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Mahantango Formation

Mahantango Formation
Stratigraphic range: Middle Devonian, ~387 Ma
Mahantango Formation Rt522 PA.jpg
Outcrop of the Mahantango on Rt. 522, Fulton County, Pennsylvania
Type Geological formation
Unit of Hamilton Group
Sub-units See: Description
Underlies Harrell Shale
Overlies Marcellus Formation
Lithology
Primary Shale
Other Limestone, Siltstone
Location
Region Appalachian Basin of
eastern North America
Extent New York to Maryland
Type section
Named for Mahantango Creek

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.

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.

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.

There are numerous marine fossils found in the Mahantango including Brachiopods, Crinoids, Trilobites, Bivalves, and Bryozoans.


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