Repeated Deformation - Lake Rufus Woods, WA
Shear stress at the base of the overriding flow is transmitted to the substrate. Soupy and clay-rich substrate material is partially sheared off, peeled away, and incorporated into the flow, becoming internally deformed (plastic deformation) as the flow comes to rest. Structures in this outcrop are fine-scale versions of "no-slip flow" features produced by large-scale turbidity currents in marine settings (Sobieskiak et al., 2018) commonly observed in high-res seismic imagery, side-scan sonar images, and detailed seafloor bathymetry. Sobieskiak et al. (2018), Styles of basal interaction beneath mass transport deposits, Marine and Petroleum Geology v. 98.
Glacial sediments and other aspects of the paleo-landscape that formerly lay beneath ice of the Okanogan Lobe are now exposed in shoreline bluffs along Lake Rufus Woods. Map modifed from Atwater (1987).
Analogous structures. For comparison, the image above and the one below are from Missoula flood rhythmites at White Bluffs in south-central Washington. Nearly identical syn-depositional structures occur in Bjornstad's "channel rhythmites" exposed along the Rhythmites Trail that leads north from the boat launch (Bjornstad, 2006).
Megabreccia landslide debris overlain by lake deposits. Age of this slide? May 2022 photos.
Brian and Captain Peg.
Huge thanks to our trip leaders, Peg and Brian.
Richard maps our progress.
My work. Each map symbol represents at least one outcrop. Note the different styles of deformation preserved in Pleistocene sediments along the Ice Age floodway. Soft sediment deformation and mass wasting is common in the Upper Columbia region, while sheeted clastic dikes are common in slackwater basins to the south. Both areas contain abundant fine sediment. The area between the two (Frenchman Hills to Sprague) is dominated by gravel, which forms poor outcrops.
One lone Atwater.
October 2020 photos at China Creek-Alameda Flat-Strahl Canyon.
Lake Rufus Woods is a 50-mile long reservoir formed by Chief Joseph Dam. The reservoir extends from Bridgeport to Grand Coulee, WA. Access the lake by boat or via short hikes from Hwy 155/Columbia River Rd (Colville Indian Reservation land), or via gravel roads leading north from Hwy 174 (Strahl Canyon Rd). Excellent golfing can be had at nearby Gamble Sands, designed by David McClay Kidd and ranked #1 Best Course in Washington by Golf Magazine in 2021.
Geology exposed in shoreline bluffs along this remote reach of the Columbia River has received little attention over the past century, despite the miles of exposure and fairly easy access. Evidence of repeated Missoula flooding appears as sandy, deformed intervals sandwiched between flat-lying clay-silt varves of Glacial Lake Columbia, the "Nespelem Silt" of Pardee (1918). Deformed zones up to several meters thick resemble deformed zones in the Sanpoil Valley, Hawk Creek, Wilmont Creek, and Upper Columbia. Loading and mass wasting deformation is also common in the section. The Okanogan Lobe overrode the area. Till uplands above the canyon are littered with huge haystack boulders of basalt. The Columbia River Basalt onlaps older granitic rocks here.
A long profile analysis of a conspicuous flight of Pleistocene-age terraces has not been made, but would make a great student project: Is there an isostatic uplift signature preserved in terrace profiles along Rufus Woods Lake?
Photos below are from a 2019 visit I made to the China Creek/Alameda Flat area.
This pattern repeats in shoreline bluffs.
Deformed sands between packages of flat-lying lakebeds.
Deformed varves (lakebeds).
Thumbs up. Soft sediment deformation structures of various types are found wherever you look.
Interesting micro-faulting in sandy units at China Creek.
Sand-filled fracture forms a clastic dike.
The barn owl nest with wing-flap marks.