--> Abstract: Ichnofossils in Alluvial Deposits and Paleosols of the Lower Eocene Willwood Formation, Bighorn Basin, Wyoming, by Jon J. Smith, Stephen T. Hasiotis, Mary Kraus, and Daniel Woody; #90039 (2005)

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Ichnofossils in Alluvial Deposits and Paleosols of the Lower Eocene Willwood Formation, Bighorn Basin, Wyoming

Jon J. Smith1, Stephen T. Hasiotis1, Mary Kraus2, and Daniel Woody2
1 University of Kansas, Lawrence, KS
2 University of Colorado, Boulder, CO

The lower Eocene Willwood Formation is composed of sandstone and mudstone interpreted as channel, avulsion, and distal- and proximal-overbank alluvial deposits, all modified by varying degrees of pedogenesis. These deposits contain a diverse and abundant record of plant, invertebrate, vertebrate ichnofossils. The ichnofossils indicate degree of pedogenesis, hidden biodiversity, paleohydrologic regime, in-situ paleoecology, and paleoclimatic setting of weakly to strongly developed paleosols and unmodified deposits.

Plant trace fossils, or rhizoliths, are found in pedogenically weakly to strongly modified deposits, but are most abundant in moderately to strongly developed paleosols. Rhizoliths include tree trunk and root steinkerns, sand- and mud-filled tubular structures, powdery to nodular rhizocretions, and redoximorphic-mottled zones; some containing carbonaceous cores.

Invertebrate ichnofossils, the most abundant animal traces, are found in unmodified deposits and in pedogenically weakly to strongly modified deposits. Ichnofossils include adhesive meniscate burrows (AMB), cf. Celliforma, Coprinisphaera and other dung beetle traces, cf. Cylindrichum, Edaphichnium lumbricatum, J- and U-shaped burrows, subvertical burrows, cf. Macanopsis, cf. Parowanichnus, cf. Planolites, Scaphichnium hamatum, Steinichnus, cf. Termitichnus and other termite traces, wasp cocoons, and various small- to large-diameter subhorizontal burrows.

Vertebrates have the least diverse record of ichnofossils in the Willwood Formation, and are found in pedogenically weakly to moderately modified deposits. Ichnofossils interpreted to have been constructed by vertebrates include Ichnogyrus, cf. Daimonelix, and tracks likely constructed by Coryphodon.

Channel and avulsion-belt paleoenvironments show the least pedogenic modification but contain a high ichnodiversity. Distal floodplain paleoenvironments show the greatest amount of pedogenesis but contain a low ichnodiversity.

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