Continental Ichnofossils of the Green River and Wasatch Formations, Eocene, Wyoming: A Preliminary Survey, Proposed Relation to Lake-Basin Type, and Application to Integrated Paleo-Environmental Interpretation

摘要

Alluvial and littoral portions of the Eocene Green River and Wasatch Formations in Wyoming contain a wide variety of trace fossils that record primarily the activity of air-breathing insects and vertebrates that respond to changes in lake level and groundwater table. Each member of the Eocene Green River and Wasatch Formations studied has a characteristic association of ichnofossils- the type, abundance, and diversity of ichnofossils vary in systematic ways that appear to be associated with lake-basin type. The Luman and. Niland Tongues and associated Wasatch Formation represent an overfilled lake basin, containing mostly epifaunal insect and tetrapod tracks and trails and few infaunal insect burrows in shoreline or lake-plain strata. Traces found here include Haplotichnus, Steinichnus, Fuersichnus, and adhesive meniscate burrows. These observations suggest persistently high water-table levels that accords with the co-occurrence of histosols, coals, and freshwater flora and fauna. The lower LaClede Bed, Green River Formation and coeval Wasatch Formation represent a balanced-fill lake basin, containing abundant traces attributable to insect activity in shoreline, floodplain, and mean-clering-fluvial strata. Ichnofossils found here include traces attributed to termite, ant, bee, caddisfly, and beetle nests and burrows, as well as others. Some meandering channel units up to 2.5-m thick contain extensive "ant nests", indicating complete withdrawal of surface and groundwater beyond the penetration of the "ant nests". Vertisols developed on floodplain strata contain traces attributed to bee nests and beetle burrows as well as tetrapod tracks, which also record fluctuations of the groundwater table. The Wilkins Peak Member, Green River Formation and Cathedral Bluffs Tongue, Wasatch Formation represent an underfilled lake basin containing abundant traces in sheet-flood deposits up to 3 m thick. Traces include those attributed to termite and beetles nests and burrows, as well as Maconopsis and Eatonichnus, indicating substantial drawdown of groundwater after deposition of the sheet-flood deposits. Bird and mammal tracks on multiple bedding planes demonstrate that multiple events deposited 30-70 cm thick bedsets with intervening water-level falls. Ichnofossil assemblages suggest soil moisture of 10-30% and minimum annual temperatures of > 4° C. Shoreline strata contain only such epifaunal trails as Scolicia and Cocblichnus, with other indices of saline to alkaline waters. Continental trace fossils provide valuable insights into details of depositional conditions of lake-margin, lake plain, fluvial, and floodplain paleoenvironments that commonly challenge conventional analyses. Ichnofossil analysis is powerful especially when combined with broader scale observations of stratal geometries, geochemistry, and lithologies in a lake-basin-type framework to provide more integrated pale-oenvironmental reconstructions that include groundwater and lake hydrology.