TRACE FOSSILS IN THE TIOGA ASH HELP DECIPHER CHANGING PALEOENVIRONMENTAL CONDITIONS IN THE MIDDLE DEVONIAN OF WEST VIRGINIA

The Middle Devonian (mid-Eifelian) Tioga Ash occurs throughout the central Appalachian Basin, both in outcrop and subsurface. In eastern West Virginia, it forms a marker horizon comprising multiple, water-lain ashfall episodes spaced closely in time. The Tioga interfingers with top of the Needmore Shale, a dark grey, organic-rich, marine unit, and is overlain by the Millboro (≈ “Marcellus”) Shale, a black, organic-rich, marine unit. The Needmore contains a moderately abundant, normal marine fauna of articulate brachiopods and trilobites with diagnostic, intensely bioturbated horizons dominated by Chondrites sp. The Millboro, noted for its depauperate fauna, typically contains miniature adult brachiopods, styliolines, and cephalopods; it is nearly devoid of trace fossils. These two shales clearly represent major differences in environmental conditions.

The Tioga falls (literally) between the Needmore and Millboro and should provide insight into the nature of the transition between the shales. Unfortunately, ashfalls are not typically known for abundant paleoenvironmental indicators. At the Cove Hill Quarry in eastern Pocahontas Co., WV, the Tioga also interfingers with a glauconitic sandstone, the Bob’s Ridge. Interbeds of the two formations are crosscut by numerous vertical trace fossils (Diplocraterion sp.) and marked by herringbone crossbedding. Presence of vertical dwelling traces intersecting alternating layers of glauconitic sand and volcanic ash in combination with probable tidal bedding structures implies relatively shallow water, perhaps a few to several tens of meters in depth, at the end of Needmore deposition. Conversely, Millboro deposition seems to represent deeper water conditions, restriction of circulation, or both. If increasing water depth is responsible, it must have been significant (perhaps hundreds of meters or more) and taken place in a relatively short period following the Tioga Ash event. No major global sea level rise is known for the mid-Eifelian, nor are there any major Appalachian Basin tectonic events at that time. This suggests that changing conditions from Needmore to Millboro may be better explained by restricted circulation and anoxia associated with organic overproduction.