Magnetostratigraphy across the Wasatchian/Bridgerian NALMA Boundary (Early to Middle Eocene) in the Western Green River Basin, Wyoming
Citation
Clyde, William C, Zonneveld, John-Paul, Stamatakos, John, Gunnell, Gregg F, and Bartels, William S, Magnetostratigraphy across the Wasatchian/Bridgerian NALMA Boundary (Early to Middle Eocene) in the Western Green River Basin, Wyoming: .
Summary
New paleomagnetic and biostratigraphic data from the western Green River Basin of Wyoming provide a magnetobio-stratigraphic framework to correlate the late Wasatchian through early Bridgerian North American Land Mammal Ages (NALMA) to the Geomagnetic Polarity Time Scale (GPTS). Three-hundred and twelve paleomagnetic samples were collected from 73 sites within a 350-m continuously exposed flat-lying section. The section is characterized by alternating packages of well-developed paleosol mudstone horizons (Wasatch Formation) and lacustrine shale/limestone horizons (Green River Formation). Fossil mammal assemblages are known from various levels within the section and constrain the location of the Wasatchian/Bridgerian NALMA boundary [...]
Summary
New paleomagnetic and biostratigraphic data from the western Green River Basin of Wyoming provide a magnetobio-stratigraphic framework to correlate the late Wasatchian through early Bridgerian North American Land Mammal Ages (NALMA) to the Geomagnetic Polarity Time Scale (GPTS). Three-hundred and twelve paleomagnetic samples were collected from 73 sites within a 350-m continuously exposed flat-lying section. The section is characterized by alternating packages of well-developed paleosol mudstone horizons (Wasatch Formation) and lacustrine shale/limestone horizons (Green River Formation). Fossil mammal assemblages are known from various levels within the section and constrain the location of the Wasatchian/Bridgerian NALMA boundary to lie between 60 and 120 m. Paleomagnetic results of paleosol horizons are generally characterized by two components, a present-day field component with low unblocking temperatures up to 400�C, and a characteristic component with unblocking temperatures up to 680�C. Characteristic magnetizations have either N-NW and steeply down directions or antipodal S-SE and up directions. The characteristic component is interpreted as primary based on antipodal polarity directions and congruence with the expected Eocene direction for western Wyoming. Results indicate that the first 30 m of section are characterized by normal polarity, the next 230 m by dominantly reversed polarity, followed by a 70 m interval of alternating polarity. Given radiometric ages for the late Wasatchian (early Lostcabinian "subage") and middle Bridgerian, as well as previous Wasatchian and Bridgerian magnetostratigraphic results, two potential correlations to the GPTS are possible. Correlation 1 places the Wasatchian/Bridgerian NALMA boundary within Chron C23r at about 52 Ma. Correlation 2 places the boundary within Chron C22r at about 50.5 Ma. Correlation 2 is tentatively preferred as it agrees most closely with previous estimates for the age of the boundary and assumes more uniform sediment accumulation rates.
Published in Journal of Geology, volume 105, issue 6, on pages 657 - 669, in 1997.