CONODONT-BASED CORRELATION OF THE MARCELLUS/MILLBORO FORMATIONS IN THE CENTRAL APPALACHIAN BASIN

The Middle Devonian black shale intervals in the Appalachian basin historically have been correlated based on relative position of their carbonate units and ash beds. Conodonts from the carbonates and calcareous intervals allow refinement of ages, correlations, and biofacies analyses. Because this interval currently is the target of intensive hydrocarbon exploration, the 3-dimensional time-space facies distribution within this lithologic package has added significance.

In the classic New York sections, key Middle Devonian carbonate intervals include, lower to higher, the Hurley, 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 Tully Limestone (Givetian). In West Virginia and Virginia, however, the black shale-rich strata are assigned to either the Millboro Shale or the presumed-correlative succession: Marcellus Shale, Mahantango Formation, and Tully Limestone, within which four named carbonate members may occur: Purcell Mbr. (Marcellus; lowest), Landes Limestone Mbr. (Mahantango), Pokejoy Mbr. (Mahantango), and Tully Limestone (highest).

A section in Highland County, VA that has an approximately 12 m thick succession of interbedded limestones, calcareous shales, and brown shales was interpreted previously to be the Eifelian Purcell Member of the Millboro, based on studies in WV and PA. Conodonts from this locality indicate that it was deposited during the lowest Givetian hemiansatus Zone or later, and thus younger than the Purcell as reported elsewhere, 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, appears to be early Givetian in age. Higher in this section, a carbonate unit previously mapped as Tully yielded early Frasnian conodonts, much younger than Tully (Givetian) in the northern part of the basin. This unit correlates with the Harrell Shale to the northeast. These new data indicate younger ages for the southern “Purcell” and “Tully” than previously mapped and demonstrate the need for further reevaluation of intrabasinal Middle Devonian correlations.