SILICEOUS COBBLES IN A GRANDE RONDE BASALT FLOW, OREGON BUTTE, BLUE MOUNTAINS, WASHINGTON

A Miocene basalt flow at Oregon Butte in Washington’s Blue Mountains possesses the unusual characteristic of having rounded cobbles of quartzite, sandstone and chert concentrated in a horizon near the top of the colonnade. The flow at Oregon Butte is part of Grande Ronde Basalt N2 (normal magnetic polarity). Grande Ronde flows were extruded upon the Tertiary land surface prior to and during the early stages of uplift of the Blue Mountains Anticline. Proterozoic and/or Paleozoic siliceous rocks (presently exposed in Idaho and Montana) are probably the original source for the cobbles, which may have occurred as Tertiary gravels beneath the Columbia River basalts (as seen in Oregon’s Wallowa Mountains), or in a floodplain when this particular flow erupted. In most exposures of lava flows over alluvium, cobbles have not been incorporated into the volcanic rock. Three possible models for the incorporation of the cobbles at Oregon Butte include: a fissure crossing a stream bed, entraining the cobbles in the lava; a landslide at the time of the flow; and burrowing of the lava under cobble-bearing sand.