USING FOSSILS TO AID IN BEDROCK MAPPING: EXAMPLES FROM WEST VIRGINIA

Bedrock mappers in eastern West Virginia contend with outcrops that are typically small, discontinuous, and poorly exposed. Add intense structural deformation, sedimentary facies changes, and superficially similar lithologies, and the field geologist is left grasping for any and every possible clue to help differentiate and identify stratigraphic units. In the 1800s and early 1900s, geologists mapping in the state used fossils primarily to identify the time period to which each rock unit belonged. Unfortunately, early workers did not distinguish index from facies fossils, confounding the time information implied by the former with the depositional/sedimentological information inferred from the latter.

For the purposes of “modern-day” mapping, we concentrate primarily on facies fossils and treat their presence (or absence) as an indicator of a particular set of environmental conditions during the deposition of individual stratigraphic units. In general, no time constraints are inferred – our purpose is to use the fossil content as an adjunct lithologic property to help us differentiate and identify rock units. To this end, we have had success mapping Paleozoic siliciclastic and carbonate rock units using both body fossils and ichnofossils. The presence or absence of individual fossils or assemblages of fossils can be equally important, as can the numbers and diversity of fossils. For example, the shales and minor sandstones of the Devonian Brallier Formation are typically devoid of body fossils but contain a unique, easily recognizable trace fossil; similar strata of the overlying Devonian Foreknobs Formation are typically highly fossiliferous and contain a diverse and prolific assemblage of trace fossils; similar strata of overlying Devonian Hampshire Formation are typically barren except for rare plant fossils. Treating fossil content as a critical component of lithology when mapping is a technique applicable to sedimentary rocks ranging in age from latest Proterozoic through Cenozoic.