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Two Generations of Clastic Dikes in Touchet Valley, WA

An excellent outcrop located along Touchet River Road North, some 13 miles north of Touchet, WA exposes two sets of clastic dikes. The older set contains silt, displays crisp vertical sheeting, and intrudes silt-pebble diamicts that are oxidized and contain abundant soil features consistent with their age (>35 ka). Dikes of the younger set contain coarse black sand, are crudely-sheeted or unsheeted, and intrude deformed slackwater rhythmites deposited by the Missoula floods (18-14 ka).

The outcrop is located near the Plucker Historical Marker pullout. Professor Nick Zentner and I visited this exposure in June 2021 to do some filming (see "Clastic dikes w/ Skye Cooley - Nick From the Field #31" on YouTube). The mid-day lighting was atrocious and I forgot what I was supposed to say. So I figured I'd explain things with a figure. Still a draft version.

Some important takeaways:

a.) Dike fills reflect the character of the host sediment available to opening fractures. Silt fills the dikes in the older, silty diamicts. Coarse sand fills the dikes in the younger Touchet Bed rhythmites. Not sure if the diamicts contain downhill flow indicators. They don't look like backflood rhythmites to me. More like locally-derived stuff swept from hillslopes onto the valley floor. Ask Pat what he thinks about it.

b.) There are two generations of clastic dikes preserved here. They are separated by a prominent erosional surface with some relief on it. The erosional contact is quite complex at the centimeter scale. Make things simple: Divide the outcrop into older and younger deposits at the unconformity. Get your eyeballs up close and follow that erosional surface across the outcrop - important details in there.

c.) Touchet Beds here are deformed. The deformation appears related to the setting in which they were deposited, low in a wet valley or possibly to some low angle shearing. Slope-parallel slip may have following drainage of Lake Lewis, creating instability similar to that created by reservoir drawdowns. Older silt rhythmites are not deformed. If clastic dikes are the products of strong seismic shaking, then the entire section should be deformed. If liquefaction/fluid escape was involved, the lower strata, which contains abundant fine sand and silt, should serve as the source for dike fills. Dikes here taper downward, were filled from the top, and have truncated tops.

d.) Hiatuses, bioturbation, and soil development are associated with flood-cut surfaces.

Photo: A sheeted clastic dike, filled with light-colored silt and sand, cuts old alluvium with a Bk soil profile (mostly reworked loess with a Stage II caliche). The top of the wedge-shaped dike (1st generation dike) is truncated by a horizontal erosion surface, cut by a glacial outburst flood. The bottom of the dike descends through stratified, locally-derived basaltic gravel seen at the bottom of the photo and terminates at the top of hard Columbia River Basalt bedrock, about 1m below. Bedrock is well exposed nearby. The dike did not rise from a sandy liquefied source bed somewhere below the surface. Touchet Beds of the Missoula flood cycle (18-14 ka), which are considerably younger than the alluvium (pre-late Wisconsin, >28 ka), lie atop the erosion surface and contain a network of other clastic dikes filled with coarse black sand (2nd generation dikes). Some might interpret the "alluvium" to be a flood deposit or "sediment sloughed from the hillside during seismic event". I find little field evidence supporting those interpretations; its simplest to call it "old alluvium" until something that distinguishes one type of deposit from the other is discovered.

e.) Once megafloods are introduced to the landscape, clastic dikes appear. None of the sediment below the prominent erosional surface and above the basalt is deformed. There is no evidence of fluid escape or stuff squirting around. Bedding remains pretty much flat. It has passively accepted fractures and dikes.

Figure: A time scale for the area can be helpful in keeping the age of Scabland deposits straight. The numbers in bold are good ones to memorize if you work in the Columbia Basin.

f.) Dikes in the Touchet Beds tend to sole into the underlying unconformity. I think the surface is slightly cemented, but due to the relief on the surface, its tough to see. Otherwise, that's a tough relationship to explain. Why do the younger dikes not cut clean through. They certainly do elsewhere in the area.

g.) Communicating with simple drawings for me often involves:

1.) A clean field sketch, which I photograph, adjust for contrast, and use as a general tracing image.

2.) Simple graphic elements and short phrases of text, which I add in Illustrator.

Photo: Richard Waitt and I visited the Plucker Historical Sign site, Rulo, Dry Hollow, Winans Rd, Reese Coulee, and others in the Touchet Valley in May 2022. One goal was to better understand 'silt diamicts' and pre-Late Wisconsin flood evidence in the area.

Here, I've tried to include a lot of information in a small space (one 8.5" x 11" page), yet everything has space to breathe - visual elbow room if you will. If at all possible, I avoid color in my outcrop sketches. Photos do a better job with that. Color doesn't reproduce well, especially on copy machines (remember those?). Also, notice that I've not included a vertical scale. The visual "real estate" right along the edges of my strat column sketch are the most value, but I don't want to crowd things there. Do you miss having a scale? Probably yes, but its not the first thing you think about. I will add it at some point - just some tick marks and numbers in the space to the left of the Deformed/Undeformed boxes.

March 2022 Update: Remarkably similar geology occurs at my Dry Hollow Site, located south of Clyde, WA. Read more at this link:

** Mistakes in figure to fix: Holocene loess/soil is not deformed; "undeformed" misspelled in 2nd gen description; Should read "indicators in lower portion" in setting description.


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