Stratigraphy, fossil distribution, and depositional environments of the upper Bridger Formation (middle Eocene) of southwestern Wyoming, and the taphonomy of an unusual Bridger microfossil assemblage

A stratigraphic and paleontologic investigation of the middle Eocene Bridger Formation determines its areal distribution, origin, tempo and mode of deposition, general depositional environments, and relations with associated Eocene Rocks in the southern Green River Basin, Wyoming. The study focuses on the Twin Buttes and Turtle Bluff Members (upper part) of the Bridger Formation. With a thickness of 428 m in the type area, the upper Bridger is dominated by green and brown mudstones and claystones, but includes scattered ribbon and sheet sandstones, widespread micritic and sparry limestones, and thin but widespread ash-fall tuffs. Fifteen marker units consisting of limestones, tuffs, and tuffaceous sandstones are described, and seven of these are used to subdivide two of the Bridger subdivisions (C and D) into lower, middle, and upper subdivisions. The seven major markers are mapped over approximately 885 km2 of the basin. Rock accumulation rates, radiometric ages of ash-fall tuffs, and fossils indicate that the Bridger Formation was deposited over about 3.2 My from about 48.8 to 45.6 Ma, and that the Bridgerian-Uintan Land Mammal Age boundary occurs at about 46 Ma. Depositional environments include fluvial, lacustrine, playa, paludal, marginal mudflat, alluvial fan, and ash-fall. The stratigraphic and radiometric data provide a detailed framework for positioning fossils, and the stratigraphic distribution of 2,800 mammalian fossils from over 350 localities is examined. The study concludes that an influx of volcaniclastic sediment to the Green River Basin during the middle Eocene led to the demise of Lake Gosiute and the development of muddy floodplains of low topographic relief which persisted for up to 85% of Bridger time. These sediments originated mostly in the Absaroka volcanic field of northwestern Wyoming, and were transported to the basin fluvially, although the mineralogy of some ash-fall tuffs indicates an origin in the Challis volcanic field in central Idaho. Occasional lapses in the flow of sediment to the basin permitted the development of shallow groundwater-fed lakes which accumulated up to four times as slowly as fluvial deposits, and these lapses decreased in frequency throughout deposition of the upper Bridger. As indicated by fossil distribution and diversity, lakes and their margins provided favorable habitats for a large variety of aquatic and terrestrial vertebrates and invertebrates, and lake margins were well vegetated and at least partially forested. Increases in volcaniclastic sediment to the basin are interpreted to have resulted from periodic wetter climatic conditions that appear to correlate with 40,000 and 200,000 year cycles. (Abstract shortened by UMI.)...