STRATIGRAPHIC SIGNIFICANCE OF A BENTONITE UNIT IN THE UPPER JURASSIC MORRISON FORMATION, NORTHEASTERN BIGHORN BASIN, WYOMING

In exposures across the western United States, the Morrison Formation documents varied paleoenvironments, associated fauna, and their evolution throughout the Upper Jurassic. While the Morrison has been well-explored, there remain gaps in knowledge about the uppermost extent of the formation. In the Bighorn Basin of northwestern Wyoming, a disconformity exists between the uppermost Morrison and the overlying Cloverly, leaving an incomplete section. In addition, limited exposure of the surviving upper Morrison beds has hindered exploration of the upper Morrison in this basin.

The Morrison Formation exposed in the Indian Pass 7.5 min Quadrangle of the northeastern Bighorn Basin is representative of the paleoenvironmental conditions in this area during the Upper Jurassic. The site encompasses wide areal exposures of the lower and upper Morrison. Bentonite beds are predominant in the upper Morrison at this site, with the only other lithologies in this interval consisting of thin, rare, and discontinuous beds of shales and sandstones. One laterally-continuous horizon within this bentonitic interval contains stromatolites, while a stratigraphically-higher horizon contains fragmented vertebrate fossils.

Ongoing chemostratigraphic (XRF and LIBS) and lithostratigraphic research indicates that this bentonitic section represents a localized subunit of the Morrison, either positioned within the upper Brushy Basin Member or directly overlying it. The absence of this subunit at other sections of the Morrison within the Bighorn Basin may indicate that the bentonite unit is positioned within the interval usually removed by the Morrison-Cloverly disconformity. The abundance of thick bentonite beds (more than forty meters of stratigraphic thickness), indicates high rates of volcanism along the western margin of the North American plate during the latest Jurassic. Bentonite thickness distribution in this subunit may also correlate to syndepositional topographic lows, such as filling of lakes with volcanic ash. Peaks in the concentration of calcium and magnesium within a stromatolitic horizon are indicative of a saline lake setting, with a decline in these elements suggesting a decrease of lake size or salinity variation during the subunit’s deposition.