EOCENE TO OLIGOCENE VOLCANOGENIC SEDIMENTARY AND VOLCANIC ROCKS EXPOSED IN THE WHITNEY 7.5' QUADRANGLE, OREGON: IMPLICATIONS FOR VOLCANO-TECTONIC EVOLUTION

The southern half of the Whitney 7.5’ Quadrangle, northeastern Oregon is dominated by a folded and faulted, ~300-m-thick succession of late Eocene to Oligocene volcanogenic sedimentary and volcanic strata that records episodes of Paleogene volcanism that were characterized by a continuum of basaltic to rhyolitic igneous products. Potentially correlative with Eocene Clarno or Oligocene to early Miocene John Day volcanism, this extensive ~450 Km² though unnamed lithostratigraphic unit overlies structurally complex, metasedimentary and metavolcanic rocks of the late Paleozoic to early Mesozoic Baker Terrane and multi-cyclic Paleogene gravel deposits. The unit is overlain by the middle Miocene Strawberry Volcanics and is variably capped by Quaternary alluvium, colluvium, and landslide deposits. Eocene to Oligocene volcanogenic sedimentary rocks are dominated by medium to very thick beds of matrix- and clast-supported volcanogenic sedimentary pebble-boulder conglomerate and breccia. Volcanogenic sedimentary gravel-dominated beds are intercalated with decimeters-thick beds of silt and fine sand that locally contain plant macrofossil imprints and meters-thick porphyritic, plagioclase- and amphibole-bearing basaltic andesite lava flows, some of which are closely associated with sedimentary beds containing petrified wood. Relatively minor, decimeters to meters thick beds of welded tuff are intercalated with the volcanogenic sedimentary deposits. Detrital gravel clasts are heterolithologic and include basalt, rhyolite, andesite, and dacite. Overall, andesite and dacite gravel clasts dominate, basalt clasts are more common in lower parts of the section with rhyolite becoming more common in the upper parts of the section. The generally massive to crudely stratified nature of the gravel-rich strata, commonly cross-stratified sand beds, and bedding geometries are interpreted as the products of debris and hyperconcentrated flows and fluvial flows that acted on broadly confined alluvial plains. Paleocurrents determined from high-angle planar cross-stratified gravel and sand beds indicate flows were directed to the north, away from an apparently nearby volcanic center and upland to the south.