THE TERTIARY RECORD OF EXPLOSIVE AND EFFUSIVE VOLCANISM IN THE PALEOGENE, CLARNO FORMATION-DEPOSITS OF THE GREENHORN 7.5' QUADRANGLE, OREGON

Volcanogenic sedimentary strata, silicic pyroclastic rocks, and intercalated basaltic to andesitic lavas that are > 0.5 km in thickness in the Greenhorn 7.5’ quadrangle of NE Oregon provide insight into the volcanic evolution and coupled volcanogenic and fluvial sedimentation in this region during the Paleogene. Detailed stratigraphic analyses indicate that Clarno Formation deposits resulted from discrete pulses of volcanism and volcanogenic sedimentation. The Clarno succession consists of beds of volcanogenic breccia and conglomerate interstratified with <3 m beds of silicic ash-flow /airfall tuffs. This succession is unconformably overlain by a laterally discontinuous <2 m bed of the middle Miocene Dinner Creek tuff (15.9-15.4Ma) and wide continuous lavas of the Strawberry Volcanics (12-4Ma). Matrix- to clast-supported conglomerate and breccia of the Clarno strata commonly have a tuffaceous sandstone matrix, and contain clasts that range from rounded to angular gravel to megaclasts (diameters >1 m). Pre-existing topography resulted in the channelization of both debris and lava flows that make their correlation difficult. Clast counts and XRF major and trace element analyses indicate that their volcanic sources were predominantly andesitic to basaltic but range from 59 to 64 weight percent silica. The rhyolitic Dinner Creek Tuff is characterized by a basal vitrophyre and increasing fiamme and a eutaxitic structures upsection. The tuff is overlain by the basaltic to andesitic flows of the Strawberry Volcanics. Although local andesitic eruptive vents have been identified in the field area, with larger scale dacitic plugs mapped to the north, location of coeval, large-scale volcanic centers is uncertain. Clast lithologies, paleocurrent indicators, bed geometries and spatial distributions of volcanogenic sedimentary deposits provide evidence for volcanic center source areas to the south/southeast. Leaf fossils and petrified wood are abundant throughout the succession. Tectonic deformation is relatively minor, with normal fault displacements of <4 m and dike emplacement appearing to affect the lower strata the most, indicating possible syntectonic sedimentation, and a prolonged period of relative tectonic quiescence prior to the explosive emplacement of the Dinner Creek Tuff.