There are two main formations encompassed by the group: the
Mahantango Formation and the
Marcellus Shale. In southwestern
Virginia, where the two sub-units are not easily distinguishable, the Hamilton Group is broadly equivalent to the Millboro Shale or Millboro Formation.[3]
The group is named for the village of
Hamilton, New York.[4][5] These rocks are the oldest
strata of the Devonian gas shale sequence.[6]
Stratigraphic Setting
In western Pennsylvania, Ohio and West Virginia the Hamilton Group consists of the
Mahantango Formation, a gray, dark gray, brown, and olive laminated shale; siltstone; and very fine-grained sandstone or claystone containing marine fossils. The uppermost clay layers of the
Mahantango Formation are particularly rich in fossils.[8] In New York, central and eastern Pennsylvania the upper sections contain several thick grey shale formations, occasionally marked with limestone stringer. It overlies the
Marcellus Shale, a
fissile gray-black to black, thinly laminated,
pyritic,
carbonaceous thin shale with sparse marine fauna and siderite concretions. The total thickness of the Hamilton Group in Pennsylvania runs about 970 feet. In New York State, it thickens from 250 feet near
Lake Erie to over 2,500 feet in
Ulster and
Greene counties. Depths ranging from outcrops to 8,000 feet below the surface of
Sullivan County, in the southeastern part of New York state.[6]
In the interior lowlands of New York and north eastern and north central Pennsylvania, the Hamilton Group contains the
Marcellus,
Skaneateles,
Ludlowville, and
Moscow Formations, in ascending order, with the
Tully Limestone above.[9]
The Mahantango Formation includes these members in descending order: Sherman Ridge, Montebello sandstone, Fisher Ridge, Dalmatia, and Turkey Ridge. In south-central Pennsylvania, it includes Clearville, Frame, Chaneysville, and Gander Run Members. Its thickness in Maryland ranges from 600 feet in the west, increasing to 1,200 feet in the east,[12] and approximately 1000 feet thick in central Pennsylvania.[8]
The Marcellus Formation contains a local limestones Purcell Member and Cherry Valley as well as
Tioga Bentonites at the base in central and eastern Pennsylvania. Its thickness in Maryland ranges from 250 feet in east, increasing to 500 feet in the west.[12]
The
Geneseo Shale is a dark grey to black shale that overlies the Tully Limestone; it includes the Filmore beds.[13]
The Tully Limestone is a shallow-water carbonaceous unit.
The
Moscow Formation (Middle Devonian / Givetian) comprises grey to black shales interbedded with limestones and calcareous mudstones and usually bioturbated.[14]
Age
Relative age dating of the Hamilton places it in the middle and lower
Devonian period. In Maryland, it rests conformably atop the Tioga Metabentonite, a thin layer of laminated shale lying over the
Needmore Shale. In other regions, the Needmore Shale is replaced by laterally equivalent units such as limestone and shale of the
Onondaga Formation, or chert of the
Huntersville Chert.[15]
The Hamilton Group lies conformably below the
Brallier Formation (formerly Woodmont Shale, dark laminated shale) at its southern extent. Further north, it is overlain by the
Tully Limestone, though some consider the Tully Limestone a subunit of the Mahantango Formation. The Tully Limestone is succeeded by the dark laminated
Harrell Shale.[16] In 2012, Read and Erikson reported the group as dating from the
Eifelian to
Famennian in Virginia.[15][12]
The Hamilton is a good source of road material,
riprap and building stone,[18] that is used locally for shale aggregate and common fill.[8]
References
^Ryder, R.T., Swezey, C.S., Crangle, R.D., Jr., and Trippi, M.T., 2008, Geologic cross
section E-E’ through the central Appalachian Basin from the Findlay Arch, Wood County, Ohio, to the Valley and Ridge Province, Pendleton County, West Virginia: U.S. Geological Survey Scientific Investigations Map SIM-2985, 2 sheets with 48-page pamphlet.
https://pubs.er.usgs.gov/publication/sim2985
^Ryder, R.T., Trippi, M.H., Swezey, C.S., Crangle, R.D., Jr., Hope, R.S., Rowan, E.L.,
and Lentz, E.E., 2012, Geologic cross section C-C’ through the central Appalachian basin from near the Findlay Arch, north-central Ohio, to the Valley and Ridge Province, Bedford County, south-central Pennsylvania: U.S. Geological Survey Scientific Investigations Map SIM-3172, 2 sheets with 70-page pamphlet.
https://pubs.er.usgs.gov/publication/sim3172
^"Map Unit Descriptions"(PDF). Virginia Department of Mines, Minerals and Energy. 2007-09-28. Archived from
the original(PDF) on 2008-02-11. Retrieved 2008-05-03.
^Wood, G.H., Trexler, J.P., Kehn, T.M., (1964). Geology of the West-Central Part of the Southern Anthracite Field and Adjoining Areas, Pennsylvania. United States Geological Survey, C-46.
^Sinclair, James P. (1993). Final Generic Environmental Impact Statement for Promulgation of 6 NYCRR Part 382: Regulations for Low-Level Radioactive Waste Disposal Facilities. DIANE Publishing.
ISBN1-56806-746-1.
Upper Warren, Lower Warren, Speechely Stray, Speechely, Balltown A, Balltown B, Balltown C, Sheffield, First Bradford, Second Bradford, Third Bradford, Kane