VEGETATIONAL HISTORY AND AGE OF THE GREEN RIVER FORMATION IN FOSSIL BASIN, WYOMING

The preserved plant microfossil assemblages provide an excellent opportunity to study the vegetational history of the Eocene Green River Formation in Fossil Basin, Wyoming. Outcrop samples collected from several measured sections, representing lake margin to lake center facies, produced a variety of palynomorph taxa.

The abundance of hardwood, riparian, and conifer taxa suggests that moist lowlands and floodplains existed around Fossil Lake with upland forests on the surrounding ridges and mountains. The palynoflora suggests that a mixed mesophytic forest grew near Fossil Lake during the early and middle phases of lacustrine sedimentation. The vegetation around the lake changed during the latest phase of lacustrine deposition as the lake became more saline and evaporation rates increased in the basin.

The co-occurrence of Bombacacidites, Eucommiidites, Pistillipollenites mcgregorii, Platycarya platycaryoides, and Momipites triradiatus support a late early Eocene (Ypresian) to early middle Eocene (Lutetian) age for the Green River Formation in Fossil Basin. Consequently, the palynoflora does not rule out the possibility that the late stages of lacustrine deposition may have continued into the early middle Eocene.