View source for Marcellus Formation

{{Short description|Middle Devonian age unit of sedimentary rock}}
{{Infobox rockunit
| name = Marcellus Formation
| age = [[Middle Devonian]]
| period = Middle Devonian
| image = MarcellusShaleBank1.jpg
| type = [[Geological formation]]
| prilithology = [[Shale]]
| otherlithology = [[Slate]], [[limestone]], [[sandstone]], [[tuff]]
| namedfor = [[Marcellus (village), New York|Marcellus, New York]]
| namedby = [[James Hall (paleontologist)|James Hall]], 1839
| region = [[Appalachian Basin]] of<br/>eastern [[North America]]<!-- | coordinates = {{coord|42.99|N|76.34|W}} -->
| caption = Marcellus shale exposure above Marcellus, N.Y. The vertical [[Joint (geology)|joint]]s create sheer cliff faces.
| unitof = [[Hamilton Group]]
| subunits = [[#Named members|See: Named members]]
| underlies = [[Mahantango Formation]] and [[Skaneatales Formation]]/Stafford Lime Mbr
| overlies = [[Huntersville Chert]], [[Needmore Shale]], and [[Onondaga Formation]]
| thickness = up to {{convert|900|ft|m|-1}}<ref name=White/>
| extent = {{convert|600|mi|km}}<ref name=Durham>{{cite web | url=https://explorer.aapg.org/story/articleid/11349/another-shale-making-seismic-waves | title=Another Shale Making Seismic Waves | date=March 2008 }}</ref>
}}

The '''Marcellus Formation''' or the '''Marcellus Shale''' is a [[Middle Devonian]] age [[Geological unit|unit]] of [[sedimentary rock]] found in eastern North America.  Named for a distinctive outcrop near the village of [[Marcellus (village), New York|Marcellus, New York]],<ref name=MGS1918/>
it extends throughout much of the [[Appalachian Basin]].<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>

The unit name usage by the U.S. Geological Survey (USGS) includes ''Marcellus Shale'' and ''Marcellus Formation''.<ref name="Geolex">See unit names marked by an asterisk at [https://ngmdb.usgs.gov/Geolex/Units/Marcellus_2635.html Geolex]</ref>  The term "Marcellus Shale" is the preferred name throughout most of the Appalachian region, although the term "Marcellus Formation" is also acceptable within the State of Pennsylvania.<ref name="Geolex"/> The unit was first described and named as the "Marcellus shales" by J. Hall in 1839.<ref>New York Geological Survey, 3rd Report, p. 295-296</ref>

==Description==
[[File:MarcellusShaleCloseUp.jpg|alt=Many irregular pieces of rock in different sizes.|thumb|left|Fragments below exposure of [[Fissility (geology)|fissile]] Marcellus black shale at [[Marcellus (village), New York|Marcellus, N.Y.]]]]
The Marcellus consists predominantly of [[black shale]] and a few [[limestone]] [[Bed (geology)|beds]] and concentrations of iron [[pyrite]]&nbsp;([[Iron|Fe]][[Sulfur|S]]<sub>2</sub>) and [[siderite]]&nbsp;(Fe[[carbonate|CO<sub>3</sub>]]).<ref name=EPA>{{cite web |url=http://www.epa.gov/superfund/sites/docrec/pdoc1724.pdf |title=HRS Documentation Record, Safety Light Corporation, EPA ID No. PAD987295276 |access-date=2008-04-02 |work=Hazard Ranking System|publisher=[[United States Environmental Protection Agency]]}}</ref>
Like most shales, it tends to split easily along the bedding plane, a property known as [[fissility (geology)|fissility]].<ref name=EPA/>
Lighter colored shales in the upper portion of the formation tend to split into small thin-edged fragments after exposure.<ref name=MacFarlane1890>{{cite book
 | author = MacFarlane, James R.
 | year = 1890
 |orig-year=1878
 | title =An American Geological Railway Guide : Giving the Geological Formation at Every Railway Station,...
 | edition = 2nd
 | pages = [https://archive.org/details/anamericangeolo00rgoog/page/n38 30]–31, 33
 | publisher = D. Appleton
 | location = New York
 | url = https://archive.org/details/anamericangeolo00rgoog
 | lccn = tmp96004299
}}</ref>
These fragments may have [[rust]] stains from exposure of pyrite to air, and tiny [[gypsum]]&nbsp;([[calcium|Ca]][[sulfate|SO<sub>4</sub>]]·2[[Hydrate|H<sub>2</sub>O]]) crystals from the reaction between pyrite and limestone particles.<ref name=MacFarlane1890/>
Fresh exposures of the pyriteiferous shale may develop the secondary [[Mineralization (geology)|mineralization]] of orange [[limonite]]&nbsp;(Fe[[Oxygen|O]]([[hydroxide|OH]])·nH<sub>2</sub>O), and the pale yellow [[efflorescence]] or bloom of [[sulfur]], associated with [[acid rock drainage]].<ref name="Milam and Cow Knob">{{cite web |url=http://www.wvgs.wvnet.edu/www/statemap/statemap06.htm |title= Milam and Cow Knob Quadrangles | author = McDowell, Ronald; Katharine Lee Avary, Eric Lewis, James Britton, Paula Hunt|author2=Paula Waggy |author3=Megan Ganak   | year = 2006| publisher =West Virginia Geological & Economic Survey |work= WVGES STATEMAP Projects |access-date=2008-07-13}}</ref>

Pyrite is especially abundant near the base,<ref name=Allensville/>
and the upper contacts of limestones, but [[framboid]]al microcrystals and [[euhedral]] crystals of pyrite occur throughout the organic-rich deposits.<ref name=Sageman2003/>
The Marcellus also contains [[uranium]],<ref name=Hand1988>{{cite journal
 | author = Hand, B.M.
 |author2=Banikowski, J.E.
 | year = 1988
 | title = Radon in Onondaga County, New York; paleohydrogeology and redistribution of uranium in Paleozoic sedimentary rocks
 | journal = Geology
 | volume = 16
 | issue = 9
 | pages = 775–778
 | url = http://geology.geoscienceworld.org/cgi/content/abstract/16/9/775
 | access-date = 2008-05-03
 | doi = 10.1130/0091-7613(1988)016<0775:RIOCNY>2.3.CO;2
|bibcode = 1988Geo....16..775H | url-access = subscription
 }}</ref><ref name=Harrell1991>{{cite journal
 | author1 = Harrell, J.A.
 | author2 = Belsito, M.E.
 | author3 = Kumar, A.
 | year = 1991
 | title = Radon hazards associated with outcrops of Ohio Shale in Ohio
 | journal = Environmental Geology
 | volume = 18
 | issue = 1
 | pages = 17–26
 | doi = 10.1007/BF01704574
 |bibcode = 1991EnGeo..18...17H | s2cid = 140653557
 }}</ref>
and the [[radioactive decay]] of the [[uranium-238]]&nbsp;(<sup>238</sup>U) makes it a [[source rock]] for radioactive [[radon]] gas&nbsp;(<sup>222</sup>Rn).<ref name="isbn0-87371-955-7">{{cite book |author=Wanty, Richard B. |title=Field studies of radon in rocks, soils, and water |publisher=C.K. Smoley |location=Chelsea, Michigan |year=1993 |page=216 |isbn=0-87371-955-7 }}</ref>

Measured total organic content of the Marcellus ranges from less than 1% in eastern New York, to over 11% in the central part of the state,<ref name=Martin/><ref name=nyahay/>
and the shale may contain enough carbon to support [[combustion]].<ref name=Dewees1878>{{cite book
 | author = Dewees, John H.
 |author2=Ashburner, Charles A.
 | year = 1878
 | title =Report of Progress in the Juniata District on the Fossil Iron Ore Beds of Middle Pennsylvania
 | url = https://archive.org/details/reportprogressi00changoog
 | publisher = Board of commissioners for the Second geological survey
 | location = Harrisburg
 | lccn = gs07000714
}}</ref>
The more organic-rich black shales can be [[Bitumen|bituminous]], but are too old to contain [[bituminous coal]] formed from land plants.<ref name=Underwood1879/>
In [[petroleum geology]], these black shales are an important [[source rock]] that filled conventional [[petroleum reservoir]]s in overlying formations, are an unconventional [[shale gas]] reservoir, and are an [[Permeability (earth sciences)|impermeable]] seal that traps underlying conventional [[natural gas]] reservoirs.<ref name=PA2006>{{cite journal
|url=http://www.dcnr.state.pa.us/info/carbon/documents/geological_assessment_rev_091806.pdf
|archive-url=https://web.archive.org/web/20110609235418/http://www.dcnr.state.pa.us/info/carbon/documents/geological_assessment_rev_091806.pdf
|url-status=dead
|archive-date=June 9, 2011
| title = Assessment of Geological Sequestration Potential in Pennsylvania
| publisher = Pennsylvania Department of Conservation and Natural Resources
| date = 2006-09-18
| access-date = 2008-07-07}}
</ref>
To the west the formation may produce liquid [[petroleum]]; further north heating during deeper burial more than 240 million years ago [[Cracking (chemistry)|cracked]] this oil into gas.<ref name=Martin/><ref name=Laughrey2004>{{cite journal
 | author1 = Laughrey, C.D.
 | author2 = Billman, D.A.
 | author3 = Canich, M.R.
 | year = 2004
 | title = Petroleum geology and geochemistry of the Council Run gas field, north central Pennsylvania
 | journal = AAPG Bulletin
 | volume = 88
 | issue = 2
 | pages = 213–239
 | doi = 10.1306/10060301104
| bibcode = 2004BAAPG..88..213L
 }}</ref>

==Geographic extent==
The Marcellus is found throughout the [[Allegheny Plateau]] region of the northern [[Appalachian Basin]] of North America. In the United States, the Marcellus Shale runs across the [[Southern Tier]] and [[Finger Lakes]] regions of [[New York (state)|New York]], in northern and western [[Pennsylvania]], eastern [[Ohio]],  through western [[Maryland]], and throughout most of [[West Virginia]] extending across the state line into extreme western [[Virginia]].<ref name=PSU>{{cite web|url=http://live.psu.edu/story/28116 |archive-url=https://web.archive.org/web/20081222111707/http://live.psu.edu/story/28116 |url-status=dead |archive-date=2008-12-22 |title=Unconventional natural gas reservoir could boost U.S. supply |date=2008-01-17 |access-date=2008-04-04 |work=Penn State Live |publisher=[[Pennsylvania State University]] }}</ref>  The Marcellus bedrock in eastern Pennsylvania<ref>{{cite web |url = http://www.fracktrack.org/index.php |title = Pennsylvania Marcellus Shale Mapping Application|access-date=2011-03-17}}</ref> extends across the [[Delaware River]] into extreme western [[New Jersey]].<ref name=White/> It also exists in the subsurface of a small portion of [[Kentucky]] and [[Tennessee]].<ref name="geology.com">{{cite web |url=http://geology.com/articles/marcellus-shale.shtml |title=Marcellus Shale Gas: New Research Results Surprise Geologists! |work=geology.com |publisher = Dr. Hobart M. King|access-date=2008-05-03}}</ref> Below [[Lake Erie]], it can be found crossing the border into [[Canada]], where it stretches between [[Port Stanley, Ontario|Port Stanley]] and [[Long Point, Ontario|Long Point]] to [[St. Thomas, Ontario|St. Thomas]] in southern [[Ontario]].<ref>[http://stopfrackingontario.wordpress.com/fracking/in-ontario/ Stop Fracking Ontario: Shale in Ontario]</ref><ref name=Singer>{{cite web |url=https://archive.org/details/hydrogeologyofso00snsiuoft |title=The Hydrology of Southern Ontario: Second Edition|access-date=2008-04-06 |author1=Singer, S.N. |author2=Cheng, C.K. |author3=Scafe, M.G. |year=2003|format=PDF |work=Hydrology of Ontario Series|publisher=Ontario Ministry of the Environment}}</ref>

===Outcrops in New York===
[[File:Middle Delaware River above Walpack Bend.jpg|alt=A placid river winds its way through farmland and forest.|thumb|upright|[[Delaware River]] above Walpack Bend, where it leaves the buried valley eroded from Marcellus Shale bedrock]]
The Marcellus appears in outcrops along the northern margin of the formation in central New York. There, the two [[Joint (geology)|joint]] planes in the Marcellus are nearly at right angles, each making cracks in the formation that run perpendicular to the bedding plane, which lies almost level.<ref name=MacFarlane1890/>
These joints form smooth nearly vertical [[cliff]]s, and the intersecting joint planes form projecting corners in the rock faces.<ref name=MacFarlane1890/>
Once exposed, the weathered faces lose most of their organic carbon,<ref name=Peucker-ehrenbrink2001/>
turning from black or dark gray to a lighter shade of gray.

Outcrops of the Marcellus  can contain very small beds that resemble [[coal]].<ref name=Vanuxem1842/> The New York outcrops, and others further south in Pennsylvania and New Jersey, were extensively excavated in the early 19th century, sometimes at great expense, in the false hope of finding minable [[coal seam]]s.<ref name=Vanuxem1842>{{cite book
 | author = Vanuxem, L.
 | year = 1842
 | title = Geology of New York, Pt. 3, Comprising the Survey of the Third Geological District
 | series = Natural History of New York.
 | publisher =Carroll and Cook
 | location = Albany, New York
 | lccn = gs07000386
}}</ref>
In [[Perry County, Pennsylvania]] along the [[Juniata River]] the false coal beds become up to {{convert|.3|m|ft|0|abbr=on}} thick, but they did not produce a valuable fuel, despite the considerable effort expended to mine it from the surrounding hills.<ref name=Lesley1892/>
[[Seaweed]] and marine plants probably formed the false coal. True coal is formed from terrestrial plants, which only began to appear in Marcellus and later fossils.<ref name=MacFarlane1873>{{cite book
 | author = MacFarlane, J.
 | year = 1875
 | title = The Coal-regions of America: Their Topography, Geology, and Development.
 | url = https://archive.org/details/ael2949.0001.001.umich.edu
 | publisher = D. Appleton and Company
 | edition = 3rd
 | page = [https://archive.org/details/ael2949.0001.001.umich.edu/page/612 612]
 | lccn = 03026433
}}</ref><ref name=Ripley1861>{{cite book
 | author = Ripley, G.
 |author2=Dana, C.A.
 | year = 1861
 | title = The New American Cyclopaedia: A Popular Dictionary of General Knowledge
 | publisher = D. Appleton and Company
 | location = New York
 | lccn = 07019446
}}</ref>

Close proximity to the surface of Marcellus bedrock south of the New York outcrops makes an east–west band running through the city of [[Syracuse, New York|Syracuse]]<ref name=Hand1988/> a high-risk area for radon as an [[Indoor air quality|indoor air pollutant]].<ref name="New York">{{cite web|url=http://www.nyradon.org/abstracts.htm#1a |title=New York State Studies (published) |work=Radon – The State of Radon in New York |publisher=NYS Department of Health Wadsworth Center |access-date=2008-05-03 |url-status=dead |archive-url=https://web.archive.org/web/20080808120456/http://www.nyradon.org/abstracts.htm |archive-date=2008-08-08 }}</ref> From the surface exposures along the northern and eastern margins, the formation descends to depths of over {{convert|2700|m|ft|-2|abbr=on}} below the surface in southern Pennsylvania.<ref name="geology.com"/>

==Geomorphological expression==
Upturned beds are exposed in sections of the [[Fold (geology)|folded]] [[Ridge-and-Valley Appalachians]],<ref name=PSU/> including exposures on the flanks and axis of the Broad Top [[Synclinorium]] in south central Pennsylvania.<ref name=Allensville>{{cite web |url=http://www.dcnr.state.pa.us/topogeo/openfile/allensville.pdf |archive-url=https://web.archive.org/web/20110609185454/http://www.dcnr.state.pa.us/topogeo/openfile/allensville.pdf |url-status=dead |archive-date=June 9, 2011 |title= Bedrock Geologic Map of the Allensville Quadrangle, Huntingdon and Mifflin Counties, Pennsylvania|work= Open File Report OFBM 07-02.0|publisher=Pennsylvania Geological Survey|author1=McElroy, Thomas A. |author2=Hoskins, Donald M. |page=12}}</ref> Exposed beds are nearly horizontal on the Allegheny Plateau,<ref name="urlOutcrops of the Marcellus Formation">{{cite web |url=http://www.geosc.psu.edu/~engelder/marcellus/marcellus.html |title=Outcrops of the Marcellus Formation|author=Engelder, T.|publisher=The Pennsylvania State University|access-date=2008-05-03}}</ref> but upturned to form slightly overturned beds found along the [[Allegheny Front]].<ref>{{cite journal
 |author1=Engelder, T. 
 |author2=Lash, G. 
 |year=2008 
 |title=Systematic joints in Devonian black shale: A target for horizontal drilling in the Appalachian Basin (draft) 
 |url=http://www.papgrocks.org/engelder%20lash.pdf 
 |access-date=2008-05-03 
 |publisher=Pittsburgh Association of Petroleum Geologists 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20110727163318/http://www.papgrocks.org/engelder%20lash.pdf 
 |archive-date=2011-07-27 
}}</ref>
From [[Wind Gap, Pennsylvania]] heading south, the [[Strike and dip|dip]] of the beds steepens, becoming vertical at [[Bowmanstown, Pennsylvania|Bowmanstown]] on the [[Lehigh River]].<ref name=Lesley1892/> Nearby, in the [[Lehigh Gap]] area of Pennsylvania, the Marcellus is extensively [[Fault (geology)|faulted]],<ref name=Epstein2006>{{cite journal
 | author = Epstein, J.B.
 | year = 2006
 | title = Geology of the Delaware Water Gap National Recreation Area, New Jersey-Pennsylvania
 | journal = Excursions in Geology and History: Field Trips in the Middle Atlantic States
 | page = 52
 | url =  https://books.google.com/books?id=vQdHRNitfOoC&q=Geology+Delaware+Water+Gap+Pennsylvania+Epstein&pg=PA47
 | access-date = 2008-07-07
 | doi = 10.1130/2006.fld008(04)
 | volume = 8
 | publisher = Geological Society of America
 | isbn = 978-0-8137-0008-3
| url-access = subscription
 }}</ref>
and the beds are steeply overturned, with a reverse [[Strike and dip|dip]] angle of up to 40° south.<ref name=Lesley1892/>

The Marcellus Shale and the fine-grained shales near the middle of the Mahantango Formation are classified by geologists as [[slope-former]]s.<ref name=Altamura1999>{{cite journal
 | author = Altamura, R.J.
 | year = 1999
 | title = Geologic Mapping UsingcRadar Imagery in the Ridge and Valley Province
 | url = http://www.dcnr.state.pa.us/topogeo/pub/pageolmag/pdfs/v30n3_4.pdf
 | archive-url = https://web.archive.org/web/20031214180447/http://www.dcnr.state.pa.us/topogeo/pub/pageolmag/pdfs/v30n3_4.pdf
 | url-status = dead
 | archive-date = December 14, 2003
 | journal = Pennsylvania Geology
 | publisher = Pennsylvania Bureau of Topographic and Geologic Survey
 | volume= 30
 | page = 9
 | access-date = 2008-07-13
 | issue = 3/4
}}</ref>
Marcellus and Mahantango shale beds dipping at 60° to 75° to the west form the west facing slopes of [[Tonoloway Ridge]] on the west flank of the [[Cacapon Mountain]] [[anticline]] in the [[Eastern Panhandle of West Virginia]].<ref name=Grimsley1916>{{cite book
 | author = Grimsley, G.P.
 |author2=White, I. C.
 | year = 1916
 | title = Jefferson, Berkley, and Morgan counties
 | series = County Reports
 | publisher = West Virginia Geological Survey
 | location = Morgantown, WV
 | pages = 123, 167, 210, 326
 | lccn = gs17000132
}}</ref>
On the eastern limb of this anticline, beds of these shales  dipping to the east at a shallower angle also form the steep slopes on the east side of [[Warm Springs Ridge]].<ref name=Grimsley1916/>

[[File:Marcellus Bedrock.gif|thumb|left|Bedrock [[geological map]] showing Marcellus bedrock in New York and Pennsylvania]]

The Marcellus is easily [[erosion|eroded]], and is also found underlying low areas between some Appalachian ridges, forming linear valleys of moderate relief. These bedrock surfaces are typically covered with [[colluvium]] from erosion of stratigraphically higher and more erosion-resistant strata that form the surrounding higher ground.<ref name=Allensville/> The soils formed from the Marcellus and the overlying Hamilton shales are deep, free of stones, and well suited for [[agriculture]].<ref name=MacFarlane1890/>
Sampling of soil formed on the Marcellus bedrock showed the dominant [[mineralogy]] consisted of [[quartz]], [[illite]], [[montmorillonite]], [[muscovite]], and [[biotite]], with phases of [[todorokite]] and [[trona]] appearing at depths closer to the bedrock.<ref name="url">{{cite web|url=http://www.juniata.edu/faculty/mathur/Fornadel_Marcellus_LAS.pdf |title=Dirty work: Soil formation and isotope fractionation behind Brumbaugh Academic Center |first=Andrew P. |last=Fornadel |author2=Ryan D. Mathur |author3=Susan L. Brantley |publisher=Juniata College Geology Department, and the Center for Environmental Kinetics Analysis, Pennsylvania State University |access-date=2008-07-13 }}{{dead link|date=January 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

Upturned beds of the soft shale also capture streams and rivers with relatively straight segments in strike valleys such as the [[Aquashicola Creek]] and [[McMichael Creek]] at the foot of [[The Poconos]],<ref name=Lesley1892/>
and the long, straight section of the [[Lost River (Cacapon River)|Lost River]] in West Virginia.<ref name=Rich1939>{{cite journal
 | author = Rich, J.L.
 | year = 1939
 | title = A Bird's-Eye Cross Section of the Central Appalachian Mountains and Plateau: Washington to Cincinnati
 | journal = Geographical Review
 | volume = 29
 | issue = 4
 | pages = 561–586
 | doi = 10.2307/209829
 | publisher = Geographical Review, Vol. 29, No. 4
| jstor=209829
| bibcode = 1939GeoRv..29..561R
 }}</ref>
Below [[Port Jervis, New York]], the Walpack Ridge deflects the [[Delaware River]] into the Minisink Valley, where it follows the southwest [[Strike and dip|strike]] of the eroded Marcellus beds along the Pennsylvania – New Jersey state line for {{convert|40|km|mi|0|abbr=on}} to the end of the ridge at Walpack Bend in the [[Delaware Water Gap National Recreation Area]].<ref name=White>{{cite web |url=http://www.njgeology.org/enviroed/newsletter/v2n1.pdf |title= Karst in the Delaware Water Gap National Recreation Area |author1=White, Ron W. |author2=Monteverde, Donald H. |work= Unearthing New Jersey Vol. 2, No. 1 | date=2006-02-01 |access-date=2008-06-07|publisher=New Jersey Geological Survey}}</ref><ref name=White1882>{{cite book
 | author = White, I.C.
 |author2=Chance, H.M.
 | year = 1882
 | title = The geology of Pike and Monroe counties
 | series = Second Geol. Surv. of Penna.
 | location = Harrisburg
 | volume = Rept. of Progress, G6
 | pages = 17, 73–80, 114–115
}}</ref>
The Minisink is a [[buried valley]] where the Delaware flows in a bed of [[glacier|glacial]] [[till]] that buried the eroded Marcellus bedrock during the [[last glacial period]]. This buried valley continues along the strike of the Marcellus southwest from the bend through [[Stroudsburg, Pennsylvania]], and northeast from Port Jervis toward the [[Hudson River]],<ref name=White1882/>
along the route of the [[Delaware and Hudson Canal]].<ref name=Lesley1892/>

[[File:Eastern North American Paleogeography Middle Devonian.gif|alt=Map of north-eastern North America showing the Illinois Basin, the Michigan Basin and to the east, the Appalachian Basin.|thumb|right|[[Palaeogeography|Paleogeographic]] reconstruction of the [[Appalachian Basin]] area during deposition of the Marcellus<ref name=autogenerated1>{{cite web |url=http://jan.ucc.nau.edu/rcb7/nam.html |title=Paleogeography and Geologic Evolution of North America |first=Ron |last=Blakey |publisher=Northern Arizona University |work=Global Plate Tectonics and Paleogeography |access-date=2008-07-04 |archive-url=https://web.archive.org/web/20080621201253/http://jan.ucc.nau.edu/rcb7/nam.html |archive-date=2008-06-21 |url-status=dead }}</ref>]]

==Stratigraphy==
[[Stratigraphy|Stratigraphically]], the Marcellus is the lowest unit of the [[Devonian]] age Hamilton Group, and is divided into several sub-units. 
In the first [[Pennsylvania Geological Survey]], begun in 1836, [[Henry Darwin Rogers]] classified the Marcellus as the "Cadent Lower Black Slate" which he numbered "No. VIII b."<ref name=Dana1895>{{cite book
 | author = Dana, J.D.
 | year = 1895
 | title = Manual of Geology: Treating of the Principles of the Science with Special Reference to American Geological History
 | publisher = American book company
 | location = New York
 | edition = 4th ed.\
 | chapter = Historical Geology
 | chapter-url = http://www.geology.19thcenturyscience.org/books/1896-Dana-ManGeol/htm/doc728.html
 | page = 728
 | lccn = 04004705
}}</ref> In the first [[New York State Geological Survey]], also begun that year, [[James Hall (paleontologist)|James Hall]] established the term "Marcellus Shale" in his 1839 report titled "''Marcellus Shales in Seneca County.''"<ref name=Bulletin66>{{cite journal
 | author = Ellis, Mary
 | year = 1903
 | title = Index to Publications of the New York State Natural History Survey and New York State Museum 1837–1902
 | publisher = University of the State of New York
 | journal = New York State Museum Bulletin
 | page = 429
 |location=Albany
 | issue = 66
 | lccn = 03028299
}}</ref> Professor Hall also argued in 1839 against formulating geological names based on observed characteristics that may vary from place to place or need revision in the future, and in favor of location-based nomenclature where "the rock or group will receive its name from the place where it is best developed."<ref>{{cite book|title=The Texan Permian and Its Mesozoic Types of Fossils|first=Charles|last=white|year=1891|publisher=United States Geological Survey|location=Washington D.C.|page=401}}</ref> His arguments proved persuasive, and the location-based name for this, and many of the other group names he published based on exposures in New York, were adopted in the second Pennsylvania survey, and are now widely accepted.

===Overlying units===
In the first New York survey, the Marcellus Shale was placed below the Hamilton Group at the base of the Erie division of the New York system, but this taxonomy is obsolete.<ref name=White1882/>

In current practice, the Marcellus Shale&nbsp;(abbr. Dm or Dms) is classified as the basal unit of the [[Hamilton Group]]&nbsp;(Dh),<ref name=Martin/> lying beneath the [[Mahantango Formation]]&nbsp;(Dmh) member of this group in Pennsylvania<ref name="explanation"/> and Maryland. In New York, the Mahantango, also of Middle Devonian age, is further divided. There the Marcellus is separated from the overlying Skaneateles Formation, a more clastic and fossiliferous dark shale, by the thin Stafford or Mottville Limestone bed.<ref name=Grasso1968/><ref name=Martin2004>{{cite journal
| author1 = Martin, J.P.
| author2 = Hill, D.G.
| author3 = Lombardi, T.E.
| year = 2004
| title = Fractured Shale Gas Potential In New York
| journal = Northeastern Geology and Environmental Sciences
| volume = 26
| issue = 1/2
| pages = 57–78
}}</ref>

In West Virginia, the Marcellus may be separated from the brown shales of the Mahantango by occasional [[sandstone]] beds and [[concretions]],<ref name=USGS2006-5198>{{cite journal
 |author1=Boughton, Carol J. |author2=McCoy, Kurt J. | publisher = [[U.S. Geological Survey]]
 | year = 2006
 | title = Hydrogeology, Aquifer Geochemistry, and Ground-Water Quality in Morgan County, West Virginia
 |journal=USGS Report |series=Scientific Investigations Report |page=81 |doi=10.3133/sir20065198 |bibcode=2006usgs.rept...81B | url = http://pubs.usgs.gov/sir/2006/5198/
 | id= Scientific Investigations Report 2006-5198
|url-access=subscription }}</ref>
or it may lie directly below the younger [[Late Devonian]] [[Harrel Formation]] (or its lateral equivalents) because of a  [[disconformity]],<ref name=Repetski2002>{{cite journal
 | author1 = Repetski, J.E.
 | author2 = Ryder, R.T.
 | author3 = Harper, J.A.
 | author4 = Trippi, M.H.
 | year = 2002
 | title = Thermal maturity patterns (CAI and% Ro) in the Ordovician and Devonian rocks of the Appalachian basin in Pennsylvania
 | journal = US Geological Survey Open-File Report
 | page = 10
}}</ref> which represents a gap in the geological record due to a period of erosion or non-deposition.
In eastern Ohio the Hamilton Group also lies disconformably beneath the Rhinestreet Shale Member of the [[West Falls Formation]], another transgressive black shale tongue with similar characteristics to the Marcellus.<ref name=Wickstrom/>

===Underlying units===
The Marcellus Shale is typically found deposited on the limestone of the [[Onondaga Formation]]&nbsp;(Don), which extends down to the end of the [[Early Devonian]] period. The contact between them may be sharp, gradational, or erosional. In southwestern Ontario, Canada, north of [[Lake Erie]], the Marcellus overlies the Dundee Formation, a lateral equivalent of the Onondaga.<ref name=Rickard1984/><ref name="Southern Ontario">{{cite web|url=http://www.mndm.gov.on.ca/mndm/mines/resgeol/southern/southwestern/geo_e.asp |title=Geology of the Southwestern District |work=Resident Geologist Program |access-date=2008-04-02 |date=2004-05-21 |publisher=Ministry of Northern Development and Mines, Government of Ontario |url-status=dead |archive-url=https://web.archive.org/web/20070922120056/http://www.mndm.gov.on.ca/mndm/mines/resgeol/southern/southwestern/geo_e.asp |archive-date=September 22, 2007 }}</ref>
In Pennsylvania, the Marcellus forms a sharp [[Unconformity|conformable]] contact with the Onondaga's Selinsgrove Limestone member.<ref name=OFBM>{{cite web |url=http://www.dcnr.state.pa.us/topogeo/openfile/midsusque.pdf |archive-url=https://web.archive.org/web/20110609184558/http://www.dcnr.state.pa.us/topogeo/openfile/midsusque.pdf |url-status=dead |archive-date=June 9, 2011 |title= Preliminary Bedrock Geologic Map of the Middle Portion of the Susquehanna River Valley, Cumberland, Dauphin, And Perry Counties, Pennsylvania|year=2007|access-date=2008-01-26|author1=Jackson, Margaret S. |author2=Hanley, Peter M. |author3=Sak, Peter B. |name-list-style=amp |publisher=[[Pennsylvania Geological Survey]]|work=Open File Report OFBM-07-05.0}}</ref>
A thin [[pyrite-carbonate]] bed is also found at the base of the Marcellus black shale in the exposures of south central Pennsylvania, above a thin calcerous green shale bed, which lies upon the Onondaga limestone.<ref name=Dewees1878/>

In eastern New York, the contact between the Marcellus and Onondaga (where present) is gradational.<ref name=Oliver1976>{{cite journal
 | author = Oliver Jr, W.A.
 | year = 1976
 | title = Noncystimorph colonial rugose corals of the Onesquethaw and Lower Cazenovia Stages (Lower and Middle Devonian) in New York and adjacent areas
 | journal = Geological Survey Professional Paper
 | volume = 869
 | pages = 1–156
}}</ref><ref name=Martin2002/>
In western New York, the Union Springs member of the Marcellus conformably overlies the Seneca member of the Onondaga Limestone,<ref name=Sageman2003/>
or the stratigraphically higher Cherry Valley Limestone member may rest directly and unconformably upon the Onondaga in the absence of the Union Springs shale.<ref name=Lash_AAPG_pg1/><ref name=Rickard1984>{{cite journal
| author = Rickard, L.V.
| year = 1984
| title = Correlation of the subsurface Lower and Middle Devonian of the Lake Erie region
| journal = Bulletin of the Geological Society of America
| volume = 95
| issue = 7
| pages = 814–828
| url = http://bulletin.geoscienceworld.org/cgi/content/abstract/95/7/814
| access-date = 2008-04-03
| doi = 10.1130/0016-7606(1984)95<814:COTSLA>2.0.CO;2
| issn = 0016-7606
|bibcode = 1984GSAB...95..814R | url-access = subscription
}}</ref>
The local disappearance of units of the Onondaga suggests that its upper contact with the Marcellus can be erosional.<ref name=Lash_AAPG_pg1>{{cite web|url=http://www.searchanddiscovery.net/documents/2007/07022lash/images/Lash_AAPG_pg1_red.pdf|title=Influence of Basin Dynamics on Upper Devonian Black Shale Deposition, Western New York State and Northwest Pennsylvania|author=Lash, Gary|access-date=2008-05-27|publisher=American Association of Petroleum Geologists}}</ref>
In [[Erie County, New York|Erie County]] in western New York, both the upper and lower contact of the Marcellus are eroded away.<ref>{{cite journal|url=http://www.buffalocrushedstone.com/uploads/uploads5400/BCSDEISAsAccepted.pdf |title=Draft Environmental Impact Statement, Modification of Permit to Mine, Buffalo Crushed Stone Quarry |author=TVGA Consultants |publisher=New York State Department of Environmental Conservation |date=2007-08-29 |page=16 |url-status=dead |archive-url=https://web.archive.org/web/20110708103420/http://www.buffalocrushedstone.com/uploads/uploads5400/BCSDEISAsAccepted.pdf |archive-date=2011-07-08 }}</ref>

In eastern West Virginia the Marcellus overlies the Onesquethaw Group, consisting of the dark gray or green, calcitic, mostly nonfissil Needmore Shale, which grades westward into the Huntersville Chert.<ref name=USGS2006-5198/>
To the south and west, the Hamilton Group grades laterally into the Millboro Shale formation in southern West Virginia and Virginia,<ref name=USGS2006-1237/>
which grades into the lower part of the Chattanooga Shale of [[Tennessee]].<ref name="Virginia"/>

The Milboro is gradational with the underlying Needmore Formation shale.<ref name="Virginia">{{cite web|url=http://www.dmme.virginia.gov/DMR3/dmrpdfs/Giles_Geologic_Descriptions.pdf |title=Map Unit Descriptions |publisher=Virginia Department of Mines, Minerals and Energy |date=2007-09-28 |url-status=dead |archive-url=https://web.archive.org/web/20080211213112/https://www.dmme.virginia.gov/DMR3/dmrpdfs/Giles_Geologic_Descriptions.pdf |archive-date=2008-02-11 }}</ref>
South of the [[Mason-Dixon line]], due to the difficulty in differentiating the Millboro and Needmore shales with the limited exposures available,<ref name=MGS1913>{{cite book
 | author = Maryland Geological Survey
 | year = 1913
 | title = Middle and Upper Devonian, text.
 | series = Reports Dealing with the Systematic Geology and Paleontology of Maryland
 | publisher = Johns Hopkins Press
 | location = Baltimore
 | pages = 40–48
 | chapter = Adoption of the Romney Formation
 | chapter-url= https://books.google.com/books?id=169LAAAAMAAJ&pg=PA46
 | lccn = gs13000808
 }}</ref>
and initial uncertainty in correlation with the New York survey, they were mapped as the Romney Formation, a unit containing all the Middle Devonian strata,<ref name=Grimsley1916/>
named for an exposure at [[Romney, West Virginia]].<ref name=MGS1918>{{cite book | author = Clark, W. B. |author-link=William Bullock Clark| title = The geography of Maryland | series = Maryland Geological Survey | volume = 10 | year = 1918 | location = Baltimore | publisher = Johns Hopkins Press}}</ref>
The correlations were established by 1916 through tracing the New York exposures across Pennsylvania and Maryland into West Virginia, so under the principle of [[scientific priority]],<ref name=Grimsley1916/>
the Romney classification is now obsolete; but its Marcellus and underlying Needmore shale members are still found grouped in an undifferentiated map unit&nbsp;(Dmn).<ref>{{cite journal
 |author1=Donovan, Joseph J. |author2=Eberhard Werner |author3=Dorothy J. Vesper |author4=Lacoa Corder | year = 2006
 | title = Springs, source water areas, and potential for high-yield aquifers along the Cacapon Mountain anticline, Morgan County, WV
 | url = http://www.hrc.nrcce.wvu.edu/HRC3/HRC3-MorganCounty-Final%20Report2006.pdf
 | access-date = 2008-05-26
 | publisher = [[West Virginia University]]
 | id = HRC-3
}}</ref>

[[File:USGS 2006 1237 Fig7b.gif|thumb|center|upright=3.75|Geologic cross section of upper to middle Devonian strata from [[Cherry Valley (town), New York|Cherry Valley]], [[New York (state)|New York]] southwest across the [[Allegheny Plateau]] and then along the [[Ridge-and-Valley Appalachians]] to [[Tennessee]].<ref name="pubs.er.usgs.gov">Milici, R.C., and Swezey, C.S., 2006, Assessment of Appalachian Basin oil and gas resources: Devonian Shale-Middle and Upper Paleozoic Total Petroleum System: U.S. Geological Survey Open-File Report 2006–1237, 70p.  http://pubs.er.usgs.gov/publication/ofr20061237</ref> Note the Marcellus grades up to the Milboro and Chattanooga black shales.<ref name=USGS2006-5198/>]]
{{Clear}}

====Tioga ash beds====
Tioga metabentonite or K-[[bentonite]]–stratigraphic unit about {{convert|.6|m|ft|0|abbr=on}} thick that consists of several discrete, relatively thin [[volcanic ash]] falls–is also included at the base of the Marcellus in eastern Pennsylvania.<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>
In 1843 it was described without being named by Hall,<ref name=Collins1979>{{cite journal
 |author=Collins, H.R.
 |year= 1979
 | title = Devonian Bentonites in Eastern Ohio: GEOLOGIC NOTES
 | doi = 10.1306/2F9181AD-16CE-11D7-8645000102C1865D
 | journal = AAPG Bulletin
 | volume = 63
}}</ref>
and more than 100 years passed before it was eventually named for the [[natural gas field]] in [[Tioga County, Pennsylvania]],<ref name=Dennison1970>{{cite journal
 | author = Dennison, J.M.
 |author2=Textoris, D.A.
 | year = 1970
 | title = Devonian tioga tuff in Northeastern United States
 | journal = Bulletin Volcanologique
 | volume = 34
 | issue = 1
 | pages = 289–294
 | doi = 10.1007/BF02597791
|bibcode = 1970BVol...34..289D |s2cid=129708915
 }}</ref>
where it was encountered when [[borehole|drilling]] [[Oil well|gas wells]]. It is a regional stratigraphic marker,<ref name=USGS2006-1237/>
used by geologists to identify the Marcellus,<ref name=Scheetz2007>{{cite journal|author=Scheetz, B.E. |author2=Ellsworth, C.J. |date=2007-07-31 |title=Preliminary Assessment of Acid Producing Rock on Future PENNDOT Construction, Final Report |url=http://www.mautc.psu.edu/docs/PSU-2005-05.pdf |access-date=2008-06-14 |id=FHWA-PA-2007-022-510401-05; PTI 2007–53 |publisher=[[National Technical Information Service]] (U.S.) |pages=41–42 |url-status=dead |archive-url=https://web.archive.org/web/20100708071702/http://www.mautc.psu.edu/docs/PSU-2005-05.pdf |archive-date=July 8, 2010 }}</ref>
and correlate laterally equivalent strata.<ref name=Rickard1984/><ref name=Dennison1976/> Difficulty in correctly identifying the more than 80 different ash falls during the Devonian period, collected in 15 or more beds,<ref name=Straeten2004/> has also led to many miscorrelations.<ref name=Indiana/>

From Virginia to New York the Tioga is widely distributed, running across the central and northern parts of the Appalachian basin,<ref name=Kaufmann2006/>
an areal extent exceeding {{convert|265000|km2|sqmi|-3|abbr=on}}.<ref name=Dennison1986/>
[[Explosive eruption]]s associated with the [[Acadian orogeny]]<ref name=Harper2004/> originating near present-day central Virginia released the ash into the atmosphere.<ref name=Dennison1970/>
It was dispersed across the [[Appalachian Basin|Appalachian]], [[Michigan Basin|Michigan]], and [[Illinois Basin|Illinois]] Basins by the southern [[trade winds]], because this area was in the southern hemisphere during the Devonian period.<ref name=Dennison1986>{{cite book
 | author = Dennison, J.M.
 | year = 1986
 | title = Tioga Bentonite in the Appalachian Basin: Final report
 | publisher = U.S. Department of Energy
 | id = DOE/ET/12139-T2
}}</ref>
The volcanic origin of the ash is evidenced by its distinctive [[mineralogy]]–the ash was deposited directly upon the water, so its angular quartz grains differ from the clastic sediments rounded through the erosion process that carries them to the sea.<ref name=Indiana/> As the volcanic ash settled to the bottom, it was [[mixture|admixed]] with these terrigenous components, producing a distinctive lithology in the sedimentary rock.<ref name=Indiana/>

[[File:Marcellus I80 MP307.jpg|thumb|Marcellus exposure along [[Interstate 80]] in eastern Pennsylvania where the formation is thickest.]]

The Tioga may appear in the formation as a gray, brown, black, or olive bed, or parting,<ref name=Indiana>{{cite web |url=http://igs.indiana.edu/geology/structure/compendium/html/comp1lb8.cfm |title=Tioga Bentonite Bed |first = John B.|last=Droste|author2=Robert H. Shaver | year = 1997 |work=Compendium of Paleozoic Rock-Unit Stratigraphy in Indiana&nbsp;— A Revision |access-date=2008-06-12}}</ref>
consisting of coarse crystal [[tuff]] or tuffaceous shale,<ref name=Dennison1970/>
thinly laminated, with sand-sized mica flakes.<ref name="Maryland1968">{{cite web |url=http://www.mgs.md.gov/esic/geo/lgdalleg.html |title=Geologic Map Legends : Allegheny Plateau and Valley and Ridge|work =Geologic Map of Maryland | year =1968| publisher = Maryland Geological Survey |access-date=2008-06-12}}</ref>
The Tioga ash bed zone consists of eight ash beds labeled according to their stratigraphic order from A (oldest) to H (youngest),<ref name=Kaufmann2006>{{cite journal
 | author = Kaufmann, B.
 | year = 2006
 | title = Calibrating the Devonian Time Scale: A synthesis of U—Pb ID—TIMS ages and conodont stratigraphy
 | journal = Earth-Science Reviews
 | volume = 76
 | pages = 175–190
 | doi = 10.1016/j.earscirev.2006.01.001
|bibcode = 2006ESRv...76..175K
 | issue = 3–4 }}</ref><ref name=Way1986>{{cite conference
 | author1 = Way, J.H.
 | author2 = Smith, R.C.
 | author3 = Roden, M.
 | year = 1986
 | title = Detailed correlations across 175 miles of the Valley and Ridge of Pennsylvania using 7 ash beds in the Tioga Zone
 | conference = 51st Annual Field Conference of Pennsylvania Geologists
 | book-title = Selected geology of Bedford and Huntington Counties
 | pages = 55–72
 | publisher = U.S. Geological Survey
}}</ref>
and another bed known as the Tioga middle coarse zone.<ref name=Straeten2004>{{cite journal
 | author = Ver Straeten, C.A.
 | year = 2004
 | title = K-bentonites, volcanic ash preservation, and implications for Early to Middle Devonian volcanism in the Acadian orogen, eastern North America
 | journal = Bulletin of the Geological Society of America
 | volume = 116
 | issue = 3–4
 | pages = 474–489
 | doi = 10.1130/B25244.1
|bibcode = 2004GSAB..116..474V }}</ref><ref name=Dennison1986/>
Its basal beds are found within the uppermost beds of the Onondaga Limestone or Needmore Shale, and the uppermost ash bed within the lowermost part of the Marcellus or Millboro Shale.<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 |work=Open-File Report Series 2006-1237| publisher=[[United States Geological Survey]]}}</ref>
In western New York state, the Tioga Ash Bed B marks the boundary between the Moorehouse and Seneca Members of the Onondaga Formation,<ref name=Alden2001>{{cite map |title=New York State Geological Highway Map |year=1990 |author=Isachsen, Y. W. |author2=T. D. Mock |author3=R. E. Nyahay |author4=W. B. Rogers |series=Educational Leaflet  No. 33 |publisher=New York State Museum |url=http://geology.about.com/library/bl/maps/n_statemap_NYexplan.htm |id=Plate 3, Legend for the Geologic Map |access-date=2008-06-13 |archive-date=2007-11-27 |archive-url=https://web.archive.org/web/20071127042335/http://geology.about.com/library/bl/maps/n_statemap_NYexplan.htm |url-status=dead }}</ref>
but in the central part of the state, and the southern part of the basin, the ash beds are actually in the Marcellus.<ref name=Martin2002/><ref name=Kaufmann2006/><ref name=Koch1981>{{cite journal
 | author = Koch Ii, W.F.
 | year = 1981
 | title = Brachiopod community paleoecology, paleobiogeography, and depositional topography of the Devonian Onondaga Limestone and correlative strata in eastern North America
 | journal = Lethaia
 | volume = 14
 | issue = 2
 | pages = 83–103
 | doi = 10.1111/j.1502-3931.1981.tb01909.x
| bibcode = 1981Letha..14...83K
 }}</ref>
This indicates that deposition of the Marcellus there began earlier,<ref name=Martin2002>{{cite journal
 |author=Martin, R.L. 
 |year=2002 
 |title=Taxonomic Revision and Paleoecology of Middle Devonian (Eifelian) Fishes of the Onondaga, Columbus and Delaware Limestones of the eastern United States 
 |access-date=2008-07-07 
 |publisher=West Virginia University 
 |url=http://kitkat.wvu.edu:8080/files/2728/Martin_Robert_dissertation.pdf 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20060907001801/http://kitkat.wvu.edu:8080/files/2728/Martin_Robert_dissertation.pdf 
 |archive-date=2006-09-07 
}}</ref>
since the ash beds represent a single epoch in geologic time.

===Thickness===
Maximum thickness of the Marcellus ranges from {{convert|270|m|ft|-1|abbr=on}} in New Jersey,<ref name=White/>  to {{convert|12|m|ft|-1|abbr=on}} in Canada.<ref name=Singer/>
In West Virginia, the Marcellus Shale is as much as {{convert|60|m|ft|-1|abbr=on}} thick.<ref name=USGS2006-5198/>
In extreme eastern Pennsylvania, it is {{convert|240|m|ft|-1|abbr=on}} thick,<ref name=White1882/>
thinning to the west, becoming only {{convert|15|m|ft|0|abbr=on}} thick along the [[Ohio River]], and only a few feet in [[Licking County, Ohio]].<ref name=dispatch>{{cite web|url=http://www.dispatch.com/live/content/science/stories/2008/03/11/Sci_shale.ART_ART_03-11-08_B4_A99I7HO.html?print=yes |title=Low down, rich and stingy |first=Kevin |last=Mayhood |access-date=2008-04-04 |date=2008-03-11 |work=The Columbus Dispatch |url-status=dead |archive-url=https://web.archive.org/web/20110523030900/http://www.dispatch.com/live/content/science/stories/2008/03/11/Sci_shale.ART_ART_03-11-08_B4_A99I7HO.html?print=yes |archive-date=2011-05-23 }}</ref>
The thinning, or [[Stratigraphy|stratigraphic]] convergence, from east to west is caused by decreasing grain size in the [[clastic rock|clastic]] deposits, which entered the basin from the east.<ref name=Dennison1976>{{cite journal
| author = Dennison, J.M.
|author2=Hasson, K.O.
| year = 1976
| title = Stratigraphic cross section of Hamilton Group (Devonian) and adjacent strata along south border of Pennsylvania
|journal=AAPG Bulletin
|volume= 60
| issue = 2
| pages = 278–287
| doi= 10.1306/83D922BC-16C7-11D7-8645000102C1865D
}}</ref>
The beds finally "pinch out" westward because deposition was limited by the [[Cincinnati Arch]],<ref name=Wickstrom>{{cite web|url=http://www.dnr.state.oh.us/Portals/10/pdf/BurgerReport.pdf |title=Geologic Assessment of the Burger Power Plant and Surrounding Vicinity for Potential Injection of Carbon Dioxide |first=Lawrence H. |last=Wickstrom |author2=Ernie R. Slucher |author3=Mark T. Baranoski |author4=Douglas J. Mullett |publisher=Ohio Geological Survey |year=2008 |access-date=2008-06-21 |location=Columbus, Ohio |id=Open-File Report 2008-1 |url-status=dead |archive-url=https://web.archive.org/web/20081231180506/http://www.dnr.state.oh.us/Portals/10/pdf/BurgerReport.pdf |archive-date=2008-12-31 }}</ref><ref name=Roen1996>{{cite book
 | author = Roen, J.
 |author2=Walker, B.
 | year = 1996
 | title = The Atlas of Major Appalachian Gas Plays
 | publisher = West Virginia Geological and Economic Survey
 | id = Publication V-25
}}</ref>
the bulge that formed the west shore of the basin. Where the formation is relatively thick, it is divided into several members, and as the formation continues to thicken to the east, these members are further divided. Some workers chose to classify the Marcellus as a subgroup, and classify some of the members as separate formations.

{| border="0" cellspacing="0" cellpadding="10" style="float:right;"
|
{| style="border-top:1px solid gray; border-bottom:1px solid gray; border-right:1px solid gray; border-left:1px solid gray;"
|
{| border="0" cellpadding="0" style="text-align: left;"
|
{| class="wikitable" style="text-align: center;"
|+'''Members of the Marcellus Formation from West to East in New York'''
|-

! Buffalo
! &nbsp;Rochester&nbsp;
! &nbsp;Syracuse&nbsp;
! Utica
! Albany
|-
| rowspan="4" colspan="2" |

Oatka Creek shale
| rowspan="3" |

Cardiff<br />dark gray<br />shale
| Pecksport shale
| (Mt. Marion Fm.)
|-
| colspan="2" | Solsville shale and sandstone
|-
| Bridgewater shale
| Otsego
|-
| colspan="2" | Chittenango black shale
| Berne
|-
| colspan="4" | Cherry Valley limestone
| rowspan="2" | Stony Hollow<br />shale & limestone
|-
| rowspan="2" | (Seneca Mb.)<br />(Onondaga Fm.)
| colspan="3" | Union Springs shale and limestone
|-
| colspan="2" | (Onondaga Fm.)
| colspan="2" | Union Springs shale and limestone
|}
|-
|Table showing approximate correlation between Marcellus Formation<br />named members from west to east across central New York state.<ref name="nyahay">{{cite web |url=http://www.searchanddiscovery.net/documents/2007/07101nyahay/index.htm | author = Nyahay, Richard |author2=James Leone |author3=Langhorne Smith |author4=John Martin |author5=Daniel Jarvie |title= Update on the Regional Assessment of Gas Potential in the Devonian Marcellus and Ordovician Utica Shales in New York|format=PDF |publisher =American Association of Petroleum Geologists | date =September 2007 |work= 2007 AAPG Eastern Section Meeting, September 16–18, Lexington, Kentucky|id= Search and Discovery Article #10136 (2007)|access-date=2008-05-25}}</ref><ref name=Grasso1968>{{cite journal
 | author = Grasso, T.X.
 | year = 1968
 | title = A new coral bed in the Hamilton Group (Middle Devonian) of central New York
 | journal = Journal of Paleontology
 | volume = 42
 | issue = 1
 | pages = 84–87
| jstor=1302130
}}</ref>
|}
|}
|}

===Named members===
A local Purcell limestone member, {{convert|15|to|30|m|ft|0|abbr=on}} of inter-bedded calcitic shale and limestone,<ref name=Dennison1976/>
divides the Marcellus in eastern Pennsylvania.<ref name="explanation"/>
The Purcell is stratigraphically equivalent to the Cherry Valley Limestone member in New York,<ref name=Stein1982>{{cite journal
 | author = Stein Jr, W.E.
 | year = 1982
 | title = Iridopteris eriensis from the Middle Devonian of North America, with Systematics of Apparently Related Taxa
 | journal = Botanical Gazette
 | volume = 143
 | issue = 3
 | pages = 401–416
 | doi = 10.1086/337316
| jstor=2474838
| s2cid = 84596464
 }}</ref>
a [[bioclast]]ic [[packstone]],<ref name=Sageman2003/> consisting of skeletal limestones, with shaly intervals<ref name=nyahay/> between its lower massive limestone layer, thick nodular limestone/[[marlstone]], and upper limestone layer.<ref name="Chestnut Street"/>
Other named members include the Bakoven Shale, Cardiff Shale, Chittenango shale, Solsville sandstone, Union Springs shale and limestone,<ref name=paleo/> and Stony Hollow shale and limestone.<ref name=paleo2>{{cite web |url=https://paleodb.org/cgi-bin/bridge.pl?action=displayStrata&group_hint=Hamilton&formation_hint=Marcellus%20Shale&group_formation_member=Stony%20Hollow |title= Stony Hollow Member|access-date=2008-05-23 |work=The Paleobiology Database}}</ref> The Union Springs, Cherry Valley, and Oatka Creek merge beneath Lake Erie, into the Bell Shale, Rockport Quarry Limestone, and Arkona Shale of Ontario.<ref name=Rickard1984/>

The Union Springs is an organic-rich, pyritiferous, thinly bedded, blackish gray to black shale with mudstone concretionary layers,<ref name="Chestnut Street">{{cite web |url=http://bingweb.binghamton.edu/~kwilson/Devonian/DevSites/Chestnut_Street/Chestnut_Street.htm |title=Lower Marcellus Formation&nbsp;– the Chestnut Street Road Cut |first=Karl A. |last =Wilson | publisher = State University of New York at Binghamton |work= New York Paleontology|access-date=2008-05-23|date=2006-09-27}}</ref> and thin silt bands at the bottom. To the east, it becomes the Bakoven Member, a darker, less organic shale with fewer limestone layers.<ref name=nyahay/> To the west the Union Springs beds thin, with its upper limestones merging with the overlying [[calcareous]] Cherry Valley Member.<ref name=nyahay/> A regional unconformity appears in western New York, as the Union Springs lenses in and out,<ref name=nyahay/> and then reappears in northwest Pennsylvania and northeast Ohio between the Onondaga and Cherry Valley.<ref name=Rickard1984/>

In Western and central New York, the uppermost member  is the dark grey to black organic-rich Oatka Creek shale. Unlike the other Devonian shales in this region, the gray shale at the top of the Oatka Creek thickens gradually to the west, as well as the east,<ref name=Lash_AAPG_pg2>{{cite web|url=http://www.searchanddiscovery.net/documents/2007/07022lash/images/Lash_AAPG_pg2_red.pdf|title=Influence of Basin Dynamics on Upper Devonian Black Shale Deposition, Western New York State and Northwest Pennsylvania, Page 2|author=Lash, Gary|access-date=2008-05-27|publisher=American Association of Petroleum Geologists}}</ref>
where it divides into the Cardiff member lying above the Chittenango member in central New York.<ref name=nyahay/>
Organic-rich, fissile, sooty black shales make up the Chittenango Member.<ref name="Chestnut Street"/>
At the base of the Chittenango,<ref name=Kasprak2008>{{cite journal
 | author = Kasprak, A.H.
 | year = 2008
 | title = Nowakia (dacryoconarida) In The Halihan Hill Bed (oatka Creek Formation, New York)
 | url = http://gsa.confex.com/gsa/2008NE/finalprogram/abstract_134728.htm
 | publisher = Geological Society of America
 | access-date = 2008-05-26
 | archive-url = https://web.archive.org/web/20110608152346/http://gsa.confex.com/gsa/2008NE/finalprogram/abstract_134728.htm
 | archive-date = 2011-06-08
 | url-status = dead
 }}</ref>
above the Bierne Member shale,<ref name=Bartholomew2007>{{cite journal
 | author = Bartholomew, A.
 | author2 = Schramm, T.
 | year = 2007
 | title = Faunal Turnover Between Two Ee Subunits: Investigating The Timing Of Large-scale Faunal Turnover In The Latest Eifelian Of Eastern North America
 | journal = 2007 GSA Denver Annual Meeting
 | url = http://gsa.confex.com/gsa/2007AM/finalprogram/abstract_132498.htm
 | access-date = 2008-05-26
 | archive-url = https://web.archive.org/web/20080608131936/http://gsa.confex.com/gsa/2007AM/finalprogram/abstract_132498.htm
 | archive-date = 2008-06-08
 | url-status = dead
 }}</ref>
lies the Halihan Hill Bed, a highly [[Bioturbation|bioturbated]] bioclastic limestone.<ref name=Sageman2003/>

Further east, the homogeneous Cardiff divides into the Bridgewater, Solsville, and Pecksport shale members, from base to top. The Bridgewater is a fissile dark silty shale with relatively rare fossils. A thin concretionary zone lies above, then the Solsville grades from a gray calcareous shale, to sandy siltstones and fine sandstones at the top, with the gray shale of the Pecksport shale and siltstone overlying it.<ref name="Swamp Road">{{cite web |url=http://bingweb.binghamton.edu/~kwilson/Devonian/DevSites/SwampRd/swamprd.htm |title=The Marcellus Formation&nbsp;— The Swamp Road Roadcut |first=Karl A. |last =Wilson | publisher = State University of New York at Binghamton |work= New York Paleontology|access-date=2008-05-23|date=2007-03-31}}</ref>

In south central Pennsylvania, the Marcellus is mapped with three members, from top to base: The Mahanoy Member&nbsp;(Dmm), a dark gray to grayish black silty shale and siltstone; the Turkey Ridge Member&nbsp;(Dmt), an olive to dark-gray fine to medium grained sandstone; and the Shamokin Member&nbsp;(Dms), a dark gray to grayish black fissile carbonaceous shale that is calcareous in places near the base.<ref name=A126>{{cite map|title=Geologic Map of the Mifflintown Qd., PA |url=http://www.libraries.psu.edu/emsl/guides/pageomaps/A126_1.html |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]] |url-status=dead |archive-url=https://web.archive.org/web/20071203022334/http://www.libraries.psu.edu/emsl/guides/pageomaps/A126_1.html |archive-date=December 3, 2007 }}</ref> The Turkey Ridge is commonly mapped in the Mahantango Formation,<ref name="ORF 95-04">{{cite map|author=MacLachlan, D.B., hoskins, D.M., Payne, D.F.|year=1995|title=Bedrock Geology of the Freeburg 7.5 minute Quadrangle, Snyder County, Pennsylvania|series=Open-File Reports|url=http://www.dcnr.state.pa.us/topogeo/pub/openfile/pdfs/of95_04_freeburg.pdf|archive-url=https://web.archive.org/web/20110609235714/http://www.dcnr.state.pa.us/topogeo/pub/openfile/pdfs/of95_04_freeburg.pdf|url-status=dead|archive-date=June 9, 2011|format=PDF|id= ORF 95-04|publisher=[[Pennsylvania Geological Survey]], Fourth Series|cartography=[[United States Geological Survey]]}}</ref> or included in the Montebello Formation&nbsp;(Dmot),<ref>{{cite map|title=Geologic Map of the Loysville Qd., PA |url=http://www.libraries.psu.edu/emsl/guides/pageomaps/A127_1.html |id=A127 |scale=1:24,000 |author=Miller, John T. |publisher=[[Pennsylvania Geological Survey]], Fourth Series |cartography=[[United States Geological Survey]] |url-status=dead |archive-url=https://web.archive.org/web/20071203022339/http://www.libraries.psu.edu/emsl/guides/pageomaps/A127_1.html |archive-date=December 3, 2007 }}</ref> and only the Shamokin correlates with the Marcellus on adjacent map sheets.<ref name=A136>{{cite map|title=Geologic Map of the Millerstown 15 Minute Quadrangle, PA |url=http://www.libraries.psu.edu/emsl/guides/pageomaps/A136_5.html |id=A136, Plate 5 |year=1976 |scale=1:24,000 |author=Hoskins, Donald M. T. |publisher=[[Pennsylvania Geological Survey]], Fourth Series |cartography=[[United States Geological Survey]] |url-status=dead |archive-url=https://web.archive.org/web/20071204015837/http://www.libraries.psu.edu/emsl/guides/pageomaps/A136_5.html |archive-date=December 4, 2007 }}</ref> In extreme eastern Pennsylvania, the Broadhead Creek member, a dark gray silty shale with dark gray shaly limestone concretions, appears above the Stony Hollow and Union Springs, in a layer up to {{convert|275|m|ft|0|abbr=on}} thick.<ref name=GQ1047>{{cite map|title=Geologic Map of the Stroudsburg Qd., PA, NJ |url=http://www.libraries.psu.edu/emsl/guides/pageomaps/GQ1047.html |id=GQ1047 |year=1973 |scale=1:24,000 |author=Epstein, Jack |publisher=[[Pennsylvania Geological Survey]], Fourth Series |cartography=[[United States Geological Survey]] |url-status=dead |archive-url=https://web.archive.org/web/20060901093048/http://www.libraries.psu.edu/emsl/guides/pageomaps/GQ1047.html |archive-date=September 1, 2006 }}</ref>

[[File:Marcellus Cephalopod 1896-Dana-ManGeol-Fig917.png|thumb|upright|Illustration of a [[Cephalopod]] ''([[Goniatites]] vanuxemi)'' fossil from the Marcellus Formation.<ref name=Dana1880>{{cite book
 | author = Dana, J.D.
 | year = 1896
 | title = Manual of Geology: Treating of the Principles of the Science with Special Reference to American Geological History
 | publisher = American Book Company
 | edition = 4th
 | page = 599
}}</ref>]]

==Fossils==
There are relatively sparse inclusions of fossilized marine [[fauna]] found in the Marcellus,<ref name="explanation"/> but these fossils are still important to paleontology. For example, the Marcellus contains the oldest known diverse collection of thin-shelled [[mollusks]] still having well preserved shell microstructure.<ref name=Carter1978>{{cite journal
 | author = Carter, J.G.
 |author2=Tevesz, M.J.S.
 | year = 1978
 | title = Shell microstructure of a Middle Devonian (Hamilton Group) bivalve fauna from central New York
 | journal = Journal of Paleontology
 | volume = 52
 | issue = 4
 | pages = 859–880
| jstor=1303906
}}</ref>
It is also where [[goniatites]], an extinct shelled swimmer similar to a squid, make their first appearance in the fossil record.<ref>{{cite book
|title=Notes on the Geology of Onondaga County, NY : Embracing a Short Description of the Various Eras, Periods and Groups|first=Philip F. | last =Schneider | year = 1894| location = Syracuse, NY|page = 37}}</ref>
Life on land also enters the fossil record in the Marcellus, with the trunks of branchless [[conifer]] trees that floated out to sea to be preserved in the black shale.<ref name=MacFarlane1873/><ref name=Ripley1861/>

Marcellus fossils include specimens of the large clam-like [[brachiopod]] Spinocyrtia.<ref name="NYPS">{{cite web |url=http://www.nyps.org/field.htm |work= New York Paleontological Society Field Trips |title=Devonian Seas of Central New York : Syracuse Area, New York |access-date=2008-05-25}}</ref>
External molds of [[crinoid]]s, plant-like animals related to starfish also known as "sea lilies,"<ref name=Iowa>{{cite web|url=http://www.igsb.uiowa.edu/Browse/flood93/flood93.htm |title=Flood of 1993 Uncovers Devonian Sea Floor |work=Iowa Geology |publisher=Iowa Department of Natural Resources |year=1994 |access-date=2008-06-14 |url-status=dead |archive-url=https://web.archive.org/web/20080616080943/http://www.igsb.uiowa.edu/Browse/flood93/flood93.htm |archive-date=2008-06-16 }}</ref>
are found in the formation,<ref name="isbn0-8018-4145-3">{{cite book |author=Burns, Jasper |title=Fossil Collecting in the Mid-Atlantic States: With Localities, Collecting Tips, and Illustrations of More Than 450 Fossil Specimens |publisher=Johns Hopkins University Press |location=Baltimore |year=1991 |page=96 |isbn=0-8018-4145-3}}</ref>
with the molds partially filled with [[limonite]]; brachiopod and [[bivalve]] (clam) molds have also been found in the shale.<ref name=Pabian1976>{{cite journal
 | author = Pabian, R.K.
 |author2=Strimple, H.L.
 | year = 1976
 | title = Middle Devonian crinoids from western Maryland
 | journal = Journal of Paleontology
 | volume = 50
 | issue = 4
 | pages = 759–762
| jstor=1303668
}}</ref>
Small conical [[Tentaculite|tentaculitid]]s are commonly found in the Chittenango Member.<ref name="Chestnut Street"/>
The Halihan Hill bed contains [[styliolinid]]s and macrofauna including brachiopods, coral-like [[bryozoan]]s, small bivalves and [[gastropod]]s (snails),<ref name=Sageman2003/>
incorporated after the [[faunal stage|faunal turnover]] when [[Emsian]] and [[Eifelian]] Schoharie/Onondaga fauna were replaced by the [[Givetian]] Hamilton fauna.<ref name=Kasprak2008/>

The Solsville member contains well preserved bivalves, gastropods, and brachiopods.<ref name="Swamp Road"/> These shellfish lived in the [[benthic zone]] at the bottom of marginal marine to open marine environments that existed west of the ancient [[Catskill Delta]].<ref name=Hill>{{cite web |url=http://www.ncsu.edu/kenanfellows/2002/hhill/chapt3/chapt3.html |title=Chapter 3 Fossils and Geologic Time |work= The Big Dig|publisher = [[North Carolina State University]], Kenan Fellows Program|author= Hill, Heidi|access-date=2008-05-25 |archive-url = https://web.archive.org/web/20071025085454/http://www.ncsu.edu/kenanfellows/2002/hhill/chapt3/chapt3.html |archive-date = October 25, 2007}}</ref>
The fossil record in this member shows the base was dominated by [[deposit feeder]]s, while the upper layers were dominated by [[filter feeder]]s.<ref name=Hill/> This can be correlated to the lithology: the finer sediments of the shales at the base of this member would contain abundant adherent organic matter for deposit feeders, but would tend to foul the gills of filter feeders when suspended; the coarser sediments of the sandstones at the top would have contained less organic matter to support deposit feeders.<ref name=Hill/>
Below the Solsville, at the base of the Otsego in eastern New York, a [[Coral reef|coral bed]] is found; another coral bed can be seen at the top of the Marcellus near [[Berne, New York]].<ref name=Oliver1951>{{cite journal
 |doi=10.2475/ajs.249.10.705 
 |author=Oliver, W.A. 
 |date=October 1, 1951 
 |title=Middle Devonian coral beds of central New York 
 |journal=American Journal of Science 
 |volume=249 
 |issue=10 
 |pages=705–728 
 |bibcode=1951AmJS..249..705O 
 |doi-access=free 
 }}</ref>

A diverse, eel-like [[conodont]] fauna occurs in the limestone of the Cherry Valley Member,<ref name=Over1997>{{cite book
 |author= Over, D.J. |author2=Klapper, G. |author3=Murphy, M.A. |author4=Talent, J.A.
 | year = 1997
 | title = Paleozoic Sequence Stratigraphy, Biostratigraphy, and Biogeography: Studies in Honor of J. Granville ('Jess') Johnson
 | publisher = [[Geological Society of America]]
 | isbn = 0-8137-2321-3
 | page = 152
}}</ref>
which is also known for its rich [[nautiloid]] and goniatite [[cephalopod]] fauna.<ref name="Chestnut Street"/>
Originally named the ''Goniatite Limestone'',<ref name=Yochelson1985>{{cite journal
 | author = Yochelson, E.L.
 |author2=Hlavin, W.J.
 | year = 1985
 | title = Coleolus curvatus Kindle (" Vermes") from the Cleveland Member of the Ohio Shale, Late Devonian (Famennian) of Ohio
 | journal = Journal of Paleontology
 | volume = 59
 | issue = 5
 | pages = 1298–1304
| jstor=1305019}}</ref>
it produces their fossilized remains with shells that can be larger than {{convert|.3|m|ft|0|abbr=on}} across.<ref name=Dana1895/>
It also contains the "Cephalopod Graveyard" in the [[Schoharie Valley]] of eastern New York, an unusual accumulation of abundant coiled and straight shells of several types of large adult cephalopods. This bed lacks juvenile fossils, indicating that if their behavior was similar to modern squid, this may have been an area where these Devonian cephalopods reproduced and died.<ref name="NYS Museum">{{cite web |url=http://www.nysm.nysed.gov/education/teacher/ancient_life_text.html |title=Ancient Life of New York : A Billion Years of Earth History |date= September 2003 |publisher= New York State Museum |access-date=2008-05-26}}</ref>
This [[stratigraphic]] interval also provides an excellent example of incursion [[epibole]]s, which are sudden appearances and disappearances of fossil [[Taxon|taxa]] in relatively thin sections of the rock unit.<ref name=Brett1997/> In the Cherry Valley, the taxa do not reoccur; instead each thin concretionary limestone bed contains different species of goniatites.<ref name=Brett1997>{{cite book
 | author = Brett, C.E.
 |author2=Baird, G.C.
 | year = 1997
 | title = Paleontological Events: Stratigraphic, Ecological, and Evolutionary Implications
 | publisher = Columbia University Press
 | isbn = 0-2310-8250-9
 | page = 269
}}</ref>
The Cherry Valley and Union Springs also contain well-preserved [[anarcestida]].<ref name="isbn0-306-45222-7">{{cite book |author1=Davis, R. Michael |author2=Landman, Neil H. |author3=Tanabe, Kazushige |title=Ammonoid paleobiology |publisher=Plenum Press |location=New York |year=1996 |page= 644|isbn=0-306-45222-7 }}</ref>

==Age==
On the [[geological timescale]], the Marcellus occurs in the [[Middle Devonian]] epoch, of the [[Devonian]] [[Period (geology)|period]], in the [[Paleozoic]] [[Era (geology)|era]], of the [[Phanerozoic]] [[Eon (geology)|eon]]. Radiometric dating of a Marcellus sample from Pennsylvania placed its age at 384 million years old, and a sample from the bentonite at the top of the Onondaga at 390 ± 0.5 million years old.<ref name="isbn0-231-08248-7">{{cite book |author1=Anstey, Robert L. |author2=Erwin, Douglas H. |title=New approaches to speciation in the fossil record |publisher=Columbia University Press |location=New York |year=1995 |isbn=0-231-08248-7 }}</ref>

Relative age dating of the Marcellus places its formation in the Cazenovia subdivision of the [[Givetian]] [[faunal stage]], or 391.9 to 383.7 million years ago&nbsp;([[Annum|Ma]]).<ref name=paleo>{{cite web |url=https://paleodb.org/cgi-bin/bridge.pl?action=displayStrata&group_formation_member=Marcellus |title= Marcellus Formation, Group|access-date=2008-04-03 |work=The Paleobiology Database}}</ref>
The Union Springs member, at the base of the Marcellus in New York, has been dated to the end of the [[Eifelian]], the stage which immediately preceded the Givetian.<ref name="isbn1-4020-2632-3">{{cite book |author=Koutsoukos, Eduardo |title=Applied Stratigraphy (Topics in Geobiology) |publisher=Springer |location=Berlin |year=2005 |page= 70 |isbn=1-4020-2632-3}}</ref>
Anoxic dark shales in the formation mark the [[Kačák Event]],<ref name=Menning2006>{{cite journal|author=Menning, M. |author2=Alekseev, A.S. |author3=Chuvashov, B.I. |author4=Davydov, V.I. |author5=Devuyst, F.X. |author6=Forke, H.C. |author7=Grunt, T.A. |author8=Hance, L. |author9=Heckel, P.H. |author10=Izokh, N.G. | year = 2006 | title = Global time scale and regional stratigraphic reference scales of Central and West Europe, East Europe, Tethys, South China, and North America as used in the Devonian—Carboniferous—Permian Correlation Chart 2003 (DCP 2003) | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 240 | issue = 1–2 | pages = 318–372 | doi = 10.1016/j.palaeo.2006.03.058 |display-authors=etal | bibcode=2006PPP...240..318M}}</ref>
a late-[[Eifelian]]-[[Faunal stage|stage]] marine [[anoxic event]] also associated with an [[extinction event]].<ref name="isbn1-86239-222-6">{{cite book |author=W. T. Kirchgasser |title=Devonian Events and Correlations : Special Publication no 278 |chapter=Recognising the Kacak Event in the Devonian terrestrial environment and its implications for understanding land-sea interactions|series=(Geological Society Special Publication) |publisher=Geological Society of London |location=London |year=2007 |pages= 133–155|isbn=978-1-86239-222-9 }}</ref> In 2012, Read and Erikson also depicted the formation as Eifelian.<ref>[http://hdl.handle.net/10919/19078 Paleozoic Sedimentary Successions of the Virginia Valley & Ridge and Plateau]</ref>

[[File:Middle Dev strat Marcellus.gif|thumb|upright=2|Generalized [[stratigraphy|stratigraphic]] nomenclature for the [[Middle Devonian]] strata in the [[Appalachian Basin]].<ref name=USGS2006-1237/>]]

==Interpretation of depositional environment==
Although black [[shale]] is the dominant [[lithology]], it also contains lighter shales and [[interbedded]] [[limestone]] layers due to sea level variation during its [[Sedimentary depositional environment|deposition]] almost {{Ma|400 }}.<ref name=Harper2004>{{cite web |url=http://www.dcnr.state.pa.us/topogeo/tbr/tbrintro.aspx |archive-url=https://web.archive.org/web/20060328021706/http://www.dcnr.state.pa.us/topogeo/tbr/tbrintro.aspx |url-status=dead |archive-date=March 28, 2006 | title = Trenton and Black River Carbonates in the Union Furnace Area of Blair and Huntingdon Counties, Pennsylvania : Introduction| date= 2004-05-26|work=Field Trip Guidebook for the Eastern Section AAPG Annual Meeting, September 10, 2003 and the PAPG Spring Field Trip, May 26, 2004 |access-date=2008-06-14| author= Harper, John A.|author2=Christopher D. Laughrey |author3=Jaime Kostelnik |author4=David P. Gold |author5=Arnold G. Doden |publisher=Pennsylvania Geological Survey}}</ref> The black shale was deposited in relatively deep water devoid of [[oxygen]], and is only sparsely [[fossiliferous]]. Most fossils are contained in the limestone members, and the fossil record in these layers provides important [[paleontology|paleontological]] insights on [[faunal stage|faunal turnovers]].<!--Like what?  A (brief) summary would be useful here-->

Early in the [[Acadian orogeny]], as the [[Acadian Mountains]] were rising up, the black and gray shales of the Hamilton Group began accumulating as erosion of the mountains deposited [[terrigenous]] sediments from the land into the sea.<ref name=Martin>{{cite web |url=http://www.searchanddiscovery.net/documents/2006/06091eastern/abstracts/martin01.htm |archive-url=https://web.archive.org/web/20080808134608/http://www.searchanddiscovery.com/documents/2006/06091eastern/abstracts/martin01.htm |archive-date=2008-08-08 |title=The Middle Devonian Hamilton Group Shales in the Northern Appalachian Basin: Production and Potential |access-date=2008-04-02 |first=John P. | last = Martin | publisher= New York State Energy Research and Development Authority}}</ref>
The Marcellus Shale was formed from the very first deposits in a relatively deep, sediment- and oxygen-starved ([[Anoxic sea water|anoxic]]), [[Trough (geology)|trough]] that formed parallel to the mountain chain.<ref name="title">{{cite web|url=http://www.iogawv.com/RamsayBarrett-Shale.pdf |title=The Depositional Setting of The Marcellus Black Shale |access-date=2008-04-02 |publisher=Independent Oil and Gas Association of West Virginia |work=Appalachian Producers Issues Seminar |first=Ramsay |last=Barrett |url-status=dead |archive-url=https://web.archive.org/web/20080715110108/http://www.iogawv.com/RamsayBarrett-Shale.pdf |archive-date=July 15, 2008 }}</ref>
These [[clastic]] fragments of rock were carried in [[braided stream]]s to the ancient [[Catskill Delta]], a [[river delta]] probably similar to the present day [[Niger Delta]] of Africa.<ref name=Mazzullo1973>{{cite journal
 | author = Mazzullo, S.J.
 | year = 1973
 | title = Deltaic depositional environments in the Hamilton Group (middle Devonian, southeastern New York State)
 | journal = Journal of Sedimentary Research
 | volume = 43
 | issue = 4
 | pages = 1061–1071
 | doi = 10.1306/74D728F1-2B21-11D7-8648000102C1865D
}}</ref>

Smaller particles remained [[suspension (chemistry)|suspended]] longer in this [[epeiric sea]], flowing offshore as [[turbidites]] in a slow but persistent underwater avalanche. They finally came to rest at the bottom of the Acadian [[foredeep]] in the [[Appalachian Basin]],<ref name="isbn0-87842-227-7"/> hundreds of meters from shore, at depths that may have been {{convert|150|m|ft|-1|abbr=on}} or more beneath the surface.<ref name=Sageman2003>
{{cite journal
 | author1 = Sageman, B.B.
 | author2 = Murphy, A.E.
 | author3 = Werne, J.P.
 | author4 = Ver Straeten, C.A.
 | author5 = Hollander, D.J.
 | author6 = Lyons, T.W.
 | year = 2003
 | title = A tale of shales: The relative roles of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle-Upper Devonian, Appalachian basin
 | journal = Chemical Geology
 | volume = 195
 | issue = 1–4
 | pages = 229–273
 | url = http://www.earth.northwestern.edu/research/sageman/PDF/03.Sageman_etal.pdf
 | doi = 10.1016/S0009-2541(02)00397-2
 | bibcode = 2003ChGeo.195..229S
 | access-date = 2008-06-28
 | archive-url = https://web.archive.org/web/20090107144939/http://www.earth.northwestern.edu/research/sageman/PDF/03.Sageman_etal.pdf
 | archive-date = 2009-01-07
 | url-status = dead
 }}
</ref>
Alternatively, the basin may have been as shallow as {{convert|50|m|ft|-1|abbr=on}} or less, if the warm water was sufficiently [[Stratification (water)|stratified]] so that oxygen rich surface water did not mix with the anoxic bottom water.<ref name="isbn0-8182-0227-0">{{cite book |author=Shultz, Charles H. |title=The geology of Pennsylvania |publisher=Pennsylvania Geological Survey |location=Harrisburg, Pa |year=1999 |pages= 125–126 |isbn=0-8182-0227-0 }}</ref>
The Marcellus deposition produced a ''transgressive'' black shale,<ref name=Wickstrom/>
because it was deposited in deepening conditions when the basin floor dropped as the mountains rose up.<ref name=Ettensohn>{{cite book
 | title=Shales and mudstones
 | publisher=E. Schweizerbart
 | location=Stuttgart
 | year=1998
 | pages= 109–128
 | isbn=3-510-65181-2
 | author = Ettensohn, F.R.
 | chapter = Compressional tectonic controls on Epicontinental black-shale deposition: Devonian-Mississippian examples from North America
 | chapter-url = http://cat.inist.fr/?aModele=afficheN&cpsidt=1809612
 | access-date = 2008-06-12
}}</ref>
[[File:Catskill Magnafacies.gif|thumb|upright=2|Generalized geologic cross section of [[Catskill Delta]] [[facies|magnafacies]] across western [[Pennsylvania]] and eastern [[Ohio]].<ref name="pubs.er.usgs.gov"/> [[Clastic]] deposits entered the [[Appalachian Basin]] from the east, where the resulting [[sedimentary rock]] is thickest.]]

The dark shale [[facies]] of the Marcellus were formed from [[flysch]], a fine mud deposited in deep water; the deepening sea that deposited the Marcellus cut off the supply of [[carbonates]] that form limestone and the fine-grained flysch sediments buried the [[Onondaga Formation|Onondaga limestone]] beds.<ref name=Patrick2004>{{cite book
 | author = Patrick, K.J.
 | year = 2004
 | title = Pennsylvania Caves and Other Rocky Roadside Wonders
 | publisher = Stackpole Books
 | isbn = 0-8117-2632-0
 | page = 18
}}</ref>
<ref name=Werne2002>{{cite journal
 | author1 = Werne, J.P.
 | author2 = Sageman, B.B.
 | author3 = Lyons, T.W.
 | author4 = Hollander, D.J.
 | year = 2002
 | title = An integrated assessment of a" type euxinic" deposit: Evidence for multiple controls on black shale deposition in the middle Devonian Oatka Creek formation
 | journal = American Journal of Science
 | volume = 302
 | issue = 2
 | pages = 110–143
 | url = http://www.ajsonline.org/cgi/content/abstract/302/2/110
 | access-date = 2008-05-27
 | doi = 10.2475/ajs.302.2.110
| bibcode = 2002AmJS..302..110W
 | doi-access = free
 }}</ref>
Organic matter, probably dominated by [[plankton]], also settled to the bottom, but the normal [[Decomposition|aerobic decay]] process was inhibited in the [[Hypoxia (environmental)|anaerobic]] environment thereby preserving the organic carbon.<ref name=McBride2004>{{cite journal
 | author = McBride, P.
 | year = 2004
 | title = Facies Analysis of the Devonian Gordon Stray Sandstone in West Virginia, M.S. Thesis
 | publisher = West Virginia University, Morgantown, West Virginia
 | page = 21
 | url = https://eidr.wvu.edu/files/3403/McBride_Patrick_thesis.pdf
}}{{dead link|date=October 2012}}</ref>
<ref name=Lash2008>{{cite journal
 | author = Lash, G.G.
 | year = 2008
 | title = Pyrite Laminae In Black Shale-An Additional Source of Permeability In Unconventional Reservoirs
 | journal = 2008 Joint Meeting of the Geological Society of America, et Al.
 | url = http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_147721.htm
 | access-date = 2008-07-14
 | archive-url = https://web.archive.org/web/20110608152301/http://gsa.confex.com/gsa/2008AM/finalprogram/abstract_147721.htm
 | archive-date = 2011-06-08
 | url-status = dead
 }}</ref>
Uranium was also incorporated in these organic muds syndepositionally,<ref name=Hand1988/>
meaning it was deposited at the same time, rather than being introduced to the formation later.<ref name="isbn1-4020-0628-4">{{cite book |author1=Smol, J. P. |author2=Last, William M. |title=Tracking environmental change using lake sediments |publisher=Kluwer Academic | volume=2: Physical and Geochemical Methods |year=2001 |page= 191|isbn=1-4020-0628-4 }}</ref>
The organic matter scavenged trace elements from the seawater,<ref name=Peucker-ehrenbrink2001>{{cite conference
 | author = Peucker-ehrenbrink, B.
 | year = 2001
 | title = Dating Weathering of Organic-rich Shales with U-Series Disequilibrium
 | conference = Eleventh Annual V. M. Goldschmidt Conference
 | volume = 1
 | bibcode = 2001eag..conf.3781P
}}</ref>
including the [[redox]]-sensitive elements uranium, [[rhenium]], [[molybdenum]], [[osmium]], [[chromium]], and [[selenium]].<ref name="Peucker-Ehrenbrink">{{cite web |url=http://www.whoi.edu/hpb/viewPage.do?id=1375&cl=8 |title= Black shale weathering |work=Bernhard Peucker-Ehrenbrink : Projects|publisher = Woods Hole Oceanographic Institution |access-date=2008-07-13}}</ref>

The Marcellus was deposited during the [[Evolutionary history of plants|development of land plants]], when atmospheric oxygen was increasing, resulting in a reduction of carbon dioxide in the atmosphere, and the seawater where it was deposited.<!--Better to use a ref from [[Evolutionary history of plants]] for this&nbsp;— and perhaps include a little more information if it's worthy of a mention.--><ref>{{cite conference
| first1 = Pat
| last1 = Wilde
| author2 = Quinby-Hunt, Mary S.
| author3 = Lyons, Timothy W.
| title = Sulfur Proxies in Type III Black Shales: Fe, Mn, Co, Cu, Ni, Zn, Sc
| conference =  2005 Annual Meeting
| date = 2005-10-13
| location = Salt Lake City, Utah
| url = http://www.marscigrp.org/gsa05whl.ppt
| id = Paper No. 76-7
| format = ppt
| publisher = Geological Society of America
| access-date = 2008-06-21}}</ref>
Named members of the Marcellus reflect two composite depositional sequences,<ref name=Desantis2003>{{cite journal
 | author = Desantis, M.K.
 | year = 2003
 | title = Eifelian (early Middle Devonian) Depositional Sequences And Bioevents In Eastern Laurentia: Appalachian Basin-wabash Platform Comparisons
 | journal = 2003 Seattle Annual Meeting
 | publisher = Geological Society of America
 | volume = 35
 | url = http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_65945.htm
 | access-date = 2008-05-26
 | issue = 6
 | archive-url = https://web.archive.org/web/20081122004354/http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_65945.htm
 | archive-date = 2008-11-22
 | url-status = dead
 }}</ref>
with a general coarsening upward cycle that continues into the base of the overlying Mahantango Formation.<ref name=Altamura1999/>
The interbedding of lighter shale and limestone members is attributed to relatively short-term oscillations in basin depth.<ref name=Arthur2005>{{cite book
 | author = Arthur, M.A.
 |author2=Sageman, B.B.
 | year = 2005
 | chapter = Sea level control on source rock development: Perspectives from the Holocene Black Sea, the mid-Cretaceous Western Interior Basin of North America, and the Late Devonian Appalachian Basin
 | title = The Deposition of Organic Carbon-Rich Sediments: Models, Mechanisms and Consequences
 | pages = 35–59
 | publisher = SEPM Society for Sedimentary Geology
 | isbn=1-56576-110-3
  | chapter-url = https://www.sepm.org/publishing/toc/sp82toc.htm
| archive-url = https://archive.today/20080229200239/https://www.sepm.org/publishing/toc/sp82toc.htm
| url-status = dead
| archive-date = 2008-02-29
}}
</ref>
Later deep water depositional sequences formed the overlying [[Brallier Formation]] and [[Harrell Formation]].<ref name="isbn0-87842-227-7">{{cite book |author=VanDiver, Bradford B. |title=Roadside geology of Pennsylvania |publisher=Mountain Press Pub. Co |location=Missoula, Mont |year=1990 |pages= 86, 193 |isbn=0-87842-227-7 | series=Roadside Geology Series}}</ref>

==Economic resources==

===Natural gas===
{{main|Marcellus natural gas trend}}
[[File:Marcellus Shale Gas Play.gif|thumb|upright=1.25|Producing gas wells in the Marcellus Shale gas play]]

The shale contains largely untapped [[natural gas]] reserves, and its proximity to the high-demand markets along the [[East Coast of the United States]] makes it an attractive target for [[energy development]] and [[export]].<ref name=rescue>{{cite web | url = http://www.petroleumnews.com/pntruncate/246893563.shtml | title = Appalachia to the rescue: Could Devonian shales deep under the Appalachians supply trillions of cubic feet of much needed natural gas for the U.S.? | author = Alan Bailey | publisher = Petroleum News |access-date = 2008-04-05}}</ref>

The Marcellus natural gas trend, which encompasses 104,000 square miles and stretches across Pennsylvania and West Virginia, and into southeast Ohio and upstate New York, is the largest source of natural gas in the United States, and production was still growing rapidly in 2013. The Marcellus is an example of [[shale gas]], natural gas trapped in low-permeability shale, and requires the well completion method of [[hydraulic fracturing]] to allow the gas to flow to the well bore. The surge in drilling activity in the Marcellus Shale since 2008 has generated both economic benefits and environmental concerns—and thus, considerable controversy.

===Iron===
The black shales also contain [[iron ore]] that was used in the early economic development of the region, and [[uranium]] and [[pyrite]] which are environmental hazards. At the base of the Marcellus, in the [[pyrite]]-[[carbonate]] bed between the carbonaceous black shale and a green calcareous shale bed,<ref name=Dewees1878/> pyrite, carbonate, and groundwater reacted to form [[gossan]] [[iron oxide]] and [[gypsum]].<ref name=Feldmann2007>{{cite journal | url=https://dx.doi.org/10.1666%2Fpleo0022-3360%282007%29081%5B0701%3ANLCLFT%5D2.0.CO%3B2 |author = Feldmann, R.M. |author2=Schweitzer, C.E. |author3=Redman, C.M. |author4=Morris, N.J. |author5=Ward, D.J. | year = 2007 | title = New Late Cretaceous Lobsters From The Kyzylkum Desert Of Uzbekistan |journal=Journal of Paleontology | volume = 81 | issue = 4 |pages = 701–713 | doi = 10.1666/pleo0022-3360(2007)081[0701:NLCLFT]2.0.CO;2 |s2cid = 130978049 | issn=0022-3360 |url-access = subscription }}</ref> As far as the ground water necessary for the conversion could penetrate, the pyrite-carbonate was converted to a usable brown [[hematite]] [[iron ore]] along the outcrops and near the bedrock surface.<ref name=Lesley1892>{{cite book
 | author = Lesley, J.P.
 | year = 1892
 | title = A summary description of the Geology of Pennsylvania
 | publisher = Pennsylvania Geol. Survey, 2d, Final Rept.
 | volume = II
 | url= https://books.google.com/books?id=TEA4AAAAMAAJ&pg=PA1233
}}</ref> The Marcellus iron ore was actively mined in south Central Pennsylvania from its discovery in the late 18th century, until it was supplanted by the rich ore beds of the [[Iron Range]] of [[Minnesota]] in the early 20th century.<ref name=Swank1965>{{cite book
 | author = Swank, J.H.
 | year = 1892
 | title = History of the Manufacture of Iron in All Ages, and Particularly in the United States From Colonial Times to 1891.
 | publisher = Ayer Publishing
 | location = Philadelphia
}}</ref> The ore was easily located and worked from shallow pits and shafts, but once the usable upper deposits were removed, or if a mine shaft entered the bed too far below the surface, only unusable unconverted pyritic deposits were found.<ref name=Dewees1878/>

Hematite ore was converted to [[pig iron]] in [[charcoal]]-fired stone [[blast furnace]]s that were constructed throughout the [[Juniata River]] region near the workable ore deposits from the Marcellus and other formations.<ref name=Swank1965/> Iron products from this area, known as "Juniata Iron," were produced during the period between the [[American Revolution]] and the [[American Civil War]]. These blast furnaces were important to the economy of the region at the time,<ref name=Philadelphia1936>{{cite book
 | author = Ashburner, Charles A.
 | chapter = A measured section of the Paleozoic Bucks of Central Pennsylvania, from the top of the Allegheny River Coal Series, down to the Trenton Limestone or the Lower or Cambro-Silurian System
 | year = 1876
 | publisher = American Philosophical Society
 | title = Proceedings of the American Philosophical Society Held at Philadelphia for Promoting Useful Knowledge
 | volume = XV, No. 96
}}</ref> but the [[cold blast]] stone furnaces typically employed were inefficient, and consumed significant amounts of timber from the nearby hardwood forests, which ultimately led to their demise.<ref name=Schallenberg1977>{{cite journal
 | author = Schallenberg, R.H.
 |author2=Ault, D.A.
 | year = 1977
 | title = Raw Materials Supply and Technological Change in the American Charcoal Iron Industry
 | journal = Technology and Culture
 | volume = 18
 | issue = 3
 | pages = 436–466
 | doi = 10.2307/3103901
 | publisher = Technology and Culture, Vol. 18, No. 3
| jstor=3103901
|s2cid=111717770
 }}</ref> A typical furnace used {{convert|2400|kg|lb|abbr=on}} of hematite ore and {{convert|7.3|m3|impbu|abbr=on}} of charcoal to produce {{convert|910|kg|lb|abbr=on}} of pig iron,<ref name=Dewees1878/> and could produce several thousand pounds per day, which required logging more than {{convert|4000|m2|acre|0|abbr=on}} of forest daily.<ref name="Little Buffalo">{{cite web |url=http://www.dcnr.state.pa.us/stateparks/parks/littlebuffalo/littlebuffalo_history.aspx |archive-url=https://web.archive.org/web/20110609235856/http://www.dcnr.state.pa.us/stateparks/parks/littlebuffalo/littlebuffalo_history.aspx |url-status=dead |archive-date=June 9, 2011 |title= Little Buffalo State Park: Park History |publisher = Pennsylvania Department of Conservation and Natural Resources|work= Pennsylvania State Parks|access-date=2008-06-21}}</ref>

The ore from the Marcellus varied in thickness, becoming unworkably thin, and even disappearing altogether in places between the workable beds.<ref name=Dewees1878/><ref name=Lesley1859/> The quality of the ore also varied,<ref name=Dewees1878/> and it was not always profitable to [[smelting|smelt]], as several furnaces built near iron ore mines in the Marcellus were abandoned before the ore and timber resources used to fuel them became scarce.<ref name=Lesley1859>{{cite book
 | author = Lesley, J. P.
 | year = 1859
 | title = The Iron Manufacturer's Guide to the Furnaces, Forges and Rolling Mills of the United States
 | publisher = J. Wiley
 | location = New York
 | page = 651
 | url = https://books.google.com/books?id=NSJDAAAAIAAJ&pg=PA651
 | lccn = 06022651
}}</ref>

Ore found interbedded in the black slaty shale contained a relatively high proportion of carbon which was burned in the furnace, and [[sulfur]], which produced a usable but "[[red-short]]" iron.<ref name=Platt1881>{{cite book
 | author = Platt, Franklin.
 | year = 1881
 | title =   The Geology of Blair County
 | url = https://archive.org/details/geologyblaircou01platgoog
 | series = Report of Progress
 | publisher = Board of commissioners for the Second geological survey
 | location = Harrisburg
 | lccn = gs07000757
}}</ref>
Red-short iron has the undesirable properties of oxidizing more easily, and a tendency to crack, especially when heated to a red-hot state.<ref name=Crookes>{{cite book
 | author = Crookes, W.
 |author2=Röhrig, E.
 | year = 1869
 | title = A practical treatise on Metallurgy
 | volume = II
 | publisher = Longmans Green
 | location = London
 | page = 695
}}</ref>
In some locations in Pennsylvania the quality of the ore was quite good, with relatively deep veins containing 45% iron, and very low sulfur.<ref name=Dewees1878/>
In Virginia, the Marcellus ore occasionally contained [[zinc]], which produced a characteristic green flame in the furnace as it was consumed, but deposited a hard mass of impure [[zinc oxide]] known as [[cadmia]], which built up over time near the top of the flue, and had to be removed periodically to keep it unobstructed.<ref name=Firmstone1874>{{cite journal
 | author = Firmstone, H.
 | year = 1879
 | title = On a Deposit of Cadmia in a Coke Blast Furnace
 | journal = Transactions of the American Institute of Mining Engineers
 | volume = VII
 | pages = 93–94
 | publisher = American Institute of Mining Engineers
}}</ref>

====Iron pigments====
Drainage that reacted with the pyrite inclusions also deposited a form of [[bog iron]] near several outcrops of the Marcellus. In the 19th century, iron ore from these deposits was used as a mineral paint pigment. After being heated in a kiln and finely ground, it was mixed with [[linseed oil]], and used to paint exterior wood on barns, covered bridges, and railroad cars.<ref name=Lesley1892/> In addition to the bog iron,  at several sloped locations in eastern Pennsylvania brown hematite was found lying on the Marcellus bedrock buried beneath the soil. These deposits were also excavated and used for mineral paint during that time.<ref name=White1882/> A bed of hematite paint ore is also found almost directly below the Marcellus, but it is actually part of the underlying Oriskany Formation.<ref name=Lesley1892/>

====Chalybeate====
Iron rich "ferruginous waters" emanating from [[chalybeate]] springs near the base of the Marcellus in [[Bedford, Pennsylvania]] were believed to have healing powers by Native Americans. The [[Bedford Springs Hotel]] was a [[mineral spa]] built in 1802 around a series of mineral springs, including one of these, its "iron spring". The Chalybeate Springs Hotel, built nearby in 1851 around three other mineral springs including another chalybeate spring,<ref name=Waymarking>{{cite web |url=http://www.waymarking.com/waymarks/WM1054 |title=Chalybeate Springs Hotel&nbsp;— U.S. National Register of Historic Places|work=Waymarking.com |access-date=2008-10-09}}</ref> became a "resort for invalids".<ref name=Stevenson1882>{{cite book|title=The Geology of Bedford and Fulton Counties|first =John James | last = Stevenson|publisher=The board of Commissioners for the 2d. Geological Survey (of Pennsylvania)|year= 1882| location=Harrisburg|pages = 342–343|lccn=gs07000758}}</ref>  The iron-rich waters were prescribed for [[anemia]] and related complications.<ref name=Hemmeter1902/> Both of these mineral springs contain iron in the form of dissolved [[iron carbonate]],<ref name=Stevenson1882/> which gives these waters a "slightly inky taste".<ref name=Hemmeter1902>{{cite book|title=Diseases of the stomach |url=https://archive.org/details/diseasesofstomac1902hemm | first = John Conrad |last = Hemmeter|page= [https://archive.org/details/diseasesofstomac1902hemm/page/327 327]|publisher = Blakiston | year = 1902}}</ref>

===Other uses===
The Marcellus has also been used locally for shale [[Construction aggregate|aggregate]] and common [[Fill dirt|fill]],<ref name=Allensville/> although the pyritic shales are not suitable for this purpose because of acid rock drainage and volumetric expansion.<ref name=Doden2008>{{cite journal
 |author       = Doden, A.G.
 |author2      = Gold, D.P.
 |author3      = Hoover, S.E.
 |author4      = Scheetz, B.E.
 |author5      = Ellsworth, C.J.
 |year         = 2008
 |title        = Road Bed And Building Heave From Alteration Of Sulfide And Sulfate Minerals
 |url          = http://gsa.confex.com/gsa/2008NE/finalprogram/abstract_134913.htm
 |publisher    = [[Geological Society of America]]
 |access-date   = 2008-05-04
 |archive-url  = https://web.archive.org/web/20110608152437/http://gsa.confex.com/gsa/2008NE/finalprogram/abstract_134913.htm
 |archive-date = 2011-06-08
 |url-status   = dead
}}</ref>
In the 19th century, this shale was used for walkways and roadways,<ref name=Underwood1879>{{cite book
 | author = Underwood, L.M.
 | year = 1879
 | title = The Geological Formations Crossed by the Syracuse and Chenango Valley Railroad: Together with a Sketch of the Hydrography and Valley Formations of Madison and Onondaga Counties
 | url = https://archive.org/details/geologicalforma00undegoog
 |publisher=Standard Pub. Co.
 |location= Syracuse, N.Y.
 | series = Ph. D. Thesis, [[Syracuse University]]
 | page = [https://archive.org/details/geologicalforma00undegoog/page/n10 8]
}}</ref>
and was considered superior "[[road metal]]" because the fine grained fragments packed together tightly, yet drained well after a rain.<ref name=Lesley1892/>

The dark slaty shales may have the necessary [[Cleavage (geology)|cleavage]] and hardness to be worked, and were quarried for low grade roofing [[slate]] in eastern Pennsylvania during the 19th century. The slates from the Marcellus were inferior to the [[Martinsburg Formation]] slate quarried further south, and most quarries were abandoned, with the last significant operation in Lancaster County.<ref name=Lesley1892/> The Marcellus black slate was also quarried in Monroe County, Pennsylvania, for [[Chalkboard|school slates]] used by students in 19th-century rural schools.<ref name=Lesley1892/>

Carbonaceous shales, such as the Marcellus, are a possible target for [[carbon capture and storage]] for [[mitigation of global warming]]. Because carbon [[Adsorption|adsorbs]] [[carbon dioxide]]&nbsp;(CO<sub>2</sub>) at a greater rate than [[methane]]&nbsp;(CH<sub>4</sub>), carbon dioxide injected into the formation for geological [[CO2 sequestration|sequestration]] could also be used to recover additional [[natural gas]] in a process analogous to [[enhanced coal bed methane recovery]], but the practical value of this theoretical technique is not yet known.<ref name=Wickstrom/> Scientists believe that adsorption would allow sequestration at shallower depths than [[Absorption (chemistry)|absorption]] in deep saline formations, which must be at least {{convert|800|m|ft|-2|abbr=on}} below the surface to maintain liquid CO<sub>2</sub> in a [[Supercritical fluid|supercritical state]].<ref name=PA2006/>

==Engineering issues==
The [[Fissility (geology)|fissile]] shales are also easily [[erosion|eroded]], presenting additional [[civil engineering|civil]] and [[environmental engineering]] challenges.
[[File:Marcellus Shale Gas Drilling Tower 1 crop.jpg|thumb|right|A horizontal drilling rig for natural gas in the Marcellus formation in eastern [[Lycoming County, Pennsylvania]].]]
Exposures from [[cut and fill]] road construction in Virginia and Pennsylvania have resulted in localized [[acid rock drainage]] due to [[oxidation]] of the pyrite inclusions.<ref name=Orndorff2004>{{cite journal
 | author1 = Orndorff, Z.W.
 | author2 = Lee Daniels, W.
 | year = 2004
 | title = Evaluation of acid-producing sulfidic materials in Virginia highway corridors
 |journal=Environmental Geology
 |volume= 46
 | issue = 2
 | pages = 209–216
 | doi = 10.1007/s00254-004-1027-y
| s2cid = 129086495
 }}</ref>
The newly exposed shale on the cut face weathers rapidly, allowing air and water into the unexcavated rock, resulting in acidic [[surface runoff]] after precipitation events.<ref name=Smith2006>{{cite journal
 | author = Smith, M.W.
 | author2 = Varner, J.P.
 | author3 = Mital Jr, J.P.
 | author4 = Sokoloski, D.J.
 | year = 2006
 | title = Remediation Of Acid Rock Drainage From Highway Construction In The Marcellus Shale, Mifflin County, Pennsylvania
 | journal = Geological Society of America Abstracts with Programs
 | volume = 38
 | issue = 2
 | page = 33
 | url = http://gsa.confex.com/gsa/2006NE/finalprogram/abstract_100515.htm
 | access-date = 2008-05-04
 | archive-url = https://web.archive.org/web/20080706054440/http://gsa.confex.com/gsa/2006NE/finalprogram/abstract_100515.htm
 | archive-date = 2008-07-06
 | url-status = dead
 }}</ref>
Acidic runoff disrupts [[aquatic ecosystem]]s, and highly [[acidic soil]] [[Soil contamination|contaminated]] by this runoff will not support vegetation, which is unsightly, and can lead to problems with [[soil erosion]].<ref name=Orndorff2004/>

Natural decomposition of the shale into smaller fragments can affect [[slope stability]], necessitating shallower slopes that require more material be disturbed in cut and fill work, exacerbating the acid rock drainage problem. The cut material cannot be used as fill beneath roads and structures due to volumetric expansion, compounding the problem.<ref name=Scheetz2007/> The Tioga ash beds contain [[bentonite]] [[clay]] which presents a [[landslide]] hazard in the unexcavated rock as well.<ref name=Scheetz2007/>

Damage to structures constructed on fill consisting of pyritic Marcellus shale has been caused by expansion from [[sulfuric acid]]&nbsp;(H<sub>2</sub>SO<sub>4</sub>) runoff reacting with the [[calcite]]&nbsp;(CaCO<sub>3</sub>) in the shale to produce [[gypsum]]&nbsp;(CaSO<sub>4</sub>), which has double the [[molar volume]].<ref name=Hoover2004>{{cite conference
 | author1 = Hoover, S.E.
 | author2 = Wang, M.C.
 | author3 = Dempsey, B.
 | year = 2004
 | title = Structural damage induced by pyritic shale
 | conference = Fifth International Conference on Case Histories in Geotechnical Engineering, April 13–17, 2004, New York, N. Y.[[Proceedings]][electronic resource]
 | pages = 7
 | url = http://www.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=2004112611434CE&recid=20049978EQ
}}</ref>
Other [[sulfate]] minerals that can be produced by reactions with pyrite include [[anhydrite]], [[melanterite]], [[rozenite]], [[jarosite]], and [[alunite]].<ref name=Doden2008/>
The reactions have generated a heave pressure on the order of 500 kPa (10,000 pounds per square foot), but may be able to generate four times this pressure &nbsp; enough to heave [[Foundation (engineering)|foundations]] in a 5-story building.<ref name=Doden2008/>
[[Limestone]], which is used to neutralize the acid drainage, can actually exacerbate the expansion problem by promoting sulfate–sulfate reactions that form the minerals [[thaumasite]] and [[ettringite]], which have even higher molar volumes.<ref name=Doden2008/>

Drilling [[borehole]]s through the Hamilton Group shales in the subsurface can be problematic. The Marcellus has a relatively low [[density]], and these shales may not be chemically compatible with some [[drilling fluid]]s. The shale is relatively fragile, and may fracture under pressure, causing a problem in circulating the drilling fluid back up through the borehole known as lost circulation. The formation may also be under-pressurized, further complicating the drilling process.<ref name=Wickstrom/>

==See also==
{{portal|Devonian|Pennsylvania}}
*[[Geology of New Jersey]]
*[[Geology of Pennsylvania]]
**[[List of mapped rock formations in Pennsylvania]]
*[[Geology of West Virginia]]
*[http://www.philly.com/philly/news/special_packages/inquirer/marcellus-shale/133701588.html Interactive timeline of gas production from the Marcellus Formation]
*[[Pittsburgh Association of Petroleum Geologists]]
*[[Shale gas]]
*[[Shale gas in the United States]]

==References==
{{Reflist|2}}

==External links==
{{commons}}
*[http://exploreshale.org/ A creative exploration of natural gas drilling and development in the Marcellus Shale]
*[https://web.archive.org/web/20110113115809/http://energy.wilkes.edu/pages/1.asp A Marcellus Shale Clearinghouse – The Institute for Energy and Environmental Research for Northeastern Pennsylvania Wilkes University]
*[http://www.geosc.psu.edu/~engelder/marcellus/marcellus.html Outcrops of the Marcellus Formation] images with captions hosted at [[Pennsylvania State University]]
*[https://web.archive.org/web/20080916091046/http://www.srbc.net/atlas/downloads/BasinwideAtlas/PDF/1340b_MarcellusShale.pdf Susquehanna River Basin Marcellus Shale Occurrence] map
*[http://www.papgrocks.org/publications.htm Pittsburgh Association of Petroleum Geologists] PAPG publications including the Marcellus Shale
*[https://web.archive.org/web/20121102093844/http://anklebytes.com/ Marcellus Shale Links] Maps and Links to Governmental and Non Governmental sites about the Marcellus Shale
*[http://www.technologyreview.com/energy/23694/page1/ Natural Gas Changes the Energy Map] MIT's Technology Review article about the gas potential of Marcellus Shale (only a brief synopsis is free)
*[https://web.archive.org/web/20090917032801/http://www.citizenscampaign.org/campaigns/marcellus.asp Citizens Campaign for the Environment]: CAMPAIGNS: Natural Gas Hydro-Fracking in Shale
*[http://www.fractracker.org/ FracTracker: Blog and Shale Gas Data and Mapping System]
*[http://www.fracktrack.org/index.php FrackTrack: Pennsylvania Marcellus Shale Mapping Application]
*[https://www.economist.com/news/special-report/21573279-shale-gas-and-oil-bonanza-transforming-americas-energy-outlook-and-boosting-its Economist on Marcellus Region]
**[http://www.bucknell.edu/script/environmentalcenter/marcellus/ Bucknell University Marcellus Shale Initiative Publications Database]

{{Good article}}
{{Shale gas}}
{{Chronostratigraphy of Maryland}}
{{Chronostratigraphy of New York}}
{{Chronostratigraphy of Ohio}}
{{Chronostratigraphy of Pennsylvania}}
{{Chronostratigraphy of Virginia}}
{{Stratigraphic column of West Virginia}}
{{Pittsburgh Metro Area}}
{{Organic Shales}}

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[[Category:Devonian System of North America]]
[[Category:Devonian southern paleotemperate deposits]]
[[Category:Devonian Pennsylvania]]