Palaeoenvironments associated with caddisfly-dominated microbial-carbonate mounds from the Tipton Shale Member of the Green River Formation: Eocene Lake Gosiute

Caddisfly-dominated microbial-carbonate mounds and avian eggshell fragments are common in a nearshore, oolite facies of the Tipton Shale Member of the Eocene Green River Formation. The fossils occur in a 9 m thick carbonate sequence exposed on the south-west flank of Essex Mountain, Sweetwater County, Wyoming. The eggshell was determined to be of avian origin by examination of the radial eggshell microstructure by scanning electron microscopy and polarized light microscopy. Common allochems in the limestone include: ooids, pisoids, oncoids, ostracods, gastropods, intraclasts, caddisfly larval/pupal cases, fish bones, avian bones and avian eggshell fragments. Carbonate mineralogy varies between 95% calcite and 95% dolomite. Oxygen stable isotopes vary between -10·1 and -1·3[per thousand] (V-PDB). Carbon and oxygen stable isotopes co-vary. Co-variance of oxygen and carbon stable isotopes indicates that Lake Gosiute was hydrologically closed during the formation of this oolite sequence. Positive excursions of oxygen stable isotope values are correlated with dolomite, caddisfly cases and avian eggshell fragments. These factors are probably associated with lake regression and more saline conditions. Negative excursions of oxygen stable isotope values are correlated with calcite, gastropods and intraclasts. These factors are probably associated with lake transgression and lake freshening. In the 9 m oolite sequence studied there are at least two inferred transgressions, three regressions and two storm events. The caddisfly cases and avian eggshell fragments occur in sediments formed only during low-stands of the lake as determined by other lake level proxies (dolomites and more positive oxygen stable isotope values). As avian eggshell fragments were formed under subaerial conditions, the avian eggshell fragments independently confirm lake regression. In addition, the occurrence of avian eggshell fragments during saline phases of the lake may indicate that the birds which produced the eggshell preferred saline conditions (or that their food preferred saline lake water).