Trace Fossils of the Eocene Green River Lake Basins, Wyoming, Utah, and Colorado

Abstract

Trace fossils produced by insects, annelids, molluscs, birds, mammals, reptiles, amphibians, and fish are diverse in the Green River and roughly correlative formations, and provide clues to interpret sedimentary environments, stratigraphy, and paleoecology of continental organisms during the Early to middle Eocene (~53–43 Ma). The Green River Formation represents an excellent case study to better understand the composition and distribution of trace fossil assemblages in continental lake basins with dynamic tectonic and climatic histories. Bioturbation representing the subaerial exposure and pedogenesis of lacustrine and lake-margin sediments is useful for the recognition of mappable stratigraphic surfaces. The traces are mainly produced by air-breathing organisms in lake-margin terrestrial environments. In shallow water deposits of the freshwater and saline Green River lakes, trace fossil assemblages include simple trails and burrows, and are generally low diversity and have high bioturbation intensities. In saline to hypersaline lakes, moderate to high diversity assemblages are concentrated in areas with freshwater input to lake-margin settings, such as the distal delta plain or shallow water areas lakeward of fluvial input. In hypersaline lake sediments deposited in shallow littoral areas and mudflats, trace fossils are typically associated with subaerial exposure, and include meniscate backfilled burrows that that cross-cut sediments deposited in shallow water. Vertebrate trace fossils are most commonly preserved in mudflat environments of the saline lakes, and include footprints of birds, mammals, and reptiles.