SEDIMENTARY AND VOLCANIC EVOLUTION OF THE MIDDLE EOCENE CLARNO FORMATION, NE OREGON

Interstratified volcanogenic sedimentary and primary volcanic deposits, with a cumulative thickness of ~500 m in the Greenhorn-Pogue Point area of NE Oregon, yield new information about the evolution of the middle Eocene Clarno Formation. 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, pyroclastic deposits, and mafic to intermediate lava flows. Abundant clast- and matrix-supported conglomerate, stratigraphic architecture, and laterally extensive beds are evidence for sedimentary gravity-flow-dominated deposition on broad alluvial plains that flanked volcanic uplands. Basaltic andesite and andesite source rocks dominate; dacite source rock contributions are minor, localized, and occur in laterally discontinuous beds and lenses. Rare argillite gravel clasts derived from accreted-terrane rocks suggest that the volume of primary volcanic detritus overwhelmed other sediment sources.

Geologic mapping, stratigraphic correlation, and XRF geochemistry show the Greenhorn-Pogue Point deposits are stratigraphically and geochemically similar to the informally defined “upper Clarno sequence” at Cougar Rock and Magone Lake, ~40-50 km to the west. ⁴⁰Ar/³⁹Ar (groundmass) ages (40.1 ± 0.38 and 41.9 ± 0.5 Ma) obtained from channelized, calc-alkaline andesite/basaltic andesite flows in the Greenhorn-Pogue Point sedimentary-dominated succession overlap chronologically with: 1) basaltic lavas (40.64 ± 0.31, 41.09 ± 0.42 Ma) capping volcaniclastic rocks at Cougar Rock and Magone Lake, 2) Clarno andesitic-dacitic flow-dome complexes and late rhyolitic flows, domes, and tuff at the Wildcat Mountain caldera, central Oregon (43.86 ± 0.89-39.35 ± 0.30 Ma), and 3) early volcanism (41.81 ± 0.5 Ma) in the ancestral Cascades Volcanic Arc, western Oregon. Improved regional documentation of the Clarno Formation and its spatial-temporal link to the onset of Cascades arc magmatism will provide a better understanding of the volcano-tectonic evolution in the PNW.