NEW PALEOMAGNETIC STRATIGRAPHY IN THE IMNAHA AND GRANDE RONDE BASALTS, COLUMBIA RIVER BASALT GROUP, NORTHEAST OREGON AND SOUTHEAST WASHINGTON, USA

The Columbia River Basalt Group (CRBG) is the world’s youngest large igneous province (LIP). Approximately 210,000 km³ of basalt erupted between ca. 17 and 6 Ma; 95% of this volume erupted between just 16.8 and 15.8 Ma. The Imnaha Basalts (IB) set the stage for the voluminous Grande Ronde Basalts (GRB), filling paleo-topography. We have collected modern paleomagnetic data to investigate spatiotemporal trends in the IB and GRB eruptive history. High-precision, Ar-Ar and U-Pb, geochronology studies have presented ages for IB eruptions from 17.18 to 16.571 Ma and from 16.570 to 16.111 Ma for GRB (e.g., Jarboe et al., 2010; Kasbohm et al., 2018 and 2023; Streck et al., 2023). Existing IB magnetostratigraphy, which utilized blanket cleaning steps and fluxgate data, indicates dominantly normal polarities, with transitional and reversed directions at the top and bottom of IB sections (Hooper et al., 1979 and 1984). This polarity pattern and the radiometric ages show that the IB erupted in the C5Cn.3n (a.k.a. N0) polarity chron (i.e., 16.637 to 16.532 Ma, Kochhann et al., 2016). The R1, N1, R2, and N2 polarity pattern of GRB are then C5Cn.2r – C5Cn.1n.

Flow-by-flow paleomagnetic measurements provide a high-resolution chronometer to assess eruptive rates of these formations utilizing paleosecular variation, and track deformation by providing multiple points of comparison. We collected 7-8 paleomagnetic cores from each flow; cores are oriented with sun and magnetic compasses. We performed detailed thermal and alternating-field demagnetization experiments at the USGS Paleomagnetics Lab. Here, we present initial results from a 21-flow section of IB (9 flows) conformably overlain by GRB (12 flows) near Imnaha, Oregon. We see a detailed normal-to-reverse polarity transition recorded in the IB, with the sequence entering true reversed polarity before the GRB which is entirely reverse polarity. We also present a section of mostly GRB working up Weissenfels Ridge, near its type section, which begins with a reverse polarity IB flow. Our future work will explore the eruptive transition between the IB and GRB using paleosecular variation and will provide an independent method for estimating the duration and length of eruptions, which is vital to understanding deformation within the ‘stable’ LIP plateau.