GEOCHEMICAL ANALYSIS OF THE CENTRAL BASIN FACIES OF THE BEAR GULCH LIMESTONE

The Bear Gulch Limestone (BGL), located in central Montana, United States, is a Late Mississippian konservat lagerstätten. Excellent preservation of hard and soft-bodied fossils provides a glimpse of life in a small marine embayment about 12°N of the paleoequator. The BGL is one of three limestone lenses in the Heath Formation composed of rhythmically alternating thick massive beds and platy fissile beds. Due to the excellent preservation, the BGL lens has been the most extensively studied. Previous research has divided the BGL into four subenvironments: filamentous algal, arborispongia, central basin, and marginal. BGL has been reconstructed as marine bay that experienced restrictive environmental conditions as indicated by the absence or rarity of typical Mississippian organisms such as trilobites, crinoids, ostracodes, corals, and bryozoans. The focus of this study is the central basin facies, which is composed of dark brown/black, millimeter scale fissile beds. Thin sections of 50 successive layers from a 220 cm² area were collected for geochemical analyses. XRD, acid-extracted phases, and analysis in PHREEQC, were performed to determine the salinity, pH, and oxygen levels. The dataset environmental parameters inform constraints that may have restricted the ecological composition of the fauna. In addition to improving the current model of the ecology of the BGL, this research provides a framework for future research on modern analogs for plattenkalk formations. This study was funded by NSF Research Experience for Undergraduates.