Hamilton Group

Hamilton Group or Millboro Shale Stratigraphic range: Middle Devonian
Type	Geological unit
Sub-units	Mahantango Formation Marcellus Formation
Underlies	Brallier Formation and Harrell Shale
Overlies	Marcellus Formation
Thickness	up to 2,500 feet (800 m)
Lithology
Primary	Shale
Other	Limestone, Siltstone, Sandstone, Claystone
Location
Region	Appalachian Basin of eastern North America
Extent	New York, Pennsylvania, Maryland, Virginia
Type section
Named for	Hamilton, New York
Named by	James Hall

The Devonian Hamilton Group is a mapped bedrock unit in New York, Pennsylvania, Maryland, Ohio and West Virginia. In Virginia, it is known as the laterally equivalent Millboro Shale. The group is named for the village of Hamilton, New York. These rocks are the oldest strata of the Devonian gas shale sequence.

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. 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 the state.

In the interior lowlands of New York, the Hamilton Group contains the Marcellus, Skaneateles, Ludlowville, and Moscow Formations, in ascending order, with the Tully Limestone above. These units are divided by the Stafford, Centerfield, and Tichenor limestones. In Ontario, Canada, the Hamilton Group formations are, in ascending order, Bell, Rockport Quarry, Arkona, Hungry Hollow, Widder, and Ipperwash; the Kettle Point Formation of the late Devonian lies unconformably above.

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