Paleolimnology and taphonomy of some fish deposits in “Fossil” and “Uinta” Lakes of the Eocene Green River Formation, Utah and Wyoming

Abstract

In four sections in the lacustrine Eocene Green River Formation in Utah (“Lake Uinta”), fish are preserved on carbonate mudflats, along high-energy carbonate shorelines, in the littoral zone, and in deeper profundal areas. Scattered bones and scales are found throughout, dropped from disintegrating floating carcasses. Gar scales typify shallow-water deposits, partly due to habitat preference. Beach concentrations of greatly abraded bones and scales grade offshore into storm lag layers and lenses, which grade to loosely packed horizons or patches of unworn remains. Littoral fish fossils range from dropped bones to good skeletons, with much floating, lifting, and scavenging. Preservation improves away from the waterline, not only into deep and cold profundal waters (where the carcasses are protected from scavenging by anoxia and prevented from floating by temperature and pressure), but also onto mudflats, where stranding and burial of carcasses can also protect them. A section through an isolated part of the Green River Formation in Wyoming (“Fossil Lake”) also ranged from profundal to littoral with minor emergence. Dropped body parts were found throughout, but preservation quality of the best specimens improved with presumed depth. The beds with the best specimens seem to have accumulated at the bottom of a deep, stratified, warm-monomictic lake. Our Lake Uinta sections were generally shallower and suggest more fluctuations in water level, but this is not relevant to broad paleolimnological interpretations because they represent marginal zones, which are necessarily more sensitive to changes in depth. Overall, fish fossils are especially informative in deeper waters, where sedimentary distinctions fail.