Mahantango Formation


The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania, West Virginia, 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.

Description

The Mahantango Formation is a gray, brown, and olive siltstone and shale, characterized by coarsening-upward cycles.

Stratigraphic Setting

Image:USGS 2006 1237 table2 Stratigraphy Middle Devonian.svg|thumb|right|350px|Generalized stratigraphic nomenclature for the Middle Devonian strata in the Appalachian Basin.
The Mahantango Formation is typically found conformably overlying the Marcellus Formation, and underlying the Tully Limestone.
The Mahantango has been divided into four members:
  • Tully Limestone: a fossiliferous calcareous shale. The Tully is a conspicuous formation, which separates the Mahantango from the Hamilton Group.
  • Sherman Ridge: Olive gray, fossiliferous, claystone and siltstone with interbedded fine sandstone coarsening upward.
  • Montebello Sandstone: Olive gray, fossiliferous, coarse to fine-grained sandstone, interbedded with siltstone and claystone, in coarsening upward cycles.
  • Fisher Ridge : silty claystone, siltstone, and very fine-grained sandstone.
The Sherman Ridge and Fisher Ridge are ridge-formers.
In south-central Pennsylvania, the Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members.

Fossils

There are numerous marine fossils found in the Mahantango including brachiopods, crinoids, trilobites, bivalves, and bryozoans.

Brachiopods

Crinoids

Trilobites

Bivalves

Bryozoa

Atactotoechus furcatus

Coral

Gastropods

Cephalopods

Age

Relative age dating of the Mahantango places it in the Middle Devonian period, being deposited between 392 and 385 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. In 2012, Read and Erikson reported the formation as Givetian in Virginia.

Interpretation of Depositional Environments

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.