THE LOWEST FLOWS OF PICTURE GORGE BASALT RECORD REVERSE GEOMAGNETIC POLARITY! IMPLICATIONS FOR DURATION OF ERUPTIONS

The Picture Gorge Basalt (PGB) is a minor component of the Columbia River Basalt Group (CRBG) in eastern Oregon. The PGB is smaller in volume and is considered coincident in time with the more voluminous Grande Ronde Basalt, which spreads across multiple Cascadian states. Recent ⁴⁰Ar/³⁹Ar geochronologic work suggests an extended period of eruption for the PGB spanning the entire main phase—Steen, Imnaha, Grande Ronde—of CRBG volcanism, and increased its mapped distribution. Since the general magnetostratigraphy of the CRBG main volcanic sequence is understood as spanning ~6 polarity intervals (R0–N0–R1–N1–R2–N2), new paleomagnetic work on the PGB can test the hypothesis of extended eruptions. The magnetostratigraphy of PGB lavas at its type section is simple: lower flows are normal polarity, and lavas transition to reverse polarity in the space of a few flows near the top of the sequence. However, the oldest lavas of the Picture Gorge Basalt are not represented at the type section. Our new work on the oldest lava flows in the PGB, below the type section, reveals a lower reversal in multiple exposures in Eastern Oregon. These magnetostratigraphic results extend the known interval of PGB volcanism on the order of a few hundred kyr; any estimate of the duration relies on comparison with the geomagnetic polarity timescale (GPTS). Either PGB volcanism lasted throughout CRBG polarity chrons R1-N1-R2, or it began in R0 penecontemporaneous with the Steens Basalt and subsequently throughout CRBG polarity chrons R0-N0-R1. The differences between the scenarios would be on the order of tens to a few hundred kyr based on when within the GPTS polarity chrons eruptions began and ended. We favor the first option based on observations of interbedded, normal polarity PGB and Grande Ronde flows at the Butte Creek section, since Grande Ronde eruptions were not occurring during N0. Our work partially vindicates the suggestion of an extended interval of PGB volcanism (Cahoon et al., 2020), however, we find that the PGB is unlikely to have lasted through the entire main phase of the CRBG without major unrecognized hiatuses since it only spans 3 polarity intervals of the 6 in the main sequence.