STRATIGRAPHIC AND GEOCHRONOLOGIC CONFIRMATION OF THE MIDDLE EOCENE CLARNO FORMATION IN NE OREGON

The Greenhorn-Pogue Point area of NE Oregon preserves discontinuously exposed sections of interstratified volcanogenic sedimentary, volcanic, and pyroclastic deposits exceeding 500 m in cumulative thickness. New ⁴⁰Ar/³⁹Ar (groundmass) ages (40.1 ± 0.38 and 41.9 ± 0.5 Ma) obtained from channelized, calc-alkaline andesite and basaltic andesite flow rocks in this sedimentary-dominated succession overlap chronologically with: 1) those reported from olivine- and clinopyroxene-phyric basaltic lavas (40.64 ± 0.31 and 41.09 ± 0.42 Ma) that cap volcaniclastic as well as andesitic and basaltic andesitic flow rocks at Cougar Rock and Magone Lake, ~40-50 km to the west, and 2) early volcanism in the ancestral Cascades of western Oregon. Updated geologic mapping, detailed physical stratigraphic assessment, and XRF-geochemical characterization of sedimentary and flow rocks in the Greenhorn-Pogue Point area suggest they are stratigraphically and geochemically similar to the informally designated “upper Clarno sequence” at Cougar Rock and Magone Lake.

Upper (?) Clarno strata in the Greenhorn-Pogue Point area unconformably overlie Paleozoic and Mesozoic accreted-terrane rocks and consist of broadly folded beds of very poorly to moderately sorted, angular to subrounded, dominantly matrix-supported, massive to crudely stratified boulder-cobble (~30 vol%) and pebble (45 vol%) conglomerate with intercalated beds and lenses of silty sandstone and mafic to intermediate lava flows. Gravel-clast counts in the Pogue Point area indicate that, at least locally, dacitic source rocks dominate the lower third of the succession whereas basaltic andesitic and andesitic source rocks dominate the upper two-thirds of the succession, suggesting possible unroofing of eastern Clarno Formation deposits in the Cougar Rock and Magone Lake vent areas. The abundance of laterally extensive beds of gravel clast-rich, clast- and matrix-supported conglomerate and stratigraphic architecture are evidence for sedimentary gravity flow-dominated deposition that occurred on broad alluvial plains flanking volcanic uplands. The rarity of argillite gravel clasts indicate that accreted-terrane rocks in the Greenhorn-Pogue Point area were largely unexposed at the time of Clarno deposition.