MAPPING AND CORRELATING ENIGMATIC PALEOGENE TO NEOGENE VOLCANOGENIC SEDIMENTARY AND VOLCANIC STRATA IN NE OREGON

The depositional history of an approximately 400 m-thick section of interstratified volcanogenic sedimentary, pyroclastic, and lava deposits in the Pogue Point 7.5’ quadrangle, NE Oregon provides insight into the Paleogene-Neogene volcano-tectonic and landscape evolution of this region. Possibly correlative with the Eocene Clarno and Oligocene-Miocene John Day Formations, these strata consist dominantly of tens-of-meters-thick volcanogenic sedimentary beds that are overlain by or contain intercalated, meters-thick, pyroclastic deposits and lavas. Volcanogenic-sedimentary strata are characterized by matrix- and clast-supported pebble-boulder conglomerate and breccia generated by sediment gravity flow and related processes (primarily via debris and hyperconcentrated flows) and gravelly sandstone and sandy conglomerate generated by tractional processes (primarily via stream flows). The tabular- to wedge-planar geometries of individual volcanogenic sedimentary beds suggest they accumulated on broadly confined to unconfined alluvial plains. These alluvial plains were cut by meters- to tens-of-meters-deep stream channels that drained volcanic uplands located to the south, as suggested by orientations of planar and trough cross-strata in gravelly sandstone and sandy conglomerate. Gravel clasts in volcanogenic sedimentary deposits are dominated by rhyodacite and dacite in the lower third of the section and andesite, basaltic andesite, and basalt in the upper two-thirds of the section. Petrified wood commonly occurs within volcanogenic sedimentary deposits that immediately overlie pyroclastic flow, surge, and airfall deposits, suggesting a genetic relation. The channels incised into the volcanogenic sedimentary deposits served as pathways for the pyroclastic flows and lavas. The lateral discontinuity of primary volcanic deposits in the section and their subsequent burial beneath often lithologically similar Miocene volcanic deposits hinders efforts to correlate or map individual lithostratigraphic units. Gravel-clast lithologic trends now documented in the volcanogenic sedimentary deposits, however, are sufficiently distinct to offer promise as correlation and mapping tools necessary to refine understanding of the regional stratigraphy.