Warden, Washington isn't the most inspiring place. For a field geologist, its particularly slim pickings. The place is flat, hot, and dusty. There are very few outcrops and the taco wagon fare is mediocre. So when I came across this spectacular exposure, I about fell over. Interesting geology in Warden - who knew?
Warden, WA is located in Lind Coulee north of Bruce and south of Ruff. The nightlife dance scene is better in Connell.
Touchet Beds. Missoula flood deposits comprise the section, but there is more to it than just Touchet Beds. A dozen or so beds present here, but the base of the exposure extends below canal level and strata in the upper third of the outcrop is quite deformed, which makes placing bedding contacts (and counting beds) a bit of a challenge. I didn't show clasts in the section, but they are there, mostly cobble or smaller, fairly sparse. Whatever I labeled "silt" has a component of sand to it; its all water-lain (not loess). The described section totals 6-7 meters.
Color Change. A prominent color change - from orange to gray - occurs about 2/3 of the way up. A conspicuous light gray mud marks an unconformity with backfilled rodent burrows just below. Burrowing rodents need time to recolonize the formerly flooded landscape. Their presence is a mark of years-long to decades-long period of land surface stability - probably an extended hiatus longer than the average interflood period. The older stuff may get its orange tint from oxidation (greater age) and/or sediment provenance. No strongly cemented layers were found, which makes me doubt there is much difference in the age from bottom to top. The oxidized concretions layer, slightly higher in the stack, marks another pause between floods (root casts?).
A prominent color change corresponds with a surface that truncates clastic dikes and shows evidence of stability. The character of the bedding changes dramatically above. Sediment provenance or age may explain the change. Fill bands in the dike contain both orange and gray sediment. Nejirigama hoe is 37cm long.
Repeated Deformation. The story here is repeated flooding and repeated deformation, the latter a result of the former. Someone reading an earthquake history into this section should get their head examined. Yes, there are liquefaction features. But every bed in this exposure is deformed. The deformation looks different in each bed. Structures vary with grainsize and probably differences volume/velocity/depth of different floods. The deformation recurrence interval here equates to the flood inundation interval. Deformation structures are not a seismic signal imposed on the sediment; they are synsedimentary. Also, its possible a wetland persisted here between floods; the fluidization, trace fossils, muds suggest floods inundated a boggy lowland.
Lower portions of rhythmites are plane bedded to rippled, which, in combination with the local topography, indicates the depositional setting was a broad "sand plain". A large clastic dike is seen at upper right. Sags at middle right. Hints of load casts at lower left in a thin, light colored bed. The exposure isn't so vertical as to thwart access, but sliding off into the canal isn't out of the question.
Watch your step.
Load Casts. Soft sediment deformation is abundant, repetitive, and takes different forms. The lower 8 rhythmites have load casts at their bases, where plane bedded sand contacts finer sediment of the rhythmite beneath. Load casts also occur within rhythmites, where the sand transitions to siltier stuff (the backflood-slackwater transition). Contorted bedding also occurs high in the section.
Sag Structures. Prominent sags in at least 2 beds (possibly as many as 4), initiated in the finer grained upper portions of rhythmites. Sags appear directly related to rapid deposition (synsedimentary).
Dike-like sags with crudely-stratified fills descend from the finer grained upper portions of rhythmites. The darker areas are wetter. There is clearly an intrusive/crosscutting relationship between structure and strata, but I'll stick with "sag" until the terminology cops come knocking.
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/water escape either during deposition or immediately after. They imply rapid deposition of wet sediment. The dishy layer also contains t-shaped mud squirts that rise from a 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 (Glacial Lake Columbia-Missoula flood deposits). Also, "Mud Squirts" is a name currently registered to 16 Ultimate Frisbee teams at liberal arts colleges in the U.S.
Mud squirt and dish structure layer.
Better view of dish structures.
Oxidized tube-like concretions, less than 3cm in diameter, are vertically oriented and look like root casts or cemented burrows. They are not coprolites. I checked - no sphinctral striations. #moonriver
Clastic Dikes. A few sizeable sheeted clastic dikes cut the exposure and are truncated by the light gray mud. They are filled with sand and silt like many others in the region. The widest dike measured 30 cm and contains about half that many fill bands. Two additional sets of very thin, short dikes descend from the sandy bases of beds low in the stack.
Mount St. Helens Ash. The 1980 ash occurs just a few inches below the modern soil surface. Its 2" thick, bright white, and unweathered.
Feel Like a Number. This is my Site 21-03. #bobseger