Rapid Paleoenvironmental Change During the Paleocene-Eocene Thermal Maximum (PETM), Bighorn Basin, Wy

Mary J. Kraus¹, Daniel Woody¹, Jon Smith², and Stephen Hasiotis^2
1Dept of Geological Sciences, University of Colorado, Boulder, CO
²Department of Geology, University of Kansas, Lawrence, KS

Interbedded alluvial paleosols and avulsion deposits were analyzed through a PETM section. Based on morphologic features, paleosol profiles can be assigned to a particular position along a paleosol continuum that ranges from well drained to poorly drained end members. Using this qualitative approach, paleosols in the main PETM interval developed under drier conditions than pre-PETM paleosols. Quantitative estimates of MAP using a paleosol climofunction confirm this conclusion. The paleosols also show that seasonality increased during the PETM. The stratigraphic distributions of manganiferous rhizoliths and crayfish burrows, both of which are absent or less common in the PETM part of the section, also show lower water tables and increased seasonality during the warming event.

The alluvial architecture also changes in the PETM interval. Avulsion deposits separate successive paleosols from one another through the section. Those in the main body of the carbon isotope excursion (CIE) are finer grained and less well developed than those below or above the PETM interval. Consequently, the avulsion deposits tend to be incorporated into the paleosols, and paleosols are more densely spaced. The thick red paleosols that characterize the PETM interval are the result of the welding of vertically superposed paleosols. Paleosol complexity also changes. Multiple horizons and intense overprinting of pedogenic features dominate paleosols in the main body of the CIE. In contrast, paleosols directly above and below the PETM interval are relatively simple, with few subdivisions of the B horizon(s) and fewer signs of pedogenic overprinting. These features suggest that sediment accumulation was slower and more episodic during the main body of the PETM interval than in deposits directly below and above. The changes in deposition are linked to climatic controls.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas