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Astronomical climate control on paleosol stacking patterns in the upper Paleocene–lower Eocene Willwood Formation, Bighorn Basin, Wyoming

¹ Faculty of Geosciences, Utrecht University, Budapestlaan 4, P.O. Box 80021, 3508 TA Utrecht, Netherlands
² Palaeoecology, Institute of Environmental Biology, Faculty of Science, Utrecht University, Laboratory of Palaeobotany and Palynology, Budapestlaan 4, 3584 CD Utrecht, Netherlands
³ Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA
⁴ Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA

The Willwood Formation of the Bighorn Basin (Wyoming, USA) is a thick succession of upper Paleocene and lower Eocene fluvial-floodplain sandstones and mudstones. Reddish paleosols, formed on the floodplain mudstones, alternate rhythmically on various scales with heterolithic intervals of small-channel sandstones and mudstones showing weak pedogenesis. Spectral analysis of redness in the Willwood successions at Polecat Bench and Red Butte reveals significant spectral peaks corresponding to cycle thicknesses of ~8 and ~3 m. The ~8 m cycle reflects distinct clusters of 3–5 paleosols. Age constraints show that the period of this cycle closely matches the ~21 k.y. climatic precession cycle. The ~3 m cycle corresponds to individual paleosols, with a period of 7–8 k.y. This period is similar to millennial-scale sub-Milankovitch cycles found in marine and lacustrine successions of Pliocene–Pleistocene age. Precession and millennial-scale climate variations probably affected paleosol development through cyclic changes from predominantly overbank to predominantly channel-avulsion deposition, with the latter periodically halting soil formation because of high sediment accumulation. A new age model was developed for the Paleocene-Eocene carbon isotope excursion (CIE) at Polecat Bench, based on the precessional origin of paleosol clusters. The main body of the CIE spans ~5.5 precession cycles, or ~115 k.y., and the recovery tail of the CIE spans 2 precession cycles, or ~42 k.y. This outcome is consistent with, and independently confirms, recent estimates of CIE duration based on deep-sea cores.

Key Words: paleosols • Milankovitch theory • spectral analysis • Bighorn Basin • avulsion