Improving the early Eocene timescale

Improving the early Eocene timescale: High-precision 40Ar/39Ar geochronology and integrated stratigraphy of the Willwood and Tatman Formations in the Bighorn Basin (Wyoming, USA)

Kruitbosch, Edwin

Molengraaff Fonds

2021-10-12

The early Eocene is characterised by transient greenhouse warming events marking perturbations of the global carbon cycle. Terrestrial geochemical proxies facilitate evaluation of these hyperthermal events - having durations in the order of 105 yr - in a high-resolution timeframe. The northern Bighorn Basin has provided such proxies for the Paleocene-Eocene Thermal Maximum up to hyperthermal I2 constrained by basin-wide magnetostratigraphy, an exceptional mammal biostratigraphy and, more recently, floating astrochronological timescales. The astrochronology is based on inferred ~21 kyr precession-control on the fluvial stacking pattern in the Willwood Fm. allowing unprecedented time control. To extend the proxies upward, the central part of the basin has to be sampled. However, age models in the central Bighorn Basin suffer from discrepancies in the age of the Willwood Ash, uncertainties in the magnetostratigraphic record and disagreement with inferred precession forcing. This thesis presents an updated chronostratigraphic framework for the central basin including 1) high-precision 40Ar/39Ar dating of the Willwood Ash (52.53 ± 0.04/0.86 Ma; 2σ internal/external error) and three new, younger tephras SB-3 (52.20 ± 0.04/0.85 Ma), SB-4 (52.08 ± 0.04/0.85 Ma) and SB-5 (51.94 ± 0.09/0.85 Ma); 2) new magnetostratigraphy interpreted as the top of Chron C24 and base of C23 and 3) new detailed stratigraphy and cyclostratigraphy of the Willwood Fm. (cyclicity of 7.8 ± 1.7 m and 4.4 ± 0.7 m period; 1 standard deviation) and overlying Tatman Fm. (6.5 ± 1.3 m). The chronostratigraphic framework suggests that the upper part of the Willwood Fm. is ~0.5 Myr younger than formerly thought. Climate and carbon cycle proxies spanning the Paleocene-Eocene Thermal Maximum to the Early Eocene Climatic Optimum can be obtained from the Bighorn Basin. However, discrepancies with previous age models remain and, consequently, precession forcing in the basin cannot be confirmed.

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Utrecht University

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