PALEOECOLOGICAL IMPLICATIONS OF BRACHIOPOD COMMUNITIES, ARCO HILLS FORMATION (CHESTERIAN), EAST-CENTRAL IDAHO

The Arco Hills Formation of east-central Idaho is Late Mississippian in age and contains abundant well-silicified brachiopods. The formation is less than one hundred meters thick and consists of cyclically-bedded mudstones to grainstones and fine- and coarse-grained siliciclastics. It is time-correlative with the Big Snowy Formation in western Montana which contains similar faunal associations in a paleogeographically shoreward environment. The brachiopods and associated other taxa are found as both in situ communities and biostratinomic accumulations. The fauna and communities bear resemblance to Carboniferous faunas of the Rocky Mountain region which have been considered to be sediment controlled. This connection is not apparent in Idaho.

Four biofacies containing three distinct communities, interpreted to be depth controlled, have been established. Preliminary community analysis reveals low specific diversity Orbiculoidea, Spriferidina/Athyrididae, and Productacea/Davidsoniacea communities. Factors such as quantitative community analysis, functional morphology, and taphonomic features provide the basis for water depth interpretation. Using the biota of the Arco Hills Formation as indicators of paleodepth, a detailed record of significant sea level changes can be compiled for measured stratigraphic sections, especially where lithology alone fails. The cyclicity of facies and brachiopod biofacies in the Arco Hills Formation records the influences of Latest Chesterian Gondwanan glacio-eustatic sea level fluctuations.