PTERIDICHNITES BISERIATUS ABUNDANCE ZONE – 25 YEARS ON

In 1997, geologists from the West Virginia Geological and Economic Survey (WVGES), while performing bedrock mapping in the Devonian strata of Pendleton Co., West Virginia, noticed unusual numbers of the trace fossil, Pteridichnites biseriatus, in the lowermost Brallier Formation. Continued work extended the observed stratigraphic range of this trace fossil from the underlying Mahantango Formation, upwards through the Harrell (or Millboro), Brallier, and Foreknobs formations. However, the high abundance of P. biseriatus appeared to be restricted to a zone one to two meters above the base of the Brallier. In 2007, WVGES geologists proposed a Pteridichnites biseriatus “Abundance Zone” as a stratigraphic marker for the recognition of the Brallier and proximity to its base. Since that time, continued reconnaissance mapping along the eastern border of West Virginia has sought to trace the geographic extent of this biozone and to investigate alternative stratigraphic positions other than the lowermost Brallier. We have been able to recognize or tentatively recognize this “Abundance Zone” along the eastern outcrop belt of the Brallier Formation from Grant and Hardy counties in the north, southwards through Pendleton and Pocahontas counties, to Greenbrier County in the south and westward to Randolph County – a northeast-southwest distance of ~150 Km and an east-west distance of ~75 km. Although structural complexity makes finding continuous stratigraphic sections difficult in eastern West Virginia, the vertical stratigraphic position of the “Abundance Zone” appears consistently near the base of the Brallier. The mappable geographic extent and the consistent stratigraphic position suggest to the authors that formal recognition of this biozone may be warranted. Thus far, no distinctive, visible sedimentary characteristics other than an abundance of a single type of trace fossil have been noted for this biozone. Geochemical samples collected from the biozone may yet yield trends in trace elements or other chemical components of the sediment to help explain the occurrence of this feature. In addition, the extent of this biozone in adjacent states has yet to be investigated.