Flow directions in Columbia River Basalt flows and paleocurrents of interbedded sedimentary rocks, South-Central Washington

Abstract

More than 150 flow directions measured mainly in inclined pipe vesicles and, more rarely, vesicle cylinders, spiracles, and inclined foreset beds in pillowpalagonits-complexes) of the upper 12 widespread Yakima Basalt flows on the Columbia River Plateau in south-central Washington indicate rather uniform transport directions from southeast to northwest. This strongly supports former ideas that the large Grande Ronde-Cornucopia dike swarm in northeast Oregon fed the huge lava floods. It is shown that all previous evidence indicating eruption centers for the basalts in the Cascada Range to the west is inconclusive. Cross-bedding directions, maximum pebble sizes, and distribution of heavy mineral suites in sedimentary deposits interbedded with the basalt sheets delineate in more detail two main paleoslopes whose deposits interfinger: a) the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene). b) the Columbia paleoslope with south-southwest paleocurrents and chiefly sphene, epidote, clinozoisite, blue-green hornblende, garnet, sillimanite, kyanite, and staurolite. Distribution of conglomerats, current directions, and heavy mineral suites support former hypotheses that the ancestral Columbia River flowed about 50 km farther west in early Pliocene time than it does today in the area of lower Yakima Valley.