashed line). Glacial ice covered the area multiple times during the Pleistocene and a blanket of outwash gravel caps the terrace surface at the top the shoreline bluff today. Map of upper Columbia Valley modified from Hanson and Clague (2016).

Hunters, WA.