ENVIRONMENTAL CONTROLS ON THE EARLY MISSISSIPPIAN (TOURNAISIAN) FAUNA OF THE LODGEPOLE FORMATION, THREE FORKS QUADRANGLE, MONTANA

Gradient analyses of marine fossil assemblages frequently identify water depth as the primary environmental variable correlated with changes in assemblage compositions. Secondary variables such as substrate consistency and wave energy have been identified when examining fossil assemblages from within the same depositional environment. This study examined the highly diverse fossil assemblages of the Lodgepole Formation in southwestern Montana to identify such secondary environmental parameters contributing to assemblage variation. Forty-six field counts were collected within a sequence stratigraphic framework from the fossiliferous deep subtidal, foreshoal, and ooid shoal depositional environments. Lithological data such as carbonate grain composition, sedimentary structures, and bioturbation were also collected to provide lithofacies interpretations and environmental context. Detrended correspondence analysis (DCA) of the complete dataset identifies the expected onshore-offshore gradient along Axis 1 from the highly diverse brachiopod assemblages in the deep subtidal to the rugose coral dominated assemblages in the foreshoal and ooid shoals. The source of variation along Axis 2 is not apparent. Lithology, wave energy, and bioturbation do not appear to be correlated with Axis 2. DCA taxon scores indicate that Platycrinites, Schuchertella, Spirifer centronatus, and fenestrate and trepostome byrozoans plot along the positive values of Axis 2, while Dictyoclostus gallatinensis, Spirifer grimesi, Platycera, and Straparollus plot along the negative values of Axis 2. The ecology of these taxa (i.e., life habit, mode of attachment, and substrate preferences) may help in understanding the source of variation along Axis 2 for both the deep subtidal and foreshoal samples. Although multiple biofacies were identified for each depositional environment, lithologic evidence was not sufficient to identify any environmental gradient beyond water depth for the Lodgepole Formation.