Diatomite Lake Invaded by Molten Lava - Frenchman Hills, WA
A pillow-palagonite complex forms when molten lava invades a body of water, whether its the ocean, a river, or a lake. Pillows are the quenched blobs of lava. Palagonite is the loose, rusty-looking mess that 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 was loose and ravelly when it formed and in modern exposures weathers quickly. 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. They can occur as one coherent zone or as a "stratified" zone of thin basalt layers interbedded with rubbly layers of palagonite.
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 Rd 9 SW roadcut is located just below the Frenchman Hills ridgeline between Dodson Road (~5 miles east) and Adams Rd (~5 miles west), both of which cross the ridge. Road 9 SW is the highest public road that traverses Royal Slope. Its near the newly-planted Stillwater Creek Vineyard. Wind your way around and you'll find it. Parking isn't great. Ask field crew bosses or local landowners for permission to park in their driveways.
The pillowed 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.
A lone pillow surrounded by palagonite. Concentric pattern and glassy rind 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. Opal commonly forms where lava encounters wet sediment. Some vein opal 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) lie on opposite sides of the Hog Ranch-Naneum anticline, the axis of which runs roughly north-south, parallel to the Cacade Range crest. The axis (watershed divide) is approximated by a dashed line. The formerly contiguous diatomite basin (Miocene) appears to pre-date folding (Pliocene-Pleistocene). Map modified from Bingham and Grolier (1966, Fig. 4).
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. So 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.
Probably the coolest thing about the roadcut is these bright white dikes. They are not clastic dikes like those in the Touchet Beds, rather they are fractures filled with white stuff (probably siliceous mud, a diatomite lake deposit) that was heated by the overriding lava and became mobile. It looks like diking was very rapid, consistent with explosive behavior commonly observed in phreatic zones. 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, long gone, was at least as deep as the pillow-palagonite complex is thick. I deduced that from the way pillows and the white stuff intermingle. The pillow in the photo is enclosed by white stuff, which means either, a.) the lava flow was invasive into a thick bed of diatomite, or b.) the white stuff became fluidized when heated, flowing through the permeable palagonite, surrounding pillows, and filling fractures. For sure, the bulk of the white stuff is high in the outcrop, not from below; no white stuff is seen at road level.
White stuff fills the matrix of palagonite surrounding a pillow.
Fractures filled with white stuff connect to and descend from large, coherent blobs of white stuff (formerly lake bottom seds). Bingham and Grolier (1966) summarize the setting,
...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...
The fracture fills contain basalt fragments and show some textures that might result from fluidized flow(?).
Pore space in the palagonite appears to have been filled by the white stuff.
See how the stringers and pockets of white stuff are found above and below pillows and in small fractures throughout the complex? To me, that speaks to a diatomite bed several meter thick. The lava has all but replaced the diatomite. 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 calcrete? It unconformably overlies the Miocene pillow-palagonite complex. Now ignore it.
Several small filled fractures crosscut a pillow located near the top of the complex, giving it a shattered look. The white stuff appears to have once been a coherent layer, more or less, but one that was displaced. Wholesale disturbance occurred when the lava invaded. The volume of lava eclipsed that of the lake bottom sediment.
A number of articles describe the Quincy diatomite 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, www.imerys.com).
Map of diatomite occurrences in western states (USGS/Wallace et al., 2006).