Alluvial Floodplain and Splay Ichnocoenoses, Eocene Willwood Formation, Bighorn Basin, Wymoing, U.S.A.

Stephen T. Hasiotis¹ and Mary J. Kraus^2
1 University of Kansas, Lawrence, KS
² University of Colorado, Boulder, CO

Distinct suites of ichnofossil communities, or ichnocoenoses, from alluvial deposits in the lower Eocene Willwood Formation, Bighorn Basin, Wyoming, can be defined based on lithologic and pedogenic features and on trace fossil assemblages that represent organisms that co-existed in the same unit. We examined transects across channel-floodplain sequences and concentrated efforts on fine-grained overbank deposits to test whether or not changes in ichnofossil assemblages change coincidently with changes in alluvial deposits and paleosol development. The diversity and distribution of trace fossils was recorded from sandstone bodies and different types of paleosols that formed on floodplain deposits.

Ichnocoenoses indicate particular environmental and physiological factors that controlled the distribution and behavior of terrestrial and freshwater burrowing organisms and rooting plants in channel, heterolithic, and fine-grained floodplain deposits and associated paleosols. Proximity to the channel and avulsion deposition also influenced the distribution of different suites of ichnofossils. Ichnofossil tiering of a soil was controlled mainly by the groundwater profile in a particular part of the landscape. The number and depth of tiers indicates the depths of the soil-water, vadose, and phreatic zones, as well as their seasonal fluctuation. The amount of time the substratum was exposed to soil formation also controlled the amount of tiering and degree of bioturbation. The intensity of ichnofossil tiering was controlled by the groundwater profile and exposure time of sediment to pedogenesis. Epiterraphilic, terraphilic, hygrophilic, and hydrophilic ichnofossil behavioral categories reflect the space inhabited by trace-making organisms in the substrate and relative amount of moisture at that depth.