SEQUENCE STRATIGRAPHIC USE OF TRACE FOSSILS FOR THE RECOGNITION OF PROGRADATIONAL SILICICLASTICS INTO CARBONATE-DOMINATED, UNDERFILLED LAKE GOSIUTE, EOCENE GREEN RIVER FORMATION, WYOMING

Trace fossils are useful for the sequence-stratigraphic interpretation of lake-basin successions, particularly in underfilled basins where lake-levels fluctuate dramatically in response to changes in climate, and sediment and water input. In the Wilkins Peak Member of the Eocene Green River Formation, Wyoming, multiple siliciclastic-dominated intervals representing ~100 k.y. climatic cycles intercalate with carbonate- and evaporite-rich lake deposits in the central Bridger subbasin. The trace fossils help to recognize subaerially exposed (e.g., vertebrate footprints) and very shallow water (e.g., Helminthoidichnites) lowstand lacustrine deposits below the clastic intervals. They are overlain by multiple progradational parasequences, which include climbing rippled sandstones and hypopycnal marls deposited in muddy, lake marginal areas. The lithofacies and trace fossil assemblages are closely associated throughout the intervals. Trace fossils within thin, U- and Th-rich carbonate beds intercalated within these parasequences record freshwater input, in contrast to normal lake conditions. Parasequences toward the tops of the clastic intervals display terrestrial burrow types (e.g., Taenidium, insect nests), and suggest increased water table depths and stability of terrestrial deposits. Each progradational sequence is overlain by a sharp flooding surface below lacustrine shales in higher topographical areas, or gradationally overlain by rippled lacustrine deposits in flooded channels.

In addition to changes that occur within individual vertical facies successions, trace fossil assemblages also exhibit predictable changes with respect to updip-downdip position within the basin. Overprinting of trace fossils produced under different conditions is rarely observed in basin-center deposits, showing that it was a high accommodation potential setting with abundant sediment supply. The development of the clastic intervals is possibly due to increased precipitation and transport of sediment to the otherwise solute-dominated lake. However, closer to the basin margins, burrows produced by air-breathing terrestrial organisms directly overprint lacustrine deposits where sediment supply was limited or bypass occurred during periods of lake-level drop.