| Hamilton Group or Millboro Shale | |
| Type: | Group |
| Prilithology: | Shale |
| Otherlithology: | Limestone, Siltstone, Claystone |
| Namedfor: | Hamilton, New York |
| Namedby: | James Hall |
| Region: | Appalachian Basin of eastern North America |
| Subunits: | Mahantango Formation Clearville Siltstone Frame Shale Donation Siltstone Crooked Creek Shale Backbone Ridge Siltstone Chaneysville Siltstone Gander Run ShaleMoscow Formation North Evans limestone Kashong Shale Tichenor LimestoneLudlowville Formation Jaycox Shale Spafford Shale Wanakah Shale, Ivy Point Shale Ledyard Shale, Otisco Shale Centerfield LimestoneSkaneatales Formation Levanna Member Stafford LimestoneMarcellus Formation Otaka Creek Member Cherry Valley Member Union Spring Member |
| Thickness: | up to 2500feet |
| Extent: | Maryland, New York, Pennsylvania, Virginia, West Virginia and Ontario, Canada |
| Age: | Eifelian - Famennian |
| Period: | Devonian |
| Underlies: | Tully Limestone and Harrell Shale |
| Overlies: | Onondaga Limestone |
The Hamilton Group is a Devonian-age geological group which is located in the Appalachian region of the United States. It is present in New York, Pennsylvania, Maryland, Ohio, West Virginia, northwestern Virginia and Ontario, Canada,[1] [2] and is mainly composed of marine shale with some sandstone.
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]
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. 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.[7]
These units are divided by the Stafford, Centerfield, and Tichenor limestones.[8]
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.[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,[10] and approximately 1000 feet thick in central Pennsylvania.
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.[10]
The Hamilton Group, Tioga Bentonites, and Needmore Shale were formerly called the Romney Formation.[10]
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.[13]
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.[14] In 2012, Read and Erikson reported the group as dating from the Eifelian to Famennian in Virginia.
Source:[15]
Tropidoleptus carinatus
Nucfeospira concinna
Mucrospirifer
Athyris
Pleurodictyum americanum
Orthonota undulate
The Hamilton is a good source of road material, riprap and building stone,[16] that is used locally for shale aggregate and common fill.[17]