PEDOGENIC OVERPRINTING OF CRAYFISH-BURROWED ALLUVIAL DEPOSITS IN THE PALEOGENE WILLWOOD FORMATION, BIGHORN BASIN, WYOMING

Pedogenically overprinted fossil burrows of freshwater crayfish (Decapoda: Cambaridae) occur in a sequence of paleosol couplets in the Paleogene Willwood Formation of the Bighorn Basin, Wyoming. The burrows are vertical to sub-vertical, 2 to 5 cm in diameter and circular in cross-section. Burrow segments up to 12 cm in length were collected, though original burrow lengths were probably much greater. Although several burrows show transverse scrape marks, and knobby and hummocky textures characteristic of crayfish burrows, the surficial morphology on most specimens is obscured by later pedogenic modification. Based on their surficial morphology, simple burrow architecture, and relatively shallow depths, these burrows are assigned to the ichnospecies Camborygma litonomos.

The crayfish burrows occur in a series of red and gray paleosol couplets, representing stacked A/B (red) and C (gray) horizons, but are most abundant in the red mudstone intervals. The red paleosols show columnar to prismatic soil structure, probably due to the abundant and closely spaced C. litonomos burrows in these units. The red paleosols also contain vertical carbonate rhizoliths with gray-green rhizohaloes, weakly developed slickensides, common adhesive meniscate burrows and small sand-filled burrows, all of which cross-cut and obscure the C. litonomos burrows. Gray units are composed of very fine-grained sandstone and contain faint yellow-brown mottles, fine powdery carbonate rhizoliths. C. litonomos burrows are less common in gray units indicating that burrow penetration was mostly restricted to the red portion of the couplet profiles.

Crayfish burrowing activity most likely occurred during and shortly after avulsion events while water tables were high and less well-drained soil conditions were present. As avulsive sedimentation subsided, drainage conditions improved and water tables fell promoting further pedogenic modification. The development of A/B horizons overprinted the original crayfish ichnofabric but was not vigorous enough to obliterate it, resulting in the columnar to prismatic soil structure and remnant C. litonomos throughout.