Slide Planes, Mud Squirts & Diamicts in Upper Grand Coulee


Brian Atwater always makes you feel like a million bucks. He invited me to join a group of geologists meeting in the Upper Grand Coulee in July 2021 to plan an October field trip for the Geological Society of America. The group made several discoveries related to various aspects of megaflooding and the former glacial margin over the course of several enjoyable days. In this video, I describe curved shear planes exposed in shoreline bluffs of Banks Lake south of Steamboat Rock. The concave features look like simple channel forms incised into the silt-clay deposits of Glacial Lake Columbia (Pardee's "Nespelem Silt"), but show clear indications of sliding, possibly caused by ice loading. Bouldery glacial till that grades to diamict and flood-related soft sediment deformation features are also described. Video shot in September 2021.



View to the NW toward Steamboat Rock. The shoreline bluff at left is your goal. Continue hiking past the bluff for additional cutbank exposures. The sandy track leaves the State Park entrance road before the guard station. It is passable by high clearance vehicles when the ground is dry. Park at the grassy area beneath large cottonwoods near the orange bushes and hike the short distance to shoreline bluffs.


Shoreline bluff exposes about a dozen sandy flood beds with olive green silt-clay varves (lake deposits) between. Thin ledges that stand proud of the outcrop are the varved, clay-rich lake beds. The thicker flood sands are slope formers. Bedding becomes thinner near the top of the section. Loess containing weakly cemented cicada burrows (trace fossils) caps the exposure. Curious diamict beds begin to appear low in the bluff where Richard Waitt is standing. I was able to follow one diamict bed, shown in photographs below, for some 20m. There are at least two of them in this section. Stratigraphy here was described by Brian Atwater in the 1980s (Atwater, 1987, p. 188-192) and later logged by Noam Greenbaum (in Waitt et al., 2021 Field Supplement, p. FS17), who sampled it for OSL dating. Greenbaum's dates indicate deposition occurred between 36-100 ka.



Atwater in July 2021.


Curved slide planes not channel forms. Bedding follows the curved surface.


Slide planes in glaciolacustrine silts.


Bedding above parallels the slide plane. Bedding below it is sheared (photos below).


Shear fabrics.




Contorted bed enclosed by flat-lying bedding occur occasionally in Banks Lake bluffs. I suspect this deformation changes character and diminished to the south, towards Coulee City, with thinning of glacial ice. Good student project.


At least two beds of this diamict (5-20cm thick) occur low in the bluff. There may be more. The diamicts pinch out laterally, but trace to a thicker body of bouldery till to the north. Glacial ice made it south of Steamboat Rock.


Brian Atwater, Walt Hunner, and I found these glacial striations on basalt exposed along the shoreline in nearby Barker Canyon (47.89409, -119.18499). Bruce Bjornstad photographed striations on granitic bedrock in Banks Lake north of Steamboat Rock (Waitt et al., 2021, Fig. 7a).


Diamict, lodgment till, and ice-scoured bedrock on the floor of Upper Grand Coulee indicate the coulee existed at roughly its current depth when the ice of the last Okanogan Lobe occupied it (Bretz, 1932; Atwater, 1987; Waitt et al., 2021).


Diamict forms a stone line of cobbles and small boulders of granite and basalt on top olive green varves (lake bottom seds).


Bouldery lodgment till with a compact, black matrix at shoreline level beneath the deformed lakebeds.


Dark gray diamict.




Varved lake beds.


Occasionally, you'll see these rubble-filled wedges in the lakebeds. They might be periglacial or just local lateral spreads.


Rubble-filled wedge at Banks Lake.

"Now hear me out. I think this all has to do with space aliens - visitors from another solar system."





Some photointerp on a set of pre-Grand Coulee Dam photos I did for the Colville Tribes back in 2005. Blue highlights the swath of sand dunes that became the site for Steamboat Rock State Park campgrounds and associated facilities. Yellow is Steamboat Rock. Orange is granite. Green is Devils Lake, the predecessor to Banks Lake. Purple is probably till veneered by dune sand in the lee of Steamboat Rock.




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