Calcrete Field Trip - Warden Canal

Directions - From Othello, drive north on Hwy 17 and turn R (east) on Hwy 170/Rd 8 SE at the '76 gas station toward Warden. Follow for 0.5 miles, turn L (north) onto Rd S SE and follow for ~6 miles, passing by the Bassett Junction trestle at a sweeping curve. Rd S SE becomes Rd 3 SE at junction. Feedlots owned by El Oro Cattle Feeders on right. Continue for another 1.3 miles, crossing the canal bridge. Turn L (southwest) onto Rd 3 SE. In a hundred meters, pull off on the wide shoulder near a guardrail. Do not block gates or farm access driveways. The cutbank exposure is below near the canal spillway outlet. A good view is had from the right bank. Or walk across the top of the bluff (left bank) and drop below the lip at a scruffy tree to inspect up close. Loose footing. Location is Weber Coulee, a feature within larger Lind Coulee. Additional nearby outcrops shown on map. I am the first to describe this important exposure.

GPS: 47.043027, 119.110941

Touchet Beds - Missoula flood rhythmites comprise the 7m-thick section, perhaps a dozen. A prominent color change - from orange to gray - occurs about 2/3 of the way up. The orange tint may be be oxidation due to local water table or a change in sediment source. A thin, but conspicuous light gray mud line marks an unconformity that truncates two sheeted clastic dikes. A bed containing backfilled rodent burrows lies just below the dike-truncating unconformity. Such burrows are common in the Touchet Beds. Burrowing rodents recolonized portions of the landscape between most if not all floods.

Contacts and deformation structures. 1 = Sags, 2 = Large sheeted clastic dike with its top truncated by an erosional surface cuts obliquely across the exposure, 3 = Truncation surface with thin gray mud above, 4 = Highly-deformed zone with dish structures and t-shaped mud squirts, 5 = Oxidized root casts, 6 = Modern soil (disturbed), 7 = 1980 Mount St. Helens ash. Lines are my interpretation of bedding contacts and deformation features. Not all contacts are traced. View looking northwest. May 2021 photo.

Load casts & large sags - Soft sediment deformation is abundant and repetitive here, but takes different forms. The lower 8 rhythmites have load casts at their bases. Load casts also occur within rhythmites (between coarse lower and fine upper portions), where sand transitions to siltier stuff. These correspond with either flood-backflood transition or backflood-slackwater transition. Large sags in at least 2 beds, initiated in the finer grained upper portions of rhythmites. Sags within individual rhythmites link rapid deposition and deformation. Sags formed during the waning stages of a flood.

T-shaped mud squirts (white mud) and dish structures (in tan sand).

Sheeted clastic dikes - A sheeted clastic dike is truncated by an erosional surface capped by a thin, light gray mud. The dike is filled with vertical sheets of sand and silt like thousands of others in the region. It measures ~30 cm wide and contains about half that many fill bands. Another dike is seen at far right, also truncated. Two additional sets of very thin, short dikes descend from the sandy bases of beds low in the stack. Clastic dikes are typically considered to be soft sediment deformation structures formed by liquefaction and fluid escape during seismic shaking. But some are not. Hundreds of thousands of clastic dikes in the Columbia Basin are not. The sheeted dikes in the Touchet Beds formed by hydraulic fracture in response to loading by floodwater and slackwater lakes. Pressurized fluid entered and expanded cracks propagated downward into the dry vadose zone sandwiched between the ground water table and the flooded ground surface. See my article in Northwest Geology (Cooley, 2020), the annual publication of the Montana-based Tobacco Root Geological Society ( or the link below, for details.

Synsedimentary Deformation - The story at Warden Canal is repeated scabland flooding and repeated deformation, the latter a result of the former. Few outcrops expose flood-caused deformation in Touchet Beds better than this one. Every bed in this exposure is deformed. Structures vary with grainsize and timing within-a-flood (downstream flood, backflood, slackwater). Differences in flood volume, velocity, and depth might have played a role, too. Beds above the top of the truncation surface appear to have remained undrained and soupy between floods; a low spot in the landscape then as now. Trace fossils, mottling, and unusual muddy character suggest a boggy lowland setting. The deformation "recurrence interval" here is the floodwater inundation interval. Dikes and sags were triggered by rapid loading and rapid sedimentation (internal trigger), not seismic shaking generated by local faults (external trigger).

Oxidized root casts mark a pause between floods long enough for plants to establish a foothold.

Dish structures & T-shaped mud squirts - Dish structures completely obscure a sandy bed located above the light gray mud/color change/unconformity. These are sedimentary structures formed by liquefaction/fluidization either during deposition of flood sediment or immediately after. They require rapid deposition of wet sediment. The dishy layer also contains t-shaped mud squirts that rise from a light gray muddy bed. The skinny, irregular structures are 10-20cm tall. Such structures are common in rapidly-deposited sediments where strong grainsize contrasts occur (sand piled quickly atop mud). Identical features are found in shoreline bluffs of the Sanpoil Arm, Rufus Woods Lake, and Banks Lake that expose varved intervals (Glacial Lake Columbia) punctuated by sandy Missoula flood deposits (Atwater, 1986; Hanson and Clague, 2016).

Dish structures in sand near top of exposure.

Sparse outcrops. Good exposures of flood deposits are few in the sandy plain surrounding Warden, WA (eastern Quincy Basin). Warden Canal is not to be missed, despite having no calcrete. The two prominent east-west ridges on the shaded relief map are Frenchman Hills (north) and Saddle Mountains (south). The large lake west of Warden is Potholes Reservoir.

Don't fall in.

Links to Other Stops:

Calcrete Field Trip Overview

Stokrose Gravel Pit

Warden Canal

Hendricks Road

Red Tank Hike

Corfu Landslide Overlook

Offramp Stop

Herman Railcut

Coyan Road

Liesle Road

Booker Road at Canal

Lemaster-Hatton Intersection

Lind Coulee Fault at O'Sullivan Reservoir

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