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

The Arco Hills Formation of east-central Idaho consists of less than one hundred meters of cyclically-bedded mudstones to grainstones and fine- and coarse-grained siliciclastics.  Detailed community analysis within the Late Mississippian Arco Hills Formation is evaluated for the following parameters: generic and species diversity, richness, and dominance.  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 which are interpreted to be depth controlled have been established.  Biofacies A is characterized by dark mudstones containing a single species in situ Orbiculoidea wyomingensis (Inarticulata) community. The lithology gives minimal indication of depositional environment, but the presence of an epifaunal  inarticulate species (and the notable absence of other fauna) implies fouled/anoxic conditions. Oxygen-poor depositional environments are often inhabited by a single species.  Biofacies B is inhabited by Productacea/Davidsoniacea (superfamily) communities which are in situ as indicated by the orientation of fossils and the integrity of delicate skeletal structures (spines).  The environment of Biofacies B was evidently very low energy and is therefore considered to be sub-storm wavebase.  The tempestites of the Arco Hills Formation Biofacies C exhibit a range of allochem types.  In most occurrences there are whole valves, often there are articulated fossils, and in some cases, skeletal debris.  Event concentrations of the Arco Hills Formation may show proximity trends (e. g. decrease in bed thickness, grain size, bioclast- and interclast-content) based on the biostratinomic condition of fossils.  Biofacies D is composed of well-sorted, rarely cross-bedded encrinite packstones and grainstones.  The well-sorted nature of echinoderm ossicles indicates there has been continual reworking by high energy waves.  Brachiopod fossils found in Biofacies D are generally disarticulated, broken, and abraded.  The assemblages are cyclic, showing influences of Gondwanan glacio-eustactic sea level fluctuations.