Sediment Chimney at Lava Flow Base

This feature is seen in a roadcut on WA SR-14 just east of Chamberlain Lake Rest Area.  We believe it is the result of a lava flow advancing in separate lobes onto a shallow marshy area.  The lobes bulldozed the marshy plant and sediment material into a row between the lobes.  As the flow continued to inflate it progressively encased the row of marsh material, causing it to form a teepee-like escape structure before it was completely encased and buried. 

Subflow Sediment Squeeze-up in SR-14 Roadcut

The feature extends upward from the base of the flow approximately 10 feet and it is clearly continuous on either side of the roadcut, which is greater than 50 feet wide.  The feature is composed of lightly metamorphosed shales interbedded with partially petrified and carbonized woody material, including tree limb debris.

The encasing and overlying basalt is easily 40-50 feet thick, massive in texture, and according to the WA Interactive Geologic Map it is part of the Wanapum member of the Columbia River Basalt Group.

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