Examination of the “Purcell Limestone” in the Millboro Shale in Highland County, Virginia
Catherine B. Enomoto, John E. Repetski, and Diana L. Rotter
U.S. Geological Survey, National Center, Reston, VA 20192, [email protected]
Recent investigations in the Middle Devonian Millboro Shale in western Virginia indicate that one of the enclosed carbonate intervals was deposited during the Givetian Stage in a shallow marine depositional environment. Previous studies of the Millboro Shale, which includes the Marcellus Shale and the Mahantango Formation, described four limestone members: the Purcell Member of the Marcellus Shale, the Pokejoy and Landes limestones of the Mahantango Formation, and the Tully Limestone. The exposure studied near US 250 in Highland County, Virginia contained about 40 feet of interbedded limestones, calcareous shales, and shales. This interval was previously interpreted to be the Purcell Member, which was deposited during the Eifelian Stage, based on studies in West Virginia and Pennsylvania.
In hand samples, the limestone facies in this exposure is medium- to dark-gray, very fine-grained mudstone, and contains gastropods, cephalopods, and tentaculites. The beds displayed pinching and swelling. USGS researchers collected several multi-kilogram samples from the limestone facies. Oriented samples were used to produce thin sections. A small quantity of the samples were subjected to X-ray diffraction (XRD) analysis. The majority of the samples were processed for conodont elements. During the processing, insoluble residue material was reserved after the dissolution and sieving were completed, but before magnetic and gravity separation were performed. This insoluble residue material was imbedded in acrylic and made into polished acrylic-based pellets.
Examination of the thin sections with a petrographic microscope showed that the fossils were broken and not in-place, indicating transport and post-mortem deposition. The thin sections and the pellets were examined with a petrographic microscope and scanning electron microscope (SEM). This examination showed the quartz cement was most likely precipitated in an early diagenetic phase of lithification. The XRD analysis indicated 60-85% calcite, and 13-24% quartz.
The recovered conodonts indicate that this member was deposited during the Polygnathus varcus Zone of the Givetian Stage. This age indicates that the carbonate interval mapped as Purcell at the Highland County section is younger than the Purcell as reported to the north in West Virginia and Pennsylvania, as well as the Cherry Valley Member of the Marcellus Shale in New York, with which it has been correlated.
AAPG Search and Discovery Article #90154©2012 AAPG Eastern Section Meeting, Cleveland, Ohio, 22-26 September 2012