Sanpoil-style Syn-sedimentary Deformation

Mobilized lacustrine muds are dragged upward into overriding sand deposited by a Missoula flood entering Glacial Lake Columbia. The "T-shaped mud squirts" formed as a result of soupy lake bottom muck being disturbed by a sand-laden density current moving across the lake floor.

Tracing of the features in the photo above.

Soft sediment deformation, caused by rapid sedimentation and loading by Missoula floodwaters, often looks like this in the Sanpoil Valley. Flood-deposited sand atop soupy lake bottom "varves" during flooding disrupts the muddy substrate, creating the structures seen here. Recall that the geologic setting is a sediment-laden flood entering Glacial Lake Columbia from the east. The sand, carried at the base of the flow, finds the lake bottom quick. We're upstream of Grand Coulee here in the Sanpoil Arm of Lake Roosevelt near Keller, WA.

The soft sed deformation structures aren't really clastic dikes, but then again maybe they are. They're not sheeted dikes like the Touchet Bed dikes, anyway. I think Brian Atwater calls them something like "T-shaped mud squirts". Sounds good to me. The sand is swirled conformably around the margins of the deformation structures (see photo).

Another example.

And another.

In articles from the 1980s-90s, the term "turbidites" is used to describe Missoula flood rhythmites at certain sites. One reason for this is the architecture of the rhythmites exposed in the Sanpoil Valley and Columbia River Valley above Grand Coulee Dam. Here, flood beds have a conspicuous coarse grained lower portion (dark color, tractive sediment load) and an equally conspicuous fine grained upper portion (light color, suspended sediment load). Beds here were deposited in the stable "stilling basin" of Glacial Lake Columbia and exhibit different sedimentary characteristics from beds deposited in ephemeral Lake Lewis to the south. People have moved away from "turbidite" in recent years, but the field geology is hard to deny. When you're up here staring at this stuff with your own eye balls, turbidite terminology is difficult to avoid. Sanpoil Arm shoreline.

Hike the west shoreline of the Sanpoil Arm during low water periods (Winter-Spring) to access a few good exposures of lake and flood beds, some deformed.

Active rotational landslides along the west shoreline of Sanpoil Arm. The scarps provide excellent views of the local stratigraphy described by Atwater in the 1980s. Some of his outcrops are gone, but new ones can be found nearby.

Whatever you call the deformation structures, they are classic loading structures found in any textbook on sedimentary structures. The energy involved with the density current/loading event/disturbance can be estimated by the thickness of the instantaneously-deposited sand bed and its grainsize. Like Brian says, we're not talking boulder gravels.

Coarse glacial outwash/kame terrace gravels with southward flow indicators unconformably overlie the sand bed and cap the terrace landform. The gravels are younger than the megaflood deposits and probably subaerial (e.g., outwash plain). The Rogue Tools hoe is 37cm long.

These same structures are commonly found along Banks Lake, Hawk-Indian Creek area, China Creek, and elsewhere in the Columbia River Valley where Glacial Lake Columbia beds are interrupted by flood sands.


Here's a printable 8.5" x 11" PDF of the map for the lower Sanpoil Valley. I made it. PDF is the same map as shown below.

Sanpoil Area Map Hwy 21
Download PDF • 2.15MB

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