Caddisfly-dominated microbial carbonate mounds from the Green River Formation

The geology and paleontology of the Eocene Green River Formation of Wyoming, Colorado and Utah has been intensely studied for over a hundred years. In the course of these studies terrestrial microbial carbonates such as stromatolites, oncolites, tufas and travertines have been commonly reported in lake margin paleoenvironments. This dissertation focuses on two Eocene Lake Gosiute occurrences of lacustrine microbial-carbonates that are primarily composed of thousands of calcified caddisfly (Insecta: Trichoptera) pupal cases. The caddisfly-dominated microbial carbonates formed mound and column shaped structures that have no modern lacustrine analogues. A single ancient analogue is known from French Oligo-Miocene Lakes. These Green River Formation caddisfly-dominated microbial carbonates have been unrecognized or misinterpreted by previous paleontologic studies.; The first example of these caddisfly-dominated microbial carbonates occurs at the base of the Laney Member (or the top of the Wilkins Peak Member) of the Green River Formation in the Greater Green River basin. Individual mounds and columns are commonly up to 1.5 m tall, however, bioherms composed of stratigraphic accumulations of caddisfly-dominated mounds and columns are up to 9 m thick and 40 m in diameter and can be traced for 250 km across the western, northern and eastern margins of Lake Gosiute. The lacustrine caddisfly-dominated mounds extend into and interfinger with fluvial/floodplain sediments of the Eocene Wasatch Formation.; Extensions of the caddisfly-dominated mounds into Wasatch Formation sediments are interpreted as long-lived lacustrine transgressions across Wasatch Formation floodplains. Relative stasis of the oxygen stable isotope trend during these transgressive excursions of Lake Gosiute indicates tectonic control over the spatial distribution of Lake Gosiute water.; The second example of caddisfly-dominated microbial carbonate mound occurs near the top of the Tipton Shale Member of the Green River Formation. These mounds are much smaller (up to 30 cm thick), and do not form extensive reef-like biohermal deposits. A multiproxy analysis of lake level changes associated with the formation of the Tipton Shale mounds shows that the mounds formed in a hydrologically closed lake that experienced frequent water level changes. Associated avian eggshell fragments are "extraclasts" that independently confirm lake level regression at the study site.