NEWLY IDENTIFIED OLIGOCENE ALKALI VOLCANICS ALONG THE EASTERN MARGIN OF THE COLUMBIA PLATEAU, LATAH AND SURROUNDING COUNTIES, IDAHO

Oligocene basaltic and trachytic volcanics have been identified, described, and mapped in Latah and surrounding counties of northern Idaho. Dating has confirmed the Oligocene age of these rocks.

Previous workers used the names “Onaway Member” and “basalt of Potlatch” for the basaltic units. These units were considered part of the Miocene Columbia River Basalt Group (CRBG) and were included in either the Wanapum or Saddle Mountains formations because of ambiguous outcrop relationships. The basalt is fine to medium grained with isolated plagioclase phenocrysts 0.5-1 cm long, common platy partings, and normal magnetic polarity; it is commonly deeply weathered to a light tan or purplish gray. Field and well data indicate several flows with a total thickness of 150 meters or more emplaced onto a steep topography developed on pre-Tertiary rocks. The basalts vary in composition, but in comparison to most of the CRBG, they have lower SiO₂ and higher TiO₂, Al₂O₃, and P₂O₅, and they are enriched in Sr, Zr, and Nb. On a total alkali-silica (TAS) diagram, the basaltic units plot in the trachybasalt and trachyandesite fields, in contrast to basalts of the CRBG that plot in the basalt, basaltic andesite, or andesite fields.

The trachytic rocks, previously mapped as Eocene Potato Hill Volcanics, crop out along Hatter Creek several kilometers south of Potlatch, in an area covering less than 2 km². They consist of pyroclastic flows, including crystal tuff and crystal lithic tuff with flattened pumice and quartzite clasts, and minor lava. Weathered lithic tuff underlies the basalt at one location, indicating the trachytic units predate the basaltic units. On the TAS diagram, these rocks plot in the trachyandesite and alkali trachyte-trachyte fields.

Three Ar-Ar dates, two from the same basalt flow and one from hornblende crystals in tuff, ranged from 25.6 to 26.32 Ma. Two K-Ar dates from basalt flows yielded similar ages. These Oligocene alkali volcanics, the first documented in northern Idaho, expand the known extent of Oligocene volcanism in the Pacific Northwest. Future publications will propose the name “Potlatch Volcanics” for the sequence and the names “Onaway Member” and “Hatter Creek Member” for the basaltic and trachytic units, respectively.