Volcanogenic sedimentation in the Greenhorn 7.5' quadrangle Oregon: relation to explosive and effusive volcanism in the Eocene Clarno formation

Emily Bogdan Cahoon

A >500 m thick succession of volcanogenic sedimentary strata, silicic pyroclastic rocks, and intercalated basaltic to andesitic lava-flow rocks provide insight into the Tertiary volcanic-tectonic evolution of the Blue Mountains region, NE Oregon. Detailed stratigraphic analyses indicate that Clarno Formation deposits in this quadrangle resulted from discrete pulses of volcanism and volcanogenic sedimentation. The Clarno succession consists of repeated 10-50 m thick beds of volcanogenic breccia and conglomerate interstratified with <3 m thick beds of silicic ash-flow and air-fall tuff. Matrix- to clast-supported conglomerate and breccia beds of the Clarno Formation commonly contain a tuffaceous sandstone matrix, rounded to angular granule to boulder clasts, and rare subrounded megaclasts (diameter >1 m). The modern landscape reflects the underlying paleotopography as the debris and hyperconcentrated flow deposits were more erodible than the underlying metamorphic terranes. Correlation of Clarno Formation strata in the Greenhorn quadrangle indicates that the geometries and spatial distributions of its volcanic and volcanogenic deposits were strongly influenced by pre-existing, fluvially incised v topography as revealed by stratal geometries and distributions of sedimentary volcanic deposits. The Clarno Formation in the Greenhorn 7.5’ quadrangle is unconformably overlain by a laterally discontinuous <2 m beds and lenses of the middle Miocene Dinner Creek Tuff (ca. 14.9 to 16.2 Ma) characterized by a basal vitrophyre and increasing fiamme and a eutaxitic structures up-section. The Clarno Formation is also unconformable capped by the laterally continuous basaltic to andesitic lava flows of the Strawberry Volcanics (ca. 16 to 12 Ma). The provenance of the volcanogenic sedimentary Clarno deposits in the Greenhorn area is difficult to reconstruct definitively because of incomplete knowledge regarding locations and compositions of possible volcanic sources. Local andesitic eruptive vents associated with the Clarno Formation have been recognized in the field area, and include a few dacitic plugs; however, gravel-clast counts and XRF major and trace element analyses indicate that the volcanic sources were predominantly andesitic to basaltic. Lithologies, paleocurrent indicators, stratal geometries and spatial distributions of volcanogenic sedimentary deposits suggest that the volcanic centers that supplied detritus to the Clarno Formation in the Greenhorn area were located to the south or southeast.

Volcanogenic sedimentation in the Greenhorn 7.5' quadrangle Oregon: relation to explosive and effusive volcanism in the Eocene Clarno formation

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