SEQUENCE STRATIGRAPHIC ARCHITECTURE DRIVES BIOFACIES DISTRIBUTION OF THE MISSISSIPPIAN LODGEPOLE FORMATION IN SOUTH-CENTRAL MONTANA
We counted over 3000 faunal specimens in 93 field counts within the sequence stratigraphic architecture. Cluster analysis of faunal counts for both sections identified four clusters representing deep to shallow water facies. Rugose corals were consistently more abundant in shallow-water facies, whereas brachiopods were more abundant in deeper-water facies. Taxonomic richness within shallow water facies is significantly less than in the deeper water facies. Similar results were established using raw abundance data plotted against stratigraphic position. Detrended correspondence analysis (DCA) revealed that the largest variability was explained by an onshore-offshore gradient, also in agreement with the cluster analysis. DCA revealed no directional change in the position along the onshore-offshore gradient within bedsets representing the same lithofacies across consecutive parasequences, but the influence of the sequence-stratigraphic architecture was visible in the faunal composition at the level of systems tracts. At Milligan Canyon, the transgressive systems tract was characterized by cyclic changes in the onshore-offshore gradient, but no detectable directional change. However, most offshore assemblages, as assigned by DCA, concentrated around the maximum flooding surface. The highstand systems tract was characterized by a rapid shift towards shallower-water communities, visible at the level of individual lithofacies corresponding to the deep subtidal and foreshoal environments.