--> Abstract: Bioturbation Without a Trace: The Loss of Ichnological Diversity from Propedisotropic Processes in Paleosols of the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, U.S.A., by Brian F. Platt and Stephen T. Hasiotis; #90039 (2005)

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Bioturbation Without a Trace: The Loss of Ichnological Diversity from Propedisotropic Processes in Paleosols of the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, U.S.A.

Brian F. Platt and Stephen T. Hasiotis
University of Kansas, Lawrence, KS

The Upper Jurassic Morrison Formation in the northeastern Bighorn Basin contains three lithofacies: a ribbon sandstone lithofacies, a sheet sandstone lithofacies, and a massive mudstone lithofacies. The massive mudstone lithofacies contains various degrees of horizonated red-, green-, and purple-mottled coloration, carbonate nodules, slickensides, and rhizoliths, indicating pedogenic modification of a fine-grained parent material. The massive mudstone lithofacies is divided into weakly to moderately developed (WMD) and well developed (WD) pedofacies based on the relative maturity of the paleosols. WMD paleosols are associated with sheet and ribbon sandstones in heterolithic intervals, interpreted as avulsion deposits. WD paleosols are interpreted as distal floodplain deposits and are found above and below heterolithic intervals.

Animal trace fossils are rare in both pedofacies, but sheet sandstones contain a diverse assemblage of vertebrate and invertebrate ichnofossils. Most of these ichnofossils are preserved in convex hyporelief on the bottoms of the sandstone beds, indicating that they were originally created in the underlying paleosols. Many vertical burrows also penetrate through sheet sandstone beds, but are absent in the mudstones directly below them. These features suggest that soil organisms were present in the paleosols and the lack of animal trace fossils represents a loss of ichnological diversity. Because plant traces are present we attribute the loss of ichnofossil diversity to bioturbation by the organisms themselves (biotic processes) and pedoturbation (restricted to abiotic processes) rather than poor preservation or an absence of organisms in the original environments. These processes are classified as propedisotropic because they act to disrupt soil horizons.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005