ROBUST PALEOMAGNETIC CORRELATION OF YELLOWSTONE HOTSPOT IGNIMBRITES ALONG THE SNAKE RIVER PLAIN

Using a statistical technique based on geomagnetic secular variation of paleomagnetic direction in stratigraphically ordered sequences, we demonstrate robust correlations of the mid-Miocene Yellowstone hotspot related Cougar Point Tuff (CPT) succession 100 km across the southwest Snake River Plain (SRP) of Idaho, USA. Geochemical, mineralogical and geochronologic data are consistent with these correlations, but the strength of correlations based on them alone is not easily quantified. Developing a regional (SRP) ignimbrite stratigraphy is essential for estimating eruption volumes and frequency, as well as evaluating causal relationships between volcanism, tectonics, and climate change. The ~450 m tall Black Rock Escarpment (BRE), exposes eight individual eruption-units (CPT III, V, VII, IX, XI, XII, XIII, XVb), six of which have been correlated ~70 km east to the Brown's Bench Escarpment (BBE), and three have been correlated ~100 km east to the Cassia Mountains (CM). Minimum volume estimates are calculated for CPT V (300 km³), CPT VII (700 km³), CPT IX (400 km³), CPT XI (1200 km³), CPT XIII (1200 km³) and CPT XVb (1200 km³) (Finn et al., 2017; JGR Solid Earth).

The CPT succession records unusually large variation in paleomagnetic direction, which enables exceptionally strong tests of correlation. The statistical method compares the probabilities that the paleomagnetic directions of potentially correlative deposits represent either (a) simultaneous or (b) “random” samplings of the ancient geomagnetic field. If Ps is the probability that two site mean directions were recorded at the same time, while Pr is the probability that they are samplings of the geomagnetic field spaced apart over a timespan that is longer than secular variation (~300 yrs), then the ratio of Ps to Pr is the most direct measure of whether two ignimbrites were emplaced at the same “geomagnetic instant”. For example, Ps/Pr = 10 indicates a 10 times larger probability that two ignimbrites were simultaneously emplaced, rather than being separated in time. Moreover, the Ps/Pr ratios for pairs of correlations multiply when considering the cumulative Ps/Pr probability that two sections are correlative. The BRE and BBE cumulative Ps/Pr ratio is 3.8 × 10⁹, demonstrating the remarkable strength of paleomagnetism in correlating ordered sequences.