Type Geological formation Underlies Harrell Shale Extent New York to Maryland Named for Mahantango Creek Unit of Hamilton Group | Sub-units See: Description Other Limestone, Siltstone Primary Slate Overlies Marcellus Formation | |
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Region Appalachian Basin of
eastern North America | ||
The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania, New Jersey, New York and Maryland. It is named for the North branch of the Mahantango Creek in Perry and Juniata counties in Pennsylvania. It is a member of the Hamilton Group, along with the underlying the Marcellus Formation Shale. South of Tuscarora Mountain in south central Pennsylvania, the lower members of this unit were also mapped as the Montebello Formation.
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Description
The Mahantango is defined as a gray, brown, and olive siltstone and shale, characterized by coarsening-upward cycles. It is typically found conformally overlying the Marcellus Formation, and underlying the Tully Limestone (where present).
The Mahantango has been divided into four members:
The Sherman Ridge and Fisher Ridge are ridge-formers.
In south-central Pennsylvania, the Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members.
Depositional environment
The Mahantango represents a terrestrial to marine transition zone that went through many transgressive-regression sequences. The fine-grained rocks represent a shallow sea environment and accounts for many of the fossils. Coarser grained sediments represent near-shore environments, beaches, or possibly delta lobes. These environments were tide-dominated and often had violent storms. The Montebello Sandstone member is an example of a storm dominated rock unit. Brachiopod fossils are scattered in massive sandstone beds throughout the rock unit, while the Sherman Ridge member is more laminated with fossilized ripple marks often indicating tidal current directions.
Fossils
There are numerous marine fossils found in the Mahantango including Brachiopods, Crinoids, Trilobites, Bivalves, and Bryozoans.
Age
Relative age dating of the Mahantango places it in the Middle Devonian period, being deposited between 392 and 385 (±3) million years ago. It rests conformably atop the Marcellus Formation shale. Its upper contact is also conformable to the Trimmers Rock Formation and Harrell Shale.