Updated Magnetostratigraphy for The Eocene Green River Formation, Wyoming

Abstract

The Green River Formation (GRF) is one of the best-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 in the GRF is particularly important as it records the Early Eocene Climatic Optimum (EECO), an analog for current greenhouse gas-driven global warming. Here we provide an updated magnetostratigraphy for the Wilkins Peak Member (WPM) of the GRF integrated with recent 238U-206Pb and 40Ar/39Ar results that more confidently identifies the geomagnetic reversals preserved in the sediments (C22n, C22r, C23n.1n, C23n1.1r, C23n.2n, and C23r) and refines their radioisotopic ages. Earlier GRF magnetostratigraphic studies were challenged by the presence of pervasive authigenic pyrrhotite in sediments of the Wilkins Peak member (Sheriff and Shive, 1982), which are confirmed in non-tuff lithologies in this study. Here, we build on the work of Tsukui and Clyde (2012) by focusing paleomagnetic sampling on ash-fall tuffs, which are more resistant to the formation of authigenic sulfides and can be dated directly using 238U-206Pb and 40Ar/39Ar techniques. The tuffs were deposited in a closed-lake basin setting and are sufficient in number to refine the stratigraphic position of geomagnetic reversals. Most tuffs show minimal post-depositional alteration and act as reliable paleomagnetic recorders. Tuffs of both normal and reversed polarity were identified using alternating field and thermal demagnetization protocols. The magnetic mineral carriers are magnetite, hematite, and their Ti-substituted equivalents and were characterized using hysteresis loops, backfield curves, and magnetic susceptibility. Fe- sulfides are present in some samples and produce secondary magnetic minerals during thermal demagnetization at temperatures >450°C. This updated terrestrial magnetostratigraphy provides an important bridge for correlating to marine records deposited across the EECO, and ongoing cyclostratigraphic work promises an even higher resolution view of the natural history preserved within the GRF.