TECTONIC SIGNIFICANCE OF ASH -FLOW TUFFS OF THE JOHN DAY FORMATION, OREGON

The John Day Formation of Oregon consists largely of dacitic and rhyolitic ash-fall and air-fall tuffs with minor rhyolite flows and domes and alkali basalts. Most of the ash-flow tuffs occur in the western facies of the formation between the Blue Mountain uplift and the Cascade Range. Six major ash-flow sheets are present in this area; two in member A and one each in members E, G, H and I. In the eastern facies there are only two ash-flow sheets, the widespread Picture Gorge Tuff and local outcrops of the equivalent of member A of the western facies. Outcrops of Member A also occur in the Horse Heaven area near the crest of the Blue Mountain uplift. All of the ash-flow tuffs except that of member I are rhyolitic and typically contain abundant phenocrysts of quartz, sanidine and plagioclase. The Picture Gorge Tuff occurs only in the eastern facies and is believed to have originated from an unknown vent in the Ochoco Mountains. Based on their distribution, thickness and lithology all of the other ash-flow tuffs originated west of their present-day outcrops, probably along the eastern edge of the Cascade Range. Numerous rhyolitic domes in this area may mark some of the former vents. The dacitic ash-fall tuffs are believed to have been derived from the early Cascade Range. The bimodal, rhyolite-alkali basalt assemblage of the western facies, which is interlayered with the air-fall deposits, represents rifting in a back-arc basin, a major change in tectonic environment from the underlying Clarno Formation. The ash flow tuff of member A extended into the eastern facies across what is now the Blue Mountain uplift. Uplift of this feature during deposition of the John Day Formation between about 37-22 Ma separated the eastern and western facies, leading to distinctly different stratigraphic sequences on opposite sides of the Blue Mountain uplift.