PALEOMAGNETIC VARIATIONS AND 40AR/39AR GEOCHRONOLOGY OF A 1,538-FOOT BOREHOLE FROM THE EASTERN SNAKE RIVER PLAIN, IDAHO

Coring of a 1,538-foot-deep borehole (in September and October 2020) located on the Eastern Snake River Plain (ESRP) revealed a nearly continuous sequence of basaltic lava flows extending from 4 feet below ground surface to 739 feet below ground surface. Over this depth range, the borehole was interpreted to have intersected forty-two (42) individual basalt flows and numerous sedimentary interbeds. Between 739 feet below ground surface and 1,139 feet below ground surface, coring exposed sediments composed predominantly of unconsolidated to moderately consolidated conglomeratic materials (alluvial materials shed from the adjacent Arco Hills). Below the conglomerate, two (2) rhyolitic ash flow or air fall tuffs were encountered, extending (separately) from about 1,140 feet below ground surface to 1,385 feet below ground surface and from 1,512 feet below ground surface to the borehole termination depth. A second layer of moderately- to well-indurated conglomeratic materials was encountered between the rhyolitic tuff units (i.e., between 1,385 feet and 1,512 feet).

Paleomagnetic analysis of one-hundred and ninety-four (194) samples from twenty-six (26) basalt flows and ⁴⁰Ar/³⁹Ar dating of a subset of ten (10) basalt flows identified in the deep boring were completed in 2022 to facilitate correlation to pre-existing boreholes from the ESRP. Paleomagnetic inclination measurements (and separate geologic mapping) indicate that the youngest lava flows in the 1,538-foot-deep boring are likely correlated to the Crater Butte surface volcanic vent (located roughly 5.6 miles [9.0 kilometers] east-southeast of the core site). Radiometric dating of the 1,538-foot-deep boring suggests a plateau age of 0.231 ± 0.014 million years before present (Ma) for these same flows. Immediately underlying flows in turn are correlated to the Pond Butte surface vent (dated here to 0.305 ± 0.053 Ma). Pond Butte is located 7.5 miles (12.0 kilometers) east-southeast of the core site (and nearby Crater Butte). Collectively, the paleomagnetic analyses and ⁴⁰Ar/³⁹Ar dating suggest that the forty-two (42) flow units identified in the borehole correlate to fifteen (15) separate flow groups (i.e., volcanic events) emplaced roughly over the last one (1) million years.