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Diatomite Lake Invaded by Molten Lava - Frenchman Hills, WA

A pillow-palagonite complex forms when molten lava invades a water body and quenches. Pillows are the quenched blobs of lava. Palagonite or hyaloclastite is the loose, rusty-looking mess that often surrounds pillows. Palagonite is composed of basaltic glass, shards of basalt rock, and maybe even some cooked sediment. Its the alteration product of 1100 degC lava coming into contact with 25 degC water and flashing to steam. The stuff is loose and ravelly when formed and tends to remain so. Most of the time, you can just paw the stuff away with your hands. Pillow-palagonite complexes in the Columbia River Basalt are typically found at the base of flows and can be quite thick.

A spectacular pillow-palagonite complex - one with a particularly interesting geologic history - is located along Road 9 SW on the south flank of the Frenchman Hills anticline. The south flank is known as Royal Slope, home to wine grape vineyards, fruit tree orchards, and a few remaining hay fields.

The roadcut along Rd 9 SW is located just below the Frenchman Hills crest between Dodson Road (~5 miles east) and Adams Rd (~5 miles west), Road 9 SW is the highest public road traversing Royal Slope. The newly-planted Stillwater Creek Vineyard is near by. Parking isn't great.

The pillow zone is about 4m thick. In this portion, the round pillows don't touch each other.

Pillows here all touch and nested together, their shapes established prior to cooling.

Concentric pattern and textbook glassy rind is due to rapid cooling.

Yellow mineralized zones contain common opal (hydrated amorphous silica). Most is not gem quality. Opal is associated with diatomite beds in surface mines west of here (IMERYS, Inc.) and with petrified wood in the Ellensburg Fm on Saddle Mountain, a favorite place of rockhounders. Some vein opal nearby appears to have formed long after the basalt cooled, the result of mineral precipitation from circulating groundwater. Another decent outcrop containing opal is found at the intersection of Beverly-Burke Rd (Rd Q SW) and Rd 9 SW, about 9 miles west.

Diatomite beds at Quincy (Q) and Squaw Creek (SC) are slightly different age and lie on opposite sides of the Hog Ranch-Naneum anticline, the axis of which parallels the Cacade crest. The structural axis and modern watershed divide is approximated by the red dashed line. The long-lived and more or less contiguous diatomite basin appears to pre-date the fold. Map modified from Bingham and Grolier (1966, Fig. 4).

Basalt flows follow valley bottoms. Diatomites form at the bottom of shallow lakes. Calcretes in Eastern Washington are closely associated with alluvium and low landscape positions. All three units indicate this landscape has been a low spot for millions of years. Washington Geologic Survey photo archives #3529.

Miocene basalt flowed into a body of water and over wet sediments (Ellensburg Fm). A baked "soil" marks the contact. Its not really a soil, because it was submerged when the lava showed up. The pillows basalt tells us that. What do you call sediment that lies on the bottom of a lake? Muck? Soil-muck? Baked soil-muck?

Here the same basalt flowed over the same wet sediment, but not into water. Baked muck-soil, but no pillows. I get the feeling the heat was attenuated pretty quickly. The intensely baked zone is thin. Exposure of this unit varies with the year.

Probably the coolest thing about the roadcut is these bright white dikes. They are not tectonic features or flood-related clastic dikes like those in the Touchet Beds, rather they are fractures filled with remobilized diatomite. Lake bottom mud was rapidly heated by the overriding lava and became mobile - a phreatic encounter. Note how the dike crosscuts the large pillow in the center of the photo. The fracture is younger than the pillow, if only by a few hours. But the fractures are not the whole story.

The depth of the Miocene lake approximates the thickness of the pillow-palagonite complex. Pillows and the mobilized diatomite intermingle. The pillow in the photo is enclosed by white, which suggests the lava flow was invasive into the thick bed of diatomite (i.e., it burrowed beneath) and fluidized the white mud which escaped into permeable cracks and pore spaces in the partially-quenched palagonite. The white stuff is high in the outcrop. The dikes taper downward, not upward. The center of the lake and main body of the diatomite seems to be mostly east of the exposure.

White diatomite surrounds pillows.

Fractures filled with mobilized diatomite descend from a larger, coherent mass.

According to Bingham and Grolier (1966) who observed this same diatomite bed at other locations,

...the diatomite bed ends against the front of a Priest Rapids flow...the lake was impounded by this flow...relationships are believed to be similar to those between the Squaw Creek and the Sentinel Gap flow...the flow advanced into the lake rather than over the dry lake bed...the Quincy lake was impounded by one...flow and destroyed by another...

An active diatomite mine is visible from Frenchman Coulee near The Feathers. Two diatomite beds are distinguished above and below the Roza flow (Schminke, 1967; Brown, 1968, Fig. 2; Tolan et al., 2009),

On the horizon across the coulee to the north are bright white piles of sediment removed from the interbed between the Roza Member and Priest Rapids Member. The Roza flow at this locality invaded the diatomite and is an invasive flow. The diatomite is not in its proper stratigraphic position, and actually belongs to the Squaw Creek Member and not the Quincy Member of the Ellensburg Formation.

Stratigraphic position of two diatomite beds in Brown (1968, Fig. 2).

The fracture fills contain basalt fragments (chunks of wall rock) and faint indications of fluidized flow.

Open pore space in the palagonite appears to have been filled by the white stuff.

See how the stringers and pockets of white are found above and below pillows and in small fractures throughout the complex? To me, that speaks to a diatomite bed several meter thick invaded and displaced by invading lava. Ignore the light gray calcrete capping the outcrop. The calcrete, a paleosol, is Pleistocene in age - millions of years younger than the basalt. See how the uppermost pillows are truncated by it? The younger calcrete unconformably overlies the Miocene pillow-palagonite complex. Now go back to ignoring it.

A network of filled fractures crosscuts a pillow near the top of the exposure, giving it a shattered look.

Map of diatomite occurrences in the Western U.S. (USGS/Wallace et al., 2006). A number of articles describe the Quincy diatomite deposit near George, WA (Silica Rd) and the smaller Squaw Creek diatomite (NW1/4 T15N, R20E) located several miles west of Vantage (Godby, 1914; Lowell, 1930; Swift, 1940; Makin, 1949, 1961; Bingham and Grolier, 1966; Livingston, 1966; Burlington Northern, 1971; Campbell, 1975, Ralston, 1984; Brunstad, 1987; Houseman, 2006; Menicucci et al., 2008; Menicucci et al., 2016). The mines near George closed decades ago, but mining continues in the Frenchman Hills today. Two busy surface operations can be seen from the Beverly-Burke Road near where it crosses the crest of the anticline (IMERYS, Diatomite, commonly called 'diatomaceous earth', is used as a filter medium, as an abrasive, an additive, and for absorption.

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