Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 40-5
Presentation Time: 9:00 AM-3:30 PM


MCCLAUGHRY, Jason D.1, GAYLORD, David R.2, FERNS, Mark L.3, FELT, Kristopher Jerome2, SPALL, Brian N.2 and CAHOON, Emily B.4, (1)Oregon Department of Geology and Mineral Industries, Baker City Field Office, 1995 3rd Street, Suite 130, Baker City, OR 97814, (2)School of the Environment, Washington State University, PO Box 642812, Pullman, WA 99164-2812, (3)College of Arts and Sciences, Eastern Oregon University, La Grande, OR 97850, (4)Department of Geology, Portland State University, PO Box 751, Portland, OR 97207

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.