A TRACE FOSSIL USEFUL FOR BEDROCK MAPPING - A UNIQUE CONCEPT?

The Upper Devonian siliciclastic strata of eastern West Virginia, western Virginia, western Maryland, and eastern Pennsylvania consist of a series of westward prograding sedimentary deposits. Vertical and lateral contacts between stratigraphic units are gradational or interfingering. Incomplete or limited exposure creates serious challenges for mapping. Similar facies and lithologies are encountered in different stratigraphic units. In particular, the Brallier and the overlying Foreknobs can easily be misidentified when observed in widely scattered outcrops with uncertain stratigraphic context.

We have been dealing with this problem since 1997 when we began a bedrock mapping program in West Virginia under the auspices of STATEMAP. During the first year’s field reconnaissance, we encountered an unusual trace fossil associated with the Upper Devonian marine shales and siltstones. The fossil, Pteridichnites biseriatus Clarke and Swartz, 1913, was first described from the Jennings Formation (now Brallier, Foreknobs, and Hampshire formations) of Maryland and was suggested as a locomotive trace of an annelid or arthropod. Over the past eight field seasons, we have observed P. biseriatus to be most abundant in the lower portion of the Brallier and infrequent throughout the remainder of the Brallier and the Foreknobs. We suggest that the restriction of this trace fossil’s maximum-abundance or acme zone to the Brallier can be used to distinguish between similar strata within the Brallier and the Foreknobs. The presence of abundant P. biseriatus thus provides an easily recognizable stratigraphic marker for differentiating the two formations.