View source for Earth:Mahantango Formation

{{Short description|Geological formation in the United States}}
{{Infobox Rockunit
| name = Mahantango Formation
| image = Mahantango_Formation_Rt522_PA.jpg
| type = [[Earth:Geological formation|Geological formation]]
| prilithology = [[Chemistry:Shale|Shale]]
| otherlithology = [[Earth:Limestone|Limestone]], [[Earth:Siltstone|Siltstone]]
| namedfor = Mahantango Creek
| namedby = 
| region = Appalachian Basin of<br />eastern [[Earth:North America|North America]]
| coordinates = 
| caption = Outcrop of the Mahantango on Rt. 522, Fulton County, Pennsylvania
| unitof = [[Earth:Hamilton Group|Hamilton Group]]
| subunits = See: Description
| underlies = [[Earth:Harrell Shale|Harrell Shale]] and West Falls Formation
| overlies = Marcellus Formation
| thickness = 
| extent = Maryland, Ohio, Pennsylvania, Virginia, and West Virginia
| age = [[Earth:Givetian|Givetian]], ~{{Fossil range|387}}
| period = Givetian
}}
The [[Earth:Devonian|Devonian]] '''Mahantango Formation''' is a mapped bedrock unit in Pennsylvania, West Virginia, and Maryland.<ref>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/sim2985</ref><ref>Ryder, 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
</ref><ref>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
</ref> It is named for the North branch of the Mahantango Creek in Perry and Juniata counties in Pennsylvania. It is a member of the [[Earth:Hamilton Group|Hamilton Group]], along with the underlying the Marcellus Formation Shale.<ref name="explanation"/> South of Tuscarora Mountain in south central Pennsylvania, the lower members of this unit were also mapped as the '''Montebello Formation'''.<ref>{{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}}</ref>
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.<ref>{{Cite web|url=https://ngmdb.usgs.gov/Geolex/Units/Mahantango_2597.html|title = Geolex — Mahantango}}</ref>

==Description==
The Mahantango Formation is a gray, brown, and olive [[Earth:Siltstone|siltstone]] and [[Chemistry:Shale|shale]],<ref>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.</ref><ref>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</ref> characterized by coarsening-upward cycles.<ref name="explanation">{{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}}</ref> 

==Stratigraphic Setting==
[[Image:USGS 2006 1237 table2 Stratigraphy Middle Devonian.svg|thumb|right|350px|Generalized [[Earth:Stratigraphy|stratigraphic]] nomenclature for the Middle Devonian strata in the Appalachian Basin.<ref name="USGS2006-1237"/>]]

The Mahantango Formation is typically found conformably overlying the Marcellus Formation, and underlying the '''Tully Limestone''' (where present).<ref name=USGS2006-1237/>

The Mahantango has been divided into four members:<ref>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.</ref>  
*Tully Limestone: a fossiliferous calcareous shale. The Tully is a conspicuous formation, which separates the Mahantango from the Hamilton Group.<ref name=USGS2006-1237>{{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}}</ref>
*Sherman Ridge: Olive gray, fossiliferous, claystone and siltstone with interbedded fine [[Earth:Sandstone|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.<ref name="url">{{cite web |url=http://www.bucknell.edu/Documents/Geology/PaleozoicStratigraphicColumn1.pdf |title= Paleozoic Stratigraphic Column of Central Pennsylvania |publisher= [[Organization:Bucknell University|Bucknell University]]|access-date=2008-05-26}}</ref>

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

==Fossils==
[[File:Phacops-enrolled.jpg|right|thumb|Enrolled ''[[Biology:Phacops rana|Phacops rana]]'' from an outcrop of the Mahantango near Milesburg, Pennsylvania, with schizochroal eye visible]]
There are numerous marine fossils found in the Mahantango including [[Biology:Brachiopod|brachiopod]]s, [[Biology:Crinoid|crinoid]]s, [[Biology:Trilobite|trilobite]]s, bivalves, and [[Biology:Bryozoa|bryozoa]]ns.

=== Brachiopods ===
* [[Biology:Athyris|Athyris]]
* [[Biology:Chonetes|Chonetes]]
* [[Biology:Mucrospirifer|Mucrospirifer]]
* Protoleptostrophia
* Tropidoleptus

=== Crinoids ===
* Ancyrocrinus

=== Trilobites ===
* Basidechenella
* [[Biology:Dipleura|Dipleura]]
* [[Biology:Eldredgeops|Eldredgeops]] (formerly Phacops)
* [[Biology:Greenops|Greenops]]

=== Bivalves ===
* Actinodesma
* Grammysioidea
* Grammysia
* Leiopteria
* Modiomorpha
* Nuculites
* Orthonota

=== Bryozoa ===
Atactotoechus

=== Coral ===
* [[Biology:Pleurodictyum|Pleurodictyum]]
* Thamnoptychia (Trachypora)

=== Gastropods ===
* [[Biology:Bembexia|Bembexia]]
* Crenistriella
* Dictyotomaria?
* Glyptotomaria
* Loxonema
* Paleozygopleura
* [[Biology:Platyceras|Platyceras]]

=== Cephalopods ===
* [[Biology:Tornoceras|Tornoceras]]
* [[Biology:Agoniatites|Agoniatites]]
* [[Biology:Spyroceras|Spyroceras]]
* [[Biology:Michelinoceras|Michelinoceras]]
* Bactrites
* Striacoceras
* Koenenites?

==Age==
Relative age dating of the Mahantango places it in the Middle [[Earth:Devonian|Devonian]] period, being deposited between 392 and 385 (±3) million years ago. It rests [[Earth:Unconformity|conformably]] atop the Marcellus Formation shale. Its upper contact is also conformable to the Trimmers Rock Formation and Harrell Shale.<ref>Berg, T.M., et al., (1983). Stratigraphic Correlation Chart of Pennsylvania: G75, Pennsylvania Geologic Survey, Harrisburg, Pennsylvania.</ref> In 2012, Read and Erikson reported the formation as Givetian in Virginia.<ref>[http://hdl.handle.net/10919/19078 Paleozoic Sedimentary Successions of the Virginia Valley & Ridge and Plateau]</ref>

==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. [[Biology:Brachiopod|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.<ref>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.</ref>

==See also==
*Geology of Pennsylvania

==References==
<references/>

==External links==

{{Navboxes
|title = Chronostratigraphic context of the Mahantango Formation by state
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[[Category:Shale formations of the United States]]
[[Category:Limestone formations]]
[[Category:Siltstone formations]]
[[Category:Shallow marine deposits]]

{{Sourceattribution|Mahantango Formation}}