Astronomical climate control on paleosol stacking patterns in the upper Paleocene-lower Eocene Willwood Formation Bighorn Basin Wyoming
article
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. © 2008 The Geological Society of America.
Topics
TNO Identifier
953998
ISSN
00917613
Source
Geology, 36(7), pp. 531-534.
Pages
531-534
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