Abstract: Integrated Fluvial Ichnology, Paleopedology, and Sedimentology of the Fort Union Formation, Wind River Basin, Wyoming

S. T. Hasiotis, R. M. Flores, W. C. Keighin

Paleosol and trace-fossil development is related to the sedimentology and paleohydrology of fluvial channel, overbank-levee, and floodplain deposits. The Paleocene Fort Union Formation in the Wind River basin contains fluvial sandstones, siltstones, and mudstones interbedded with coals and carbonaceous shales that were formed in mires. Vertically stacked fluvial-channel sandstones display modified zones containing rhizoliths and mature paleosol profiles; locally, sandstones were bioturbated by freshwater burrowing crayfish. These features are also well developed in the proximal overbank and levee deposits. Distal floodplain deposits lateral to the fluvial-channel complex also contain carbonaceous, rooted horizons and immature paleosols, but they lack crayfish burrows. The fluvial-chan el complex was deposited by high-energy channelized flow characterized by episodic vertical and lateral accretion. After each episodic depositional event, the water table dropped to pre-overbank level, permitting subaerial pedogenesis and creating a niche partition for primary burrowing crayfish. The vertical extent of the crayfish burrows, which is controlled by the depth of the water table, provides a measure of paleo-water table levels, distribution, and fluctuations.

In contrast, the absence of both crayfish burrows and more mature paleosols in the floodplain strata was caused by slow, continuous vertical accretion of fine-grained sediments under sustained high-water table conditions, as evidenced by accumulation of organic deposits in low-lying mires. Coarsening-upward crevasse-splay mudstones, siltstones, and sandstones deposited by lateral accretion during floods commonly interrupt the floodplain and mire deposits. The splays contain well-developed crayfish burrow horizons, which indicate a return of the water table to lower, preflood levels.

Our study demonstrates that integration of fluvial ichnology, paleopedology, paleohydrology, and sedimentology aids in the detailed and comprehensive interpretation of alluvial architecture and depositional history, as well as providing information on paleo-water table fluctuations.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994