A FAR-TRAVELED LATE MIOCENE BASALT FLOW FROM CENTRAL OREGON

Two widely separated basalt lava flow sequences in central and northern Oregon are likely the products of a single eruptive episode. The Tetherow Butte Basalt (of the Deschutes Formation) issued from Tetherow Butte (lava km zero). It is a plains-forming basalt with nearly continuous outcrops as far north as Gateway (lava km 55). Similar lava is found again near Maupin (lava km 97), where it lies in a slightly incised ancestral Deschutes River canyon. The next known outcrops, mapped as basalt of Fulton Ridge (in the Dalles Formation) are near the confluence of Deschutes and Columbia Rivers (lava km 156) and extend several miles west along the south slope of the Columbia (lava km 167). Back-of-envelope calculations suggest a minimum erupted volume of 10 km³ (uncorrected to dense rock equivalent). The Tetherow Butte and Fulton Ridge basalts are chemically and petrographically similar and appear to be distinct from other regional lava flows, such as the those of the Columbia River Basalt Group, Deschutes Formation, and Dalles Formation. These two basalt units are low in aluminum, high in iron and titanium, have sparse to no olivine phenocrysts, contain 0.5-mm clinopyroxene microphenocrysts in a fine-grained groundmass, and lack diktytaxitic texture. Radiometric dating places them in the same age range. ⁴⁰Ar/³⁹Ar ages from the Tetherow Butte basalts by other researchers during the past 30 years range from 5.17-5.57±0.10 Ma. Our new ⁴⁰Ar/³⁹Ar ages from the Fulton Ridge basalt are 5.43±0.09 Ma and 5.45±0.08 Ma. Paleomagnetic measurements show a stable, normal-polarity direction at six sites sampled; this age-polarity association is at odds with the present geomagnetic polarity time scale. Paleomagnetic directions from lava flows in the north (Fulton Ridge) have a counterclockwise declination (D=359.4°; I=+59.6°; a95=7.9°) compared to those in the south (Tetherow Butte, D=008.2°; I=+55.0°; a95=4.8°), perhaps due to the numerous intervening northwest-striking lateral faults in northern region of basalt outcrops. Geochemical, geochronological, and paleomagnetic data suggest these widely separated outcrops are the same lava sequence, spanning a distance at least 160 km along the ancestral Deschutes River and downstream Columbia River and establishing a stratigraphic tie between the Deschutes and Dalles Formations.