Tully Limestone

Mahantango Formation Stratigraphic range: Middle Devonian, ~387 Ma PreЄ Є O S D C P T J K Pg N ↓
Outcrop of the Mahantango on Rt. 522, Fulton County, Pennsylvania
Type	Geological formation
Unit of	Hamilton Group
Sub-units	See: Description
Underlies	Harrell Shale and West Falls Formation
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. Details of the type section and of stratigraphic nomenclature for this unit as used by the U.S. Geological Survey are available on-line at the National Geologic Map Database.

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.