SIGNIFICANCE OF DIFFERENT TYPES OF TRACE FOSSILS AND ICHNOCOENOSES TO INTERPRETING PALEOHYDROLOGICAL AND PALEOENVIRONMENTAL SETTINGS OF PALEOSOLS IN THE PALEOGENE WILLWOOD FORMATION, BIGHORN BASIN, WYOMING, U.S.A

Different types of trace fossils and ichnocoenoses provide unparalleled information on the paleohydrological and paleoenvironmental settings of paleosols in the Paleogene Willwood Formation, Bighorn Basin, Wyoming. To understand better the significance of trace fossils and ichnocoenoses with respect to seasonal and annual soil wetness, the morphology, distribution, and occurrence patterns of traces produced by modern soil biota in alluvial environments were compared to those of the Willwood Formation.

In general, sediments that remained beneath the water table were rarely bioturbated and contained abundant primary bedding. Burrows, when present, were mostly small diameter (< 5 mm) and horizontal, and produced by organisms in aquatic settings. Sediments exposed above the water table were bioturbated variably depending on the amount of time exposed prior to burial by sediments or submersion by water due to flooding or shallowing of the water table. The longer the sediments were exposed, the greater the amount and the more penetrative the bioturbation, which included rooting by plants and burrowing or nesting by animals. If the sedimentation was frequent and abundant, then bioturbation was not thorough or penetrative. If the sedimentation was infrequent and negligible, then bioturbation was thorough and penetrative, unless soil-moisture and water-table levels were high.

Adhesive meniscate burrows (AMB) abundant in many fine-grained Willwood paleosols are nearly identical in morphology to traces produced by soil bugs, which produce backfilled burrows in sediment with 7-37% moisture. Red paleosols with abundant AMB, small-diameter subvertical burrows, and rhizoliths imply well-drained sediment. Green or purple paleosols with abundant AMB, small-diameter subvertical burrows, and rhizoliths suggest previous well-drained conditions overprinted by poorly drained and reducing conditions. Green or gray paleosols with few rhizoliths, large-diameter shallow subvertical burrows, and few to rare AMB indicate poorly drained and reducing conditions punctuated by short intervals of better-drained conditions. AMB in better-drained conditions also co-occurs with pellets and pellet-filled burrows produced by earthworms, and small- to large-diameter balls produced by dung beetles.