WHEN IS A MOUND NOT A MOUND? A CASE STUDY IN THE MISSISSIPPIAN LODGEPOLE LIMESTONE FROM WEST-CENTRAL MONTANA

During the Mississippian, the Lodgepole Limestone of the Madison Group was deposited in a large marine embayment in central Montana. Over the years, ongoing discussion of local buildup features within the Madison Group has led to many interpretations from different sites, including Waulsortian mounds, reefs, and carbonate buildups.

We visited one of these structures in the Little Belt Mountains of west-central Montana, approximately 16 miles west of the town of Utica. This site, referred to as “Elk Creek”, has been reported as a reef like buildup with adjacent fore-reef and back-reef deposits. Previous workers interpreted a prominent, massive, “mounded” outcrop along the north side of Elk Creek as the reef, smaller well-bedded outcrops west of the “mound” as the back-reef, and small outcrops to the east of the buildup as the fore-reef. However, we found the topographic buildup to be nothing more than modern, erosional, topographic relief on a hillside outcrop and not a past reef buildup or other topographic feature.

Several measured sections from west to east along the outcrop extent revealed that beds throughout the Elk Creek site strike approximately east-west and consistently dip gently (less than ten degrees) to the north-northeast. We traced several marker beds that clearly follow this dip and do not dip away (west) from the main buildup feature as observed by other workers. Several sedimentary structures, including soft sediment deformation, indicate movement towards the buildup, which would be upslope according to past interpretations. Furthermore, conodont residues from dissolved limestone yielded many euhedral garnet crystals, particularly in the buildup feature itself. Transport of abundant garnet crystals upslope is unlikely, and therefore the buildup must represent downslope deposition. These garnet crystals originated from Proterozoic garnet gneiss that forms the basement in the area and was partially exposed in nearby Proterozoic highs during the Mississippian.

Overall, we suggest, instead of being a reef, buildup, or mound, that the Elk Creek site is a succession of layered to massive, mainly crinoidal limestone. Diversity changes within this succession are due to different limestone facies that define several smaller scale cycles.