THE HUNTERSVILLE CHERT – WEST VIRGINIA’S GEOLOGIC “PROBLEM CHILD”

In 1929, the Devonian Huntersville Chert was proposed as a formal stratigraphic unit by Paul Price, former West Virginia State Geologist. Its age was determined by stratigraphic position – overlying the Lower Devonian Oriskany Sandstone and underlying the Middle Devonian Marcellus Shale – and by its content of “Oriskany age” fossils. Since 2008, we have been mapping the bedrock along the eastern border of WV in proximity to the type locality, the village of Huntersville, WV. Having observed numerous Huntersville outcrops and having conducted petrographic and geochemical investigations of the unit, we have reached conclusions that appear to be at odds with the accepted picture of the Huntersville.

Using stratigraphic position and fossil content to constrain the age of the Huntersville assumes that the generation of the unit follows the rules of depositional superposition as do the units below, above, and laterally adjacent to it. This is a problem because the Huntersville is not a typical sedimentary formation. The contacts between the Huntersville and neighboring units only approximate continuous stratigraphic horizons, not always traceable beyond outcrop. Contacts can best be described as “ragged” or discontinuous with chert sometimes appearing on the “wrong” side of the boundaries of adjacent units. More importantly, silicified relict bedding and fossils from these units (Millboro, Bobs Ridge, Tioga, Needmore, or Oriskany) are often found within what would be called Huntersville.

Some geologists have proposed the chert of the Huntersville is the product of an extensive die-off of siliceous sponges; others have suggested the chert’s silica is dissolved windblown quartz silt. Both of these theories ignore the ubiquitous presence of volcanic ash layers -Tioga eruptive event(s) - associated with the Huntersville. We believe the source of the silica for the Huntersville Chert is dissolution of this ash.

We propose that the Huntersville Chert is primarily of diagenetic origin. It was formed by the silicification of pre-existing sedimentary strata by pore fluids derived from dissolved Devonian volcanic ash. Best described in terms of ore genesis, the Huntersville is a stratiform deposit, probably diachronous over its geographic extent, and possibly significantly younger than Middle Devonian.