TRILOBITE BIOFACIES ALONG A CARBONATE MUD-MOUND TO BASIN GRADIENT IN SOUTHWESTERN VIRGINIA (ORDOVICIAN; SANDBIAN)

The Upper Ordovician Effna carbonate mud-mound buildups are exposed in the Valley and Ridge province of the southwestern Virginia Appalachians. The Effna is fringed and overlain by a dark nodular limestone (Botetourt Fm.), and a basinal shale and limestone (Liberty Hall Fm.). A series of trilobite biofacies are defined with non-metric multidimensional scaling (Q and R mode), rarefaction, and ANOSIM analyses of relative abundance data. These data represent 16 collections, comprising thousands of trilobite sclerites, from two localities.

The biohermal mudstones and biostromal grainstones of the Effna are characterized by a high diversity, illaenid-dominated biofacies. However, different buildups may vary in the dominant illaenid genus (e.g., Thaleops or Bumastoides). This biofacies grades into the lower Botetourt, and is eventually superseded by a high diversity nileid-asaphid-raphiophorid biofacies in the upper Botetourt. A low diversity basinal biofacies dominated by Isotelus is confined to the mudstones and shales of the Liberty Hall Formation.