SILURIAN-DEVONIAN BOUNDARY LOCATION NARROWED AT MUSTOE, VA OUTCROP ON THE BASIS OF STROMATOPOROID GENERA, SPECIES

Stromatoporoids dominate the invertebrate fauna in the upper Silurian-lower Devonian Keyser Formation’s Upper Limestone Member/Jersey Shore Member at an outcrop on County Road 607 near Mustoe, Highland County, Virginia. This slightly folded exposure contains a bioherm capped by a wave-cut surface and two overlying biostrome layers. Above these two units is a laminated and bimodally cross-bedded micritic unit contains nodular stromatoporoids in the lower portion. This micritic unit is recognized as the LaVale Member. These Keyser beds are all overlain by the calcarenitic New Creek limestone. Stromatoporoids were first sampled in 1978 by one of the authors and resampled in 2016 specifically from the two biostrome layers and the fossiliferous portion of the micrite. This was undertaken to identify stromatoporoid genera and species from these units. Preliminary identification of lower biostrome species include 3 specimens of Parallelostroma keyserense, and 1 specimen of Densastroma pexisum. Upper biostrome species includes 1 specimen of Parallelostroma longicolumnum. 2 specimens of Habrastroma centrotum are found in the basal fossiliferous part of the micritic LaVale unit and are considered to be lower Devonian species. On the basis of these tentatively identified stromatoporoids, we suggest that the Silurian – Devonian boundary falls between the bottom of the LaVale micrite layer and within the upper biostrome of the Jersey Shore Member.