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.


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 a diagenetic change from oxidation (orange = older floods), a change in sediment source, or accelerated weathering due to a high local water table (recent). A thin, but conspicuous light gray mud line marks an unconformity that truncates two sheeted clastic dikes. Sheeted clastic dikes identical to these occur within Ice Age floodway basins from Hunters, WA to Salem, OR.


A bed containing backfilled rodent burrows lies just below the dike-truncating unconformity. Burrowing rodents need time to recolonize the landscape between floods. Their presence indicates a years-long period of dry, upland surface stability.


Contacts and deformation structures. 1 = Sags, 2 = Large sheeted clastic dike with truncated top 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 certain 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 assumes different forms. The lower 8 rhythmites have load casts at their bases, where plane bedded sand (lower portion of rhythmite) contacts finer sediment of the upper portion of the rhythmite beneath. Load casts also occur within rhythmites (between upper and lower 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 appear directly related to rapid deposition during the waning stages of a flood.



T-shaped mud squirts.


Sheeted clastic dikes - A sizable sheeted clastic dike cuts the exposure and is truncated by and 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. These dikes, in fact, are not. The sheeted dikes in the Touchet Beds formed by hydrofracture in response to loading by floodwater and slackwater lakes. Pressurized fluid entered and expanded cracks propagated 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 (www.trgs.org), 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 and the deformation takes a variety of forms. Structures vary with grainsize and within-a-flood timing (flood, backflood, slackwater). Differences in 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. Trace fossils, mottling, and unusual muddy character suggest a boggy lowland setting. The deformation recurrence interval here is the floodwater inundation interval. The trigger was rapid loading by floodwaters (internal trigger), not seismic shaking generated by local faults (external trigger).



Oxidized concretions (root casts), slightly higher in the stack, mark another long pause between floods.


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 flatlands 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 map are Frenchman Hills (north) and Saddle Mountains (south). The large lake west of Warden is Potholes Reservoir.


Don't fall in.

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