Correlation of the Marcellus/Millboro Formations from the Northern to the South-Central Appalachian Basin
Repetski, John E.; Over, D. J.; Enomoto, C. B.
Correlations within the Middle Devonian black shale-rich interval in the Appalachian basin historically have been based primarily on physical attributes, e.g., relative positions of the contained carbonate units and ash beds. Recent investigations involve efforts to extract microfossils from the carbonates and calcareous intervals, to permit refinements of the ages and correlations within this stratigraphic package.
In New York, key Middle Devonian carbonate intervals include, lower to higher, the Cherry Valley Limestone (upper Eifelian) and Halihan Hill Bed (lowermost Givetian) within the Marcellus Formation, Stafford Limestone (base of Skaneateles Fm.; Givetian), Centerfield Limestone (base of Ludlowville Fm.), and the Tully Limestone (Givetian). In eastern West Virginia and western Virginia, however, these black shale-rich strata are assigned to either the Millboro Shale or to the presumed-correlative succession: Marcellus Shale, Mahantango Formation, Tully Limestone and Harrell Shale. Within this interval, four named carbonate members may occur: Purcell Member (Marcellus) (lowest), Landes Limestone Mbr. (Mahantango), Pokejoy Mbr. (Mahantango), and Tully Limestone (highest).
A section along US 250 in Highland County, VA contained about 12 m (40 ft) of interbedded limestones, calcareous shales, and shales. This unit was previously interpreted to be the Purcell Member of the Millboro, which was deposited during the Eifelian Stage, based on studies in WV and PA. However, the conodonts recovered at this locality indicate that this member was deposited during the Polygnathus hemiansatus Zone, or later, of the Givetian Stage. This age indicates that the carbonate interval at this location is younger than the Purcell as reported in WV and PA, and younger than the Cherry Valley Member of the Marcellus Shale in NY, with which it has been correlated.
Likewise, at a section in Pocahontas County, WV, the lowest carbonate of the Millboro, mapped as Purcell but here expressed as large concretions, appears to be early Givetian in age. Higher in this section, the carbonate unit mapped previously as Tully yielded early Frasnian conodonts, much younger than Tully in the northern part of the basin. In fact, this unit correlates with the Harrell Shale to the northeast. These new data indicate younger ages for these "southern" Purcell and Tully than previously mapped and point out a need for further reevaluation of intrabasinal Middle Devonian correlations.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013