NEOGENE BASALT FLOW STRATIGRAPHY NEAR PRINEVILLE, OREGON: INTERACTION WITH THE ANCESTRAL CROOKED RIVER

Neogene basalt flows of the Deschutes Formation and Prineville basalt, exposed near Prineville, show a close spatial association between flow distribution and ancestral channel(s) of the Crooked River. Late Miocene to Pliocene basalts are correlated with the Deschutes Formation, which has been characterized at its type section near Madras as a sedimentary and tuff dominated succession, with subordinate basalt flows, largely derived from the adjacent Cascade Range between 7.4 Ma to 4.0 Ma. By contrast, correlative strata near Prineville are dominated by basalt flows, contain an overall paucity of sedimentary rocks, and a majority of the section lies stratigraphically beneath the late Miocene (7.05 Ma) Rattlesnake ash-flow tuff. Deschutes Formation basalt flows were erupted locally from at least 7 vents and, based on stratigraphic relations and chemical affinity, form 14 mappable flow packages. The flows are typically open-textured, olivine-phyric, and are characterized by average (n = 46) major element compositions of 49.9 wt. % SiO₂, 16.1 wt. % Al₂O₃, 1.6 wt. % TiO₂, 11.3 wt. % FeO*, and 7.3 wt. % MgO. The thickest accumulation of Deschutes basalt is exposed in the modern Crooked River canyon, where flows are juxtaposed against or fill channels incised into the middle Miocene (15.7 Ma) Prineville basalt. The Prineville basalt consists of closed-textured, iron-rich basaltic andesites with elevated concentrations of P₂O₅ (1.25-2.02 wt. %) and Ba (1695-3202 ppm). Exposures of the Prineville basalt reach a maximum composite thickness of 210 m within the Crooked River canyon near Bowman Dam. The section thins abruptly away from the canyon; flows that cap highlands form thin, shoe-string exposures that are interbedded with and burrow into underlying late to middle Miocene sedimentary rocks. Map patterns, combined with stratigraphic relations in the canyon indicate: 1) Neogene basalt flow distribution was strongly controlled by paleotopography and ancestral channel(s) of the Crooked River, and 2) the Crooked River has held the same general geographic position since at least the middle Miocene. Locally palagonitic vent facies within the local Deschutes Formation and the well-exposed Bowman maar complex at Prineville Reservoir also indicate that vents interacted with water at or near the surface during eruptions.