Magnetostratigraphy of the Eocene Green River Formation, Wyoming

Abstract:

The Green River Formation (GRF) is one of the most well-preserved continuous Eocene terrestrial records in the world, allowing researchers to track phenomena in high resolution related to climate, vegetation, tectonics, and geomorphology. The preservation of the early Eocene is of particular interest as it spans the Early Eocene Climatic Optimum (EECO), an analog for current greenhouse gas-driven global warming. Geochronologic constraints in the section are under development, limiting the temporal resolution of ongoing research, especially studies of climate cycles. Magnetostratigraphy of the GRF is challenging due to the pervasive growth of authigenic pyrrhotite in sediments of the Wilkins Peak member (Sheriff & Shive, 1982). In this study, we build on the work of Tsukui and Clyde (2012) by collecting paleomagnetic data from ash-fall tuffs, which are more resistant to authigenic sulfide growth and can be dated directly using U/Pb and ⁴⁰Ar/³⁹Ar geochronologic techniques. The tuffs were deposited in closed-lake basin settings in southwestern Wyoming and are sufficient in number to refine the stratigraphic position of geomagnetic reversals. Geomagnetic polarity was determined using alternating field and thermal demagnetization protocols and we can now infer the position of the five geomagnetic reversals encompassed by chrons 23R and 23N. The samples’ magnetic mineral carriers are magnetite, hematite, and their titanium-substituted equivalents and were characterized using hysteresis loops, backfield curves, and magnetic susceptibility. Fe-sulfides are present in some samples, and repeated susceptibility measurements show that secondary magnetic minerals form during thermal demagnetization steps >450°C, but most samples generally show minimal post-depositional alteration and still retain reliable paleomagnetic information. The timing of the observed geomagnetic reversals in the GRF will be used to further refine the geochronology of the section, which will in turn provide a framework for ongoing research into Eocene cyclostratigraphy and ultimately enable correlation with the marine record as part of a larger multi-institution initiative.