Petrology and chemistry of a Columbia River basalt section, Rocky Canyon, west-central Idaho

Abstract

Field characteristics, petrography, and major-element analyses are given for 17 of the 19 flows of the Columbia River Group present in a canyon in the southeast corner of the Columbia Plateau. The 17 flows are divided into 4 flows of Imnaha Basalt at the base, 12 flows of lower Yakima Basalt above, and 1 flow of middle Yakima Basalt at the top. The 4 Imnaha Basalt members are feldsparphyric with lower SiO 2 (48.0 to 52.0 percent) and lower K 2 O (0.60 to 1.14 percent) than the lower Yakima Basalt. They may be subdivided into the upper two flows of Rock Creek chemical type that have less SiO 2 (48.8 to 50.5 percent) than the other two members. The Rock Creek flow has uniquely high values of Al 2 O 3 and MgO associated with high modal olivine. The lower two members may be distinguished from each other by their TiO 2 contents. The lower Yakima Basalt flows are fine grained and nonporphyritic with an intergranular to intersertal texture, SiO 2 varies from 52.0 to 56.6 percent and K 2 O varies from 1.14 to 2.00 percent. The earlier nine members show an increase in SiO 2 and K 2 O and a decrease in MgO upward; the top three members show a reversal of this trend. The less siliceous flows contain small subhedral olivine crystals or their pseudomorphs, and the more siliceous contain a pigeonitic in addition to an augitic clinopyroxene. Each member has a unique set of properties that is consistent vertically through the thickness of a flow and laterally between two sections 4 km apart and separated by a major fault. The top flow exposed, with phenocrysts of plagioclase and olivine, falls within the Pomona chemical type previously recorded only from upper Yakima Basalt. Flow-by-flow correlation can be made of the lower Yakima Basalt with the Whitebird section 20 km to the south. Thinning and cutting out of flows from north to south suggests that many flows are local in origin. Similar types of Imnaha Basalt occur at Rocky Canyon and Whitebird, but flow-by-flow correlation is less convincing.