COMPARATIVE PALEOECOLOGY OF A DISTAL CARBONATE RAMP TO BASIN: EARLY MISSISSIPPIAN LODGEPOLE FORMATION, MONTANA

The Early Mississippian Lodgepole Formation of western Montana was deposited east of the Antler orogenic belt. Well-preserved fossils are distributed throughout the formation, facilitating the study of potential facies-related faunal distributions of marine invertebrates along a distal carbonate ramp to basin transition.

Species diversity and abundance data were collected from 18 stratigraphic horizons distributed throughout the Lodgepole with the intent of identifying faunal associations. Nearly 1000 specimens representing over 40 species of brachiopods, bryozoans, corals, crinoids, and gastropods were analyzed from over 60 bedding surfaces. Two-way cluster analysis and non-metric multidimensional scaling were employed to identifiy three primary faunal associations within the formation.

The three Lodgepole faunal associations exhibit strong temporal control, but are also consistent with bathymetric changes and associated facies variability. The lower faunal association is dominated by bryozoans and crinoids and is associated with wackestone and packstone facies deposited on and near outer ramp mud mounds. The middle faunal association is brachiopod-dominated and preserved in thin-bedded laminated mudstones deposited on the distal slope to basin (distal turbidites). The upper faunal association is dominated by spirifers and corals, including large colonial rugosans, and found in thicker bedded mudstones and wackestones that represent proximal turbidites. Rarefied diversity curves indicate that proximal turbidites record the highest species richness, and that the outer ramp mud mounds preserve the lowest; however, the latter environment may be undersampled.

Analysis of species distributions through the Lodgepole demonstrate that first occurrences are concentrated immediately above flooding surfaces (consistent with Holland's models). This pattern of first occurrences appears to influence the paleoecological patterns observed in the data: temporally similar samples group together, sometimes from disparate facies. However, reanalysis of the dataset, including only those species that range through the formation, reveals the same general distribution of taxonomic groups and faunal associations.