Calcrete Field Trip 2021 - 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. Flood deposits here are sandier, the products of south-directed, "downhill" flows typical of the central Channeled Scabland. Rhythmites in the Warden-Quincy area differ from those near Walla Walla, Mabton, Cecil, and Lewiston. 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 (post-depositional diagenesis) or a change in sediment source. These beds were likely deposited by floods out of Grand Coulee/Ephrata Fan. Oxidized, sandy rhythmites are common in Quincy Basin. 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 the sand transitions to siltier stuff. These correspond with either the flood-backflood transition or backflood-slackwater transition. Kind of tough to say given the location, but clusters of sags like these are very common in muddy deltaic deposits worldwide. The outcrop sits at ~335m elevation, which is still within the Lake Lewis basin (hydraulic ponding upstream of Wallula Gap during Missoula floods), but near its northern margin. Large sags in at least 2 beds, initiated in the finer grained upper portions of the rhythmites and are syndepositional structures. Sags within individual rhythmites associate in time ouverland flooding, rapid sediment deposition, and soft sediment deformation. Sags here formed during the last few Missoula floods through the Channeled Scabland.

Sags. Sag structures are in part gravitational, but clearly have a component of fluid pressure drive.

Squirts and dishes. T-shaped mud squirts (white mud) and dish structures (tan-gray sand). Upward-intruding forms in Touchet Beds are rare. Where found, they look like this (small, incidental, unsheeted). You could technically call these mud dikes, but you probably shouldn't.

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. The 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 floodwater. See my article in Northwest Geology (Cooley, 2020), the annual publication of the Montana-based Tobacco Root Geological Society (www.trgs.org) or the link below, for details.

https://www.skyecooley.com/single-post/2020/09/15/Sheeted-Clastic-Dikes-in-the-Megaflood-Region

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). Brian Sherrod (USGS Seattle), who I introduced to this site during my calcrete field trip in September 2021, spent some time inspecting the exposure. I imagine his team will revisit the outcrop and attempt to reinterpret syndepositional structures as seismites. Prediction made.

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

Dish structures & T-shaped mud squirts - Dish structures in a sandy bed located above the light gray mud/color change/unconformity. These structures formed during deposition of the sand or immediately after. Dish structures require rapid deposition of wet sediment and are commonly found in rapidly-deposited sediments like turbidites (Stow and Smillie, 2020). Dish structures are found in Bouma layer C and sometimes B. They are not diagnostic of seismicity, but of rapid dewatering of rapidly-deposited, fully-saturated sediment. These dish structures are confined to a specific sandy bed near the top of the section and appear entirely related to its deposition, not dewatering of beds below or above - and certainly not the entire section as might be expected if deformed by seismic shaking. The dishy layer also contains skinny, irregular, t-shaped mud squirts 10-20cm tall that rise from a light gray muddy bed, other evidence of fluid escape. The t-shaped structures also form as a response to rapid deposition where strong grainsize contrasts commonly occur (i.e., sand piled quickly atop mud). Identical features to these are found in shoreline bluffs of the Sanpoil Arm, Rufus Woods Lake, and Banks Lake where thick section of muddy varves (Glacial Lake Columbia) is punctuated by sandy, turbidite-like flood deposits that inundated the lake bottom (Atwater, 1986; Hanson and Clague, 2016).

Dish structures. Dish structures in sand near top of exposure. All evidence in the outcrop argues for water escape during rapid deposition, not for wholesale liquefaction by earthquake shaking sometime after deposition. Dish structures are not common in Touchet Beds, but do tend to appear in sandy sections (they require sand to form) along the margins of major floodways where flows broaden out. Preservation of scabland deposits tends to be best there (just beyond the highest energy zones). Note these are very delicate structures that crumble at a touch. Could these have survived numerous earthquakes over the past 12,000 years? What is the Late Pleistocene-Holocene recurrance interval for >M6 earthquakes in Eastern Washington, anyway?

Dishing on turbidites. Megaflood deposits can, in places, be understood from a turbidite perspective (rapid, subaqueous, overland flows). Turbidite facies models don't entirely work, but can provide a useful framework for comparing rhythmites in different parts of the floodway region (same flood, different deposit depending on location). For example, rhythmites in the Glacial Lake Columbia vs. eastern Quincy Basin vs. Walla Walla Valley vs. Rufus vs. Willamette Valley. Lowe includes dish structures in unit S3 of his turbidite facies model (above), clearly indicating their formation is syndepositional. Facies model variants (i.e., Bouma, Lowe, Stow) are summarized in Stow and Smillie (2020).

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. Othello Channels scabland is just south of the lake.