THE BIRDS AND THE BEES: INSECT AND VERTEBRATE ICHNOFOSSIL EVIDENCE FOR FLUCTUATING GROUNDWATER TABLE AND LAKE LEVEL, GREEN RIVER AND WASATCH FORMATIONS, EOCENE, WYOMING

Continental ichnofossils record aspects of air-breathing animal behavior that respond to changes in lake level and groundwater table. Details of continental depositional settings, such as water-table levels, soil moisture, soil consistency, organic matter, and minimum annual temperature, are interpreted by analogy with extant insects and vertebrates. Integrating ichnofossil assemblages with larger-scale stratal patterns reveals hydrologic history, supporting analyses of lake level variations using a lake-basin type framework.

The Wilkins Peak Member, Green River Formation and Cathedral Bluffs Tongue, Wasatch Formation (underfilled lake basin), contain abundant insect traces (termites, beetles, Maconopsis, Eatonichnus, etc.) in sheetflood deposits up to 3 m thick, indicating substantial drawdown of groundwater after deposition. Bird and tetrapod tracks on multiple bedding planes demonstrate that multiple events deposited 30-70 cm thick bedsets with intervening water-level fall. Burrow assemblages suggest soil moisture of 10-30% and minimum annual temperatures of >4°C. Shoreline strata contain only epifaunal trails (Scolicia, Cochlichnus), with other indices of saline to alkaline waters.

The lower LaClede Bed, Green River Formation and coeval Wasatch Formation (balanced-fill lake basin) contain abundant insect traces in shoreline, floodplain, and meandering-fluvial strata (termite, ant, bee, caddisfly, and beetle nests and burrows, etc.). Some meandering channel units up to 2.5-m thick contain extensive ant nests, indicating complete withdrawal of groundwater. Vertisols with associated floodplain strata contain bee and beetle burrows and tetrapod tracks which also record fluctuations of the groundwater table.

The Luman and Niland Tongues and associated Wasatch Formation (overfilled lake basin) contain mostly epifaunal insect and tetrapod tracks and trails and few infaunal insect burrows in shoreline or lake-plain strata (Haplotichnus, Steinichnus, Fuersichnus, adhesive meniscate burrows). These observations suggest persistently high water levels that accords with the co-occurrence of histosols, coals, and freshwater flora and fauna.