MACROFLORAL ASSEMBLAGES OF THE CLOVERLY FORMATION, BIGHORN BASIN, WYOMING

Macrofloral assemblages recovered near the boundary between the Morrison Formation and the overlying Cloverly Formation are scarce in Wyoming and Montana. However, identifying such floras is critical to accurate determination of the age of the deposits, and might prove helpful for determining the stratigraphic placement of the boundary between them. In addition, documentation of plant paleoecology of this region during the Late Jurassic and Early Cretaceous provides a baseline against which to evaluate changes in terrestrial floras in response to the radiation of flowering plants (angiosperms) at this paleolatitude.

During recent fieldwork I located and collected three macrofloral deposits from the Cloverly Formation in the northern Bighorn Basin, Wyoming. I placed these deposits into the existing stratigraphic frameworks for the formation in this area (Ostrom, 1970; Kvale, 1986). The lowest deposit is in a grey mudstone and siltstone. This deposit has the lowest diversity of plants, with only four species recovered so far. The middle macrofloral deposit is in a clay and siltstone channel-levee system within the next highest unit in the Cloverly Fm. The deposit has a high abundance of plants specimens, with a much larger diversity of plant groups then the underlying deposit. Dominated by taxodiaceous conifers, multiple specimens of Gingko also have been recovered, but no angiosperms to date. The highest deposit is in a large sand channel complex near the top of the Cloverly Fm. The plant fossils are found in a grey claystone adjacent to and surrounded by the sandstone. This deposit contains the highest diversity of plants, with abundant angiosperms as well as ferns.