The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania, West Virginia, and Maryland.Ryder, R.T., Swezey, C.S., Crangle, R.D., Jr., and Trippi, M.T., 2008, Geologic cross section E-E’ through the central Appalachian Basin from the Findlay Arch, Wood County, Ohio, to the Valley and Ridge Province, Pendleton County, West Virginia: U.S. Geological Survey Scientific Investigations Map SIM-2985, 2 sheets with 48-page pamphlet. https://pubs.er.usgs.gov/publication/sim2985Ryder, R.T., Crangle, R.D., Jr., Trippi, M.H., Swezey, C.S., Lentz, E.E., Rowan,

E.L., and Hope, R.S., 2009, Geologic cross section D-D’ through the central Appalachian basin from the Findlay arch, Sandusky County, Ohio, to the Valley and Ridge province, Hardy County, West Virginia: U.S. Geological Survey Scientific Investigations Map SIM-3067, 2 sheets with 52-page pamphlet. https://pubs.er.usgs.gov/publication/sim3067

Ryder, R.T., Trippi, M.H., Swezey, C.S., Crangle, R.D., Jr., Hope, R.S., Rowan, E.L.,

and Lentz, E.E., 2012, Geologic cross section C-C’ through the central Appalachian basin from near the Findlay Arch, north-central Ohio, to the Valley and Ridge Province, Bedford County, south-central Pennsylvania: U.S. Geological Survey Scientific Investigations Map SIM-3172, 2 sheets with 70-page pamphlet. https://pubs.er.usgs.gov/publication/sim3172

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.{{cite map|title=Geologic Map of the Mifflintown Qd.|id=A126|scale= 1:24,000|author1=Conlin, Richard R. |author2=Hoskins, Donald M. |publisher=Pennsylvania Geological Survey, Fourth Series|cartography=United States Geological Survey}}

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.{{Cite web|url=https://ngmdb.usgs.gov/Geolex/Units/Mahantango_2597.html|title = Geolex — Mahantango}}

Description

The Mahantango Formation is a gray, brown, and olive siltstone and shale,Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers, (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.Milici, R.C., and Swezey, C.S., 2014, Assessment of Appalachian Basin oil and gas resources: Devonian gas shales of the Devonian Shale-Middle and Upper Paleozoic Total Petroleum System, in Ruppert, L.F., and Ryder, R.T., eds., Coal and Petroleum Resources in the Appalachian Basin: Distribution, Geologic Framework, and Geochemical Character: U.S. Geological Survey Professional Paper 1708, chapter G.9, 81p. http://pubs.usgs.gov/pp/1708/g9/pdf/pp1708_g9.pdf characterized by coarsening-upward cycles.{{cite web |url=http://www.dcnr.state.pa.us/topogeo/map1/explanation.pdf |archive-url=https://web.archive.org/web/20030422135023/http://www.dcnr.state.pa.us/topogeo/map1/explanation.pdf |url-status=dead |archive-date=April 22, 2003 |title= Explanation of Geologic Units |access-date=2008-01-26 |publisher=Pennsylvania Geological Survey}}

Stratigraphic Setting

Image:USGS 2006 1237 table2 Stratigraphy Middle Devonian.svg 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 (where present).

The Mahantango has been divided into four members:MacLachlan, D.B., Hoskins, D.M., Payne, D.F., (1995). Bedrock Geology of the Freeburg 7.5 minuet Quadrangle, Snyder County, Pennsylvania. ORF 95-04, The Pennsylvania Geologic Survey, Harrisburg, PA.

Tully Limestone: a fossiliferous calcareous shale. The Tully is a conspicuous formation, which separates the Mahantango from the Hamilton Group.{{cite web |url=http://pubs.usgs.gov/of/2006/1237/of2006-1237.pdf|title= Assessment of Appalachian Basin Oil and Gas Resources: Devonian Shale–Middle and Upper Paleozoic Total Petroleum System |author1=Milici, Robert C. |author2=Swezey, Christopher S. |year=2006|access-date=2008-04-05 |publisher=United States Geological Survey}}
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 (also described as the Dalmatia and Turkey Ridge): silty claystone, siltstone, and very fine-grained sandstone.

The Sherman Ridge and Fisher Ridge are ridge-formers.{{cite web |url=http://www.bucknell.edu/Documents/Geology/PaleozoicStratigraphicColumn1.pdf |title= Paleozoic Stratigraphic Column of Central Pennsylvania |publisher= Bucknell University|access-date=2008-05-26}}

In south-central Pennsylvania, the Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members.

Fossils

File:Phacops-enrolled.jpg from an outcrop of the Mahantango near Milesburg, Pennsylvania, with schizochroal eye visible]]

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

= Brachiopods =

= Crinoids =

Ancyrocrinus

= Trilobites =

= Bivalves =

= Bryozoa =

Atactotoechus furcatus

= Coral =

Pleurodictyum
Thamnoptychia (Trachypora)

= Gastropods =

= Cephalopods =

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.Berg, T.M., et al., (1983). Stratigraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania. In 2012, Read and Erikson reported the formation as Givetian in Virginia.[http://hdl.handle.net/10919/19078 Paleozoic Sedimentary Successions of the Virginia Valley & Ridge and Plateau]

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.Prave, A.R., Duke, W.L., Slattery, W. (1996). A depositional model for storm- and tide-dominated prograding siliciclastic shorelines from the Middle Devonian of the central Appalachian foreland basin, USA. Sedimentology, 43, 611-629.