PALEOGENE-NEOGENE STRATIGRAPHY, VOLCANISM, AND VOLCANOGENIC SEDIMENTATION IN THE GREENHORN 7.5' QUADRANGLE, OR

An unusually well exposed >300 m-thick succession of Paleogene to Neogene strata along the North Fork of the Burnt River in the Greenhorn 7.5’ quadrangle of NE Oregon reveals episodes of rhyodacitic to basaltic volcanism, alluvial volcanogenic sedimentation, and fluvial incision. Although not yet constrained by radiometric ages, the lithologic diversity, stratigraphic arrangement and sedimentary history of these deposits are consistent with those of Eocene Clarno and Oligocene-Miocene John Day Formation strata exposed to the west. The strata examined here consist of meters-thick beds of generally tabular, matrix- to clast-supported volcanogenic conglomerate and breccia and gravelly sandstone that are interstratified with laterally continuous, <2 m thick successions of silicic ash-flow tuff and airfall deposits and discontinuous, <4 m thick beds and lenses of andesitic to basaltic lava. Detailed section measuring and lithologic assessment of these deposits suggest that at least ten episodes of eruptive ash-flow and airfall deposition were followed by accumulation of volcanogenic sedimentary gravity flow pebble-boulder conglomerate and breccia (debris flow and associated hyperconcentrated flow) gradationally overlain by multiple stacked beds of tractionally transported, broadly confined to locally channelized, stratified and cross-stratified gravelly sandstone and sandy conglomerate. Paleocurrent indicators are uncommon and suggest largely south to north transport. Eruptive-sedimentary episodes are most common in the lower and upper one-third of the stratigraphic succession, where apparently genetically related petrified wood also is abundant. Gravel clast counts suggest that volcanogenic sedimentary deposits were supplied dominantly by basaltic and basaltic andesitic sources, though rhyodacitic and dacitic clasts are locally abundant near the middle of the section. Post-depositional tectonic deformation and dike intrusion has been relatively minor and likely accompanied regional Neogene volcano-tectonism. Understanding the overall geometry and ultimately the ages of these deposits provides the stratigraphic details necessary to map and reconstruct the volcano-tectonic evolution of the area.