REVISITING RED BEDS: MAGNETIC ANALYSES OF THE SPOKANE, GRINNELL, AND APPEKUNNY FORMATIONS, BELT-PURCELL SUPERGROUP, MT

The Boring Billion was bookended by the breakup and formation of two supercontinents, Nuna and Rodinia. The paleogeographic position of Laurentia is key for reconstructing these supercontinents due to its early assembly in the Paleoproterozoic and minimal subsequent differential motion between provinces. However, between 1460 - 1300 Ma, the position is largely constrained by red bed data from the Belt-Purcell Supergroup developed in the 1970s and 1980s (Elston et al. 2002). Red beds are both excellent and challenging for paleomagnetism because they record both detrital and chemical remanence; the latter can form soon after deposition or much later. Existing data from the Belt-Purcell Supergroup are based on low-resolution demagnetization schemes that do not allow for discrimination between components. Additionally, the Belt-Purcell Supergroup has distinct directions compared to co-eval rocks from Laurentia and Greenland, raising questions about which data are correct. To address these questions, we sampled red beds from three stratigraphic units in the lower half of the Belt-Purcell Supergroup: the Appekunny Formation and overlying Grinnell Formation in Glacier National Park, MT, and the correlated Spokane Formation in the Helena Embayment, MT. Ongoing rock and paleomagnetic measurements highlight that almost all samples have a detrital hematite component and an authigenic hematite component with statistically overlapping directions. A magnetite-held remanence often appears detrital; an exception is at Wolf Creek, which was noted as overprinted by Elston et al. (2002). Our paleomagnetic analyses corroborate this prior work, although in some cases we were able to untangle additional detrital components beyond overprints. We have also digitized raw measurements from Elston et al. (2002) to make this data open-access and to allow for a more detailed statistical comparison that will aid in assessment of other archival data. Due to early hematite formation and clear overprinted directions recognized by previous authors, our updated paleomagnetic pole (once finalized) will bolster confidence in use of the Belt-Purcell Supergroup for paleogeographic reconstructions of Laurentia during this time period.

Elston, D. P., Enkin, R. J., Baker, J., & Kisilevsky, D. K. (2002). GSA Bulletin, 114(5), 619-638.