Marcellus Formation

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Marcellus Formation
Marcellus shale exposure above Marcellus, N.Y.
Marcellus shale exposure above Marcellus, N.Y.
Type Geological formation
Unit of Hamilton Group
Sub-units See: Named members
Extent 600 miles (970 km)[1]
Thickness up to 900 feet (270 m)[2]
Lithology
Primary Shale
Other Slate, Limestone, Sandstone
Location
Named after Marcellus, New York
Region Appalachian Basin of
eastern North America

The Devonian age Marcellus Formation (abbr. Dm), also classified as the Marcellus Subgroup of the Hamilton Group (Dh), Marcellus Member of the Romney Formation, or simply the Marcellus Shale (Dms), is a mapped sedimentary bedrock unit in eastern North America. Named for an outcrop found near the town of Marcellus, New York during a geological survey in 1839,[3] it extends throughout much of the Appalachian Basin.[4] The energy development community is developing its subsurface layers deep beneath the Allegheny Plateau as a significant source of largely untapped natural gas reserves located near the high-demand markets along the East Coast of the United States.[5]

The Marcellus is the lowest unit of the Hamilton Group, and is divided into several named sub-units. Although black shale is the dominant lithology, there are lighter shales and interbedded limestone layers that geologists attribute to tectonic activity during its deposition. The black shale was deposited almost 400 million years ago in deep water devoid of oxygen, making it only sparsely fossiliferous. Most fossils are contained in the limestones, and the fossil record in these layers provide important paleontologal insights on faunal turnovers. The black shales also contain iron ore, uranium and pyrite, and the fissile shales are easily eroded, presenting civil and environmental engineering challenges.

Contents

[edit] Description

The Marcellus Formation is a radioactive, carbonaceous black shale that may contain limestone beds and pyrite and siderite concentrations.[6] The iron pyrite (FeS2) is especially abundant near the base,[7] and the upper contacts of limestones, but framboidal microcrystals and euhedral crystals of pyrite occur throughout the organic rich deposits.[8] The Marcellus is uraniferous,[9][10] and the radioactive decay of the uranium-238 (238U) makes it a source rock for radioactive radon gas (222Rn).[11]

Fragments below exposure of fissile Marcellus black shale at Marcellus, N.Y.
Fragments below exposure of fissile Marcellus black shale at Marcellus, N.Y.

Measured total organic content of the Marcellus Formation ranges from less than 1%, to over 11% in New York state,[12] and the shale may contain enough carbon to support combustion.[13] The more organic rich black shales can be bituminous, but are too old to contain bituminous coal.[14] Bedding in the Marcellus is moderately well developed, and like most shales is fissile,[6] so it tends to split easily along the bedding plane.

The Marcellus shale runs across the southern tier and Finger Lakes regions of 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 in the United States.[15] It also extends below Lake Erie and across the border into Canada, running between Port Stanley and Long Point to St. Thomas in southern Ontario.[16] The Marcellus bedrock in eastern Pennsylvania extends below the Delaware River into extreme western New Jersey.[2] It also exists in the subsurface of a small portion of Kentucky and Tennesee.[17]

The Marcellus appears in outcrops along the northern margin of the formation in central New York, and these contain very small beds that resemble coal.[18] These outcrops, and others further south, were extensively excavated in the early 19th century, sometimes at great expense, in the false hope of finding minable coal seams.[18][19] Its close proximity to the surface makes an east-west band running through the city of Syracuse[9] a high risk area for radon as an indoor air pollutant.[20] From the surface exposures along the northern and eastern margins, the formation descends to depths of over 9,000 feet (2,700 m) below the surface in southern Pennsylvania.[17]

Upturned beds are exposed in sections of the folded Ridge-and-Valley Appalachians,[15] including exposures on the flanks and axis of the Broad Top Synclinorium in south central Pennsylvania.[7] Exposed beds are nearly horizontal on the Allegheny Plateau,[21] but upturned to slightly overturned beds are found along the Allegheny Front.[22]

Delaware River above Walpack Bend, where it leaves the buried valley lying on a bed of Marcellus Shale
Delaware River above Walpack Bend, where it leaves the buried valley lying on a bed of Marcellus Shale

The Marcellus shale, like other shales, is classified by geologists as a slope-former. It is easily eroded, and is also found underlying low areas between some Appalachian ridges, forming linear valleys of moderate relief. The bedrock surface is typically covered with colluvium from erosion of stratigraphically higher and more erosional resistant strata that form the surrounding higher ground.[7] Upturned beds of the soft shale also capture streams and rivers, creating relatively straight segments where the bedrock is covered in alluvial valleys.

Below Port Jervis, New York the Wallpack Ridge deflects the Delaware River into the Minisink Valley, where it follows the southwest strike of the eroded Marcellus beds for 25 miles (40 km) to the end of the ridge at Wallpack Bend in the Delaware Water Gap National Recreation Area.[2][23] The Minisink is a buried valley where the Delaware flows in a bed of 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.[23]

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.[24]The newly exposed shale on the cut face weathers rapidly, allowing air and water into the unexcavated rock, resulting in acidic runoff after precipitation events.[25] Damage to structures constructed on fill consisting of pyritic Marcellus shale has been attributed to expansion from sulfuric acid (H2SO4) runoff reacting with the calcite (CaCO3) in the shale to produce gypsum (CaSO4), which has double the molar volume.[26]

[edit] Deposition

The black and gray shales of the Hamilton Group were formed early in the Acadian orogeny after terrigenous sediments (originating on land) from erosion of the Acadian Mountains were deposited into the sea.[12] The Marcellus Shale was formed from the very first deposits in a relatively deep, sediment starved, and oxygen starved (anoxic), trough.[27] These tiny clastic fragments of rock were carried to the river deltas, and continued flowing offshore as turbidites, coming to rest at the bottom of the Acadian foredeep basin,[28] hundreds of meters from shore, at depths that may have been 150 meters (500 ft) or more beneath the surface.[8] Alternatively, the water may have been as shallow as 150 feet (50 m) or less, if the water column was sufficiently stratified so that oxygen rich surface water could not mix with the anoxic bottom water.[29] In either case, the Marcellus is a transgressive black shale,[30] because it was deposited in deepening conditions.[31]

The dark shale is composed of flysch, a fine mud deposited in the deep water, burying the underlying Onondaga limestone beds,[32] as the deepening sea cut off the supply of carbonates that formed it.[33] Uranium was also incorporated in these organic muds syndepositionally,[9] meaning it was deposited at the same time, rather than being introduced to the formation later.[34] The Marcellus was deposited during a period of 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.[35]

The members of the Marcellus reflect two composite depositional sequences.[36] The interbedding of lighter shale and limestone members is attributed to relatively short-term oscillations in basin depth.[37] The deep water depositional sequences continued to form the overlying Brallier Formation and Harrell Formation.[28]

[edit] Age

On the geological timescale, the Marcellus occurs in the Middle Devonian epoch, of the Devonian period, in the Paleozoic era, of the Phanerozoic eon. 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 (Ma).[38] Radiometric dating of a Marcellus sample from Pennsylvania placed its age at 384 Ma, and a sample from the bentonite at the top of the Onondaga at 390 ± 0.5 Ma.[39] The anoxic dark shales in the upper reaches of the formation mark the Kačák Event,[40] a late Eifelian stage marine anoxic event also associated with an extinction event.[41]

Generalized stratigraphic nomenclature for the Middle Devonian strata in the Appalachian basin.
Generalized stratigraphic nomenclature for the Middle Devonian strata in the Appalachian basin.[42]

[edit] Stratigraphy

In the first Pennsylvania Geological Survey, begun in 1836, the Marcellus was originally classified as the "Cadent Lower Black Slate" and given the abbreviation "No. XIII b."[43] In the first New York State Geological Survey, also begun that year, James Hall established the term "Marcellus Shale" in his 1839 report titled "Marcellus Shales in Seneca County."[44] 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 his location based nomenclature where, "the rock or group will receive its name from the place where it is best developed".[45] 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, are now universally accepted.

[edit] Overlying

The Marcellus is the basal unit of the Hamilton Group (Dh),[12] lying beneath the Mahantango Formation (Dmh) member of this group in Pennsylvania,[46] and Maryland. In New York, the Mahantango is further divided, and the Marcellus is separated from the overlying Skaneateles Formation, a more clastic and fossiliferous dark shale, by the thin Stafford or Mottville Limestone bed.[47][48] In West Virginia, the Marcellus may be separated from the brown shales of the Mahantango by occasional sandstone beds and concretions.[49] In Ohio it is disconformably beneath the Rhinestreet Shale Member of the West Falls Formation, another transgressive black shale tongue with similar characteristics to the Marcellus.[30]

[edit] Underlying

The Marcellus shale is typically found deposited on the limestone of the Onondaga Formation (Don), but the contact 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.[50][51] In Pennsylvania, the Marcellus forms a sharp conformable contact with the Onondaga's Selinsgrove Limestone member.[52] An 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.[13]

In eastern New York, the contact between the Marcellus and Onondoga (where present) is gradational.[53][54] In western New York, the Union Springs member of the Marcellus conformably overlies the Seneca member of the Onondaga Limestone,[8] or the stratigraphically higher Cherry Valley Limestone member may rest directly and unconformably upon the Onondaga in the absence of the Union Springs shale.[55][50] The local disappearance of units of the Onondaga suggests that its upper contact with the Marcellus can be erosional.[55] In Erie County in western New York, both the upper and lower contact of the Marcellus are eroded away.[56] 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.[49] To the south and west, the Hamilton Group grades laterally into the Millboro Shale formation in southern West Virginia and Virginia,[42] which grades into the lower part of the Chattanooga Shale of Tennessee.[57]

The Milboro is gradational with the underlying Needmore Formation shale.[57] South of the Mason-Dixon line, due to the difficulty in differentiating the Millboro and Needmore shales with the limited exposures available,[58] they were mapped as the Romney Formation, named for an exposure at Romney, West Virginia.[3] In this area, the Romney classification is now obsolete, but its Marcellus and underlying Needmore shale members are still grouped in an undifferentiated map unit (Dmn).[59]

[edit] Tioga ash beds

Tioga metabentonite or K-bentonite – a stratigraphic unit about 2 feet (0.6 m) thick that consists of several discrete, relatively thin volcanic ash falls – is also included at the base of the Marcellus in eastern Pennsylvania.[46] The Tioga is a regional stratigraphic marker,[42] used by geologists and paleontologists to correlate equivalent strata.[50][60] It was first described, but not named, by Hall in 1843,[61] and was finally named for the gas fields in Tioga County, Pennsylvania more than 100 years later.[62]

Marcellus exposure along Interstate 80 in eastern Pennsylvania where the formation is thickest
Marcellus exposure along Interstate 80 in eastern Pennsylvania where the formation is thickest

The Tioga is widely distributed across the central and northern parts of the Appalachian basin from Virginia to New York,[63] covering an area of over 102,000 sq mi (265,000 sq km).[64] Explosive eruptions centered in what is now central Virginia released the ash into the atmosphere.[62] It was dispersed across the Appalachian, Michigan, and Illinois Basins by the southern trade winds, because this area was in the southern hemisphere in the Devonian period.[64]

The Tioga ash bed zone consists of eight ash beds numbered according to their stratigraphic order from A (oldest) to H (youngest),[63][65] and another bed known as the Tioga middle coarse zone.[64][66] 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.[42] In eastern and western New York state, the Tioga ash bed B marks the boundary between the Moorehouse and Seneca Members of the Onondaga Formation,[63] but in the central part of the state, the ash beds are actually in the Marcellus.[67][54] This indicates that deposition of the Marcellus there began earlier,[54] since the ash beds represent a single epoch in geologic time.

[edit] Thickness

Maximum thickness of the Marcellus ranges from 900 feet (270 m) in New Jersey,[2] to 12 metres (40 ft) in Canada.[16] In West Virginia, the Marcellus Formation is as much as 200 feet (60 m) thick.[49] In extreme eastern Pennsylvania, it is 800 feet (240 m) thick,[23] thinning to the west, becoming only 50 feet (15 m) thick along the Ohio River, and only a few feet in Licking County, Ohio.[4] The thinning, or stratigraphic convergence, from east to west is caused by decreasing grain size in the clastic deposits, which entered the basin from the east.[60] The beds finally "pinch out" westward because deposition was limited by the Cincinnati Arch,[30][68] 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.


Geologic cross section of upper to middle Devonian strata from Cherry Valley, New York south-southhwest across the Allegheny Plateau and then along the Ridge-and-Valley Appalachians to Tennessee. Note the Marcellus grades up to the Milboro and Chattanooga black shales.
Geologic cross section of upper to middle Devonian strata from Cherry Valley, New York south-southhwest across the Allegheny Plateau and then along the Ridge-and-Valley Appalachians to Tennessee. Note the Marcellus grades up to the Milboro and Chattanooga black shales.[49]


Members of the Marcellus Formation from West to East in New York
Buffalo Rochester Syracuse Utica Albany

Oakta Creek shale

 Cardiff shale 

 Pecksport shale (Mt. Marion Fm.)
Solsville sandstone
Bridgewater shale Otsego
Chittenango Shale Berne
Cherry Valley limestone Stony Hollow
(Seneca Fm.) Union Springs shale and limestone
(Seneca Fm.) Union Springs shale
Table showing the approximate correlation between the named members of
the Marcellus Formation from west to east across central New York state.
(Items in parentheses are from other formations).[47][69]

[edit] Named members

The Marcellus is divided by the local Purcell limestone member in eastern Pennsylvania,[46] a 50 to 100 foot (15 to 30 m) layer of inter-bedded calcitic shale and limestone,[60] which is stratigraphically equivalent to the Cherry Valley Limestone of New York.[70] The Cherry Valley is a bioclastic packstone,[8] consisting of skeletal limestones, with shaly intervals[71] between its lower massive limestone layer, thick nodular limestone/marlstone, and upper limestone layer.[72] Other named members include the Bakoven Shale, Cardiff Shale, Chittenango shale, Solsville sandstone, Union Springs shale and limestone,[38] and Stony Hollow shale and limestone.[73] The Union Springs, Cherry Valley, and Oatka Creek merge beneath Lake Erie, into the Bell Shale, Rockport Quarry Limestone, and Arkona Shale of Ontario.[50]

The Union Springs is an organic-rich, pyritiferous, thinly bedded, blackish gray to black shale with mudstone concretionary layers,[72] and thin silt bands at the bottom. To the east, it becomes the Bakoven Member, a darker, less organic shale with fewer limestone layers.[69] To the west the Union Springs beds thin, with its upper limestones merging with the overlying calcareous Cherry Valley Member.[69] A regional unconformity appears in western New York, as the Union Springs lenses in and out,[69] and then reappears in northwest Pennsylvania and northeast Ohio between the Onondaga and Cherry Valley.[50]

The uppermost member in Western and central New York is the dark grey to black organic-rich Oatka Creek shale, which becomes the Cardiff and Chittenango members in Central and Eastern New York as the formation thickens.[71] Organic-rich, fissile, sooty black shales make up the Chittenango Member.[72] At the base of the Chittenango,[74] above the Bierne Member shale,[75] lies the Halihan Hill Bed, a highly bioturbated bioclastic limestone.[8]

Further east, as the formation continues to thicken, the Cardiff into the 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 overlying it.[76] 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.[77]

In south central Pennsylvania, the Marcellus is mapped with three members, from top to base: The Mahanoy Member (Dmm), a dark gray to grayish black silty shale and siltstone; the Turkey Ridge Member (Dmt), an olive to dark-gray fine to medium grained sandstone; and the Shamokin Member (Dms), a dark gray to grayish black fissile carbonaceous shale that is calcareous in places near the base.[78] The Turkey Ridge is commonly mapped in the Mahantango Formation,[79] or included in the Montebello Formation (Dmot),[80] and only the Shamokin correlates with the Marcellus on adjacent map sheets.[81] 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 900 feet (275 m) thick.[82]

Illustration of a Cephalopod (Goniatites Vanuxemi) fossil from the Marcellus Formation.
Illustration of a Cephalopod (Goniatites Vanuxemi) fossil from the Marcellus Formation.[83]

[edit] Fossils

There are sparse inclusions of fossilized marine fauna found in the Marcellus,[46] including specimens of the large brachiopod Spinocyrtia.[84] External molds of crinoids are found in the Marcellus,[85] with the molds partially filled with limonite; brachiopod and bivalve molds have also been found in the shale.[86] Small conical tentaculitids are commonly found in the Chittenango Member.[72] The Halihan Hill bed contains styliolinids and macrofauna including brachiopods, bryozoans, small bivalves and gastropods,[8] incorporated after the faunal turnover when Emsian and Eifelian Schoharie/Onondaga fauna were replaced by the Givetian Hamilton fauna.[74]

The Solsville member contains well preserved bivalves, gastropods, and brachiopods.[76] These animals lived in the benthic zone at the bottom of marginal marine to open marine environments that existed west of the ancient Catskill Delta.[87] The fossil record in this member shows the base was dominated by deposit feeders, while the upper layers were dominated by filter feeders.[87] 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.[87]

A diverse conodont fauna occurs in the limestone of the Cherry Valley Member,[88] which is also known for its rich nautiloid and goniatite cephalopod fauna,[72] including the "Cephalopod Graveyard" in the Schoharie Valley of eastern New York. This an unusual accumulation of abundant coiled and straight shells of several types of large adult cephalopods, with a lack of juveniles, indicating that if their behavior was similar to modern squid, this may have been an area where these Middle Devonian cephalopods reproduced and died.[89] This stratigraphic interval provides an excellent example of incursion epiboles, which are sudden appearances and disappearances of fossil taxa in relatively thin sections of the rock unit.[90] In the Cherry Valley, the taxa do not reoccur; instead each thin concretionary limestone bed contains different species of goniatites.[90] The Cherry Valley and Union Springs also contain well-preserved anarcestida.[91]

[edit] Economic uses

Marcellus Formation shale
Region: Allegheny Plateau
Country: United States
Offshore/Onshore: Onshore
Operators: Chesapeake Energy, Chief Oil and Gas, Range Resources
Extent of other Devonian shales (green) with Marcellus shale (gray) and thickness isopachs (in feet)
Field History
Start of production: 2000s
Production
Estimated GIIP (Bcf): 168×1012 cu ft (4,800 km³) – 516×1012 cu ft (14,600 km³)[5]
Producing Formations: Marcellus Formation

[edit] Fossil fuel

The impervious limestone layers of the Onondaga directly below the Marcellus, and the Tully Limestone at the top of the Hamilton Group, have trapped valuable natural gas reserves in this formation.[92] The gas is produced by thermogenic decomposition of organic materials in the sediments under the high temperature and pressure generated after the formation was buried deep below the surface of the earth. The rock holds most of the gas in the pore spaces of the shale, with vertical fractures or joints providing some additional storage as well as pathways for the gas to flow; some gas is also adsorbed on mineral grains.[17]

The United States Geological Survey had estimated that the Marcellus contained only 1.9 trillion cubic feet (54 km³) of technically recoverable natural gas in a 2002 publication.[93] In 2008, Terry Engelder, a Pennsylvania State University geosciences professor called his estimate of 168 trillion cubic feet (4,800 km³) conservative.[93] State University of New York at Fredonia geology professor Gary Lash has calculated that more than 500 trillion cubic feet (14,000 km³) of natural gas may be contained in the Marcellus black shale beds that lie between New York state and West Virginia.[94] At the present level of technology, he believes approximately 10% of this, or 50 trillion cubic feet (1.4 trillion m³), could be recovered.[94] This is enough to satisfy approximately two years' of total U.S. consumption,[93] or a total value of approximately one trillion United States dollars.[15]

Before 2000, low production gas wells were completed to the Marcellus, but these had a low rate of return, requiring a relatively long capital recovery period, although they did have a very long productive life.[17] Now, to extract the shale gas at more commercially viable rates,[15] directional drilling is done to depths of 7,000 feet (2,000 m) to 10,000 feet (3,000 m) underground to reach the formation, then water is pumped into the rock under high pressure in a process known as hydraulic fracturing to release the gas from the low permeability shale.[95] This is a technique that was only perfected around the turn of the 21st century outside Fort Worth, Texas to tap the reserves in the Barnett Formation shales of the Bend Arch-Fort Worth Basin, that are now the most prolific source of unconventional gas in the continental United States.[93] The Marcellus covers several times more area,[96] stretching some 600 miles (970 km), compared to a linear extent of only 170 miles (270 km) for the Barnett.[1]

Several companies, many that have specialized in tapping the Barnett, are working to do the same in the Marcellus.[97] As of 2008 only a few new hydro fractured wells tapping the Marcellus reserves were actually in operation, with at least one reported to be producing more than 3 million cubic feet (85 thousand m³) per day.[93] Engelder, who has studied natural rock fractures in this area for more than 25 years, also suggested drilling horizontally through the Marcellus shale perpendicular to these vertical fractures, to exploit these natural features for increased production.[15] Early results show that horizontal wells in this formation are producing gas at a rate more than double that of vertical wells, and at slightly lower cost overall,[96] despite the much higher initial cost of drilling.[15]

Since parts of this region have been producing gas from wells drilled to the deeper Oriskany Formation sandstone, older wells that are no longer viable can be reused, either by fracturing the Marcellus layer in the existing well bore, or by refinishing the bore using horizontal drilling into the Marcellus.[93] Re-use of the existing infrastructure has both environmental as well as economic benefits,[15] because in addition to avoiding development of new drilling sites and wells, existing transportation facilities can be used as well.[96]

Environmental concerns around the drilling and extraction processes have sparked opposition to development of this resource. One concern is the release to the environment of underground naturally occurring radioactive material through the drilling fluids and equipment.[98] The hydraulic fracturing process also uses large quantities of surface water, and potentially threatens underground aquifers.[98] A large portion of the formation underlies the environmentally sensitive Chesapeake Bay Watershed, and the Susquehanna River Basin Commission, which oversees this portion of the watershed, shut down drilling of several wells in May of 2008 because surface water was being diverted by the drillers without the necessary permits.[99] The practices of "land men" who acquire leases for drilling and extraction rights have also been widely questioned.[100]

[edit] Iron ore

In the pyrite-carbonate bed at the base of the Marcellus, pyrite, carbonate, and groundwater reacted to form iron oxide and gypsum.[101] As far as the ground water necessary for the conversion could penetrate, the pyrite-carbonate was converted to usable brown hematite iron ore along the outcrops and near the bedrock surface.[19] 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.[102] 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.[13]

The hematite ore was converted to pig iron in charcoal-fired stone blast furnaces that were constructed throughout the Juniata River region near the workable ore deposits from the Marcellus and other formations.[102] 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,[103] but the cold-blast stone furnaces typically employed were inefficient, and consumed significant amounts of timber from the nearby hardwood forests, which ultimately lead to their demise.[104] A typical furnace used 5,200 pounds (2,400 kg) of hematite ore and 200 imperial bushels (7.3 m³) of charcoal to produce 2,000 pounds (910 kg) of pig iron,[13] and could produce several thousand pounds per day, which required logging more than one acre (4000 m2) of forest daily.[105]

[edit] Paint

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.[19] 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.[23] A bed of hematite paint ore is also found almost directly below the Marcellus, but it is actually part of the underlying Oriskany Formation.[19]

[edit] Other

The Marcellus has also been used locally for shale aggregate and common fill,[7] although the pyritic shales are not suitable for this purpose because of acid rock drainage and volumetric expansion.[106] In the 19th century, this shale was used for walkways and roadways,[14] and was considered superior "road metal" because the fine grained fragments packed together tightly, yet drained well after a rain.[19]

The dark slaty shales may have the necessary 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.[19] The Marcellus black slate was also quarried in Monroe County, Pennsylvania for school slates used by students in 19th century rural schools.[19]

[edit] See also

[edit] External links

[edit] References

  1. ^ a b Durham, Louise S. (2008-03). Appalachian Basin’s Marcellus – the new target : Another Shale Making Seismic Waves. AAPG Explorer. American Association of Petroleum Geologists. Retrieved on 2008-04-06.
  2. ^ a b c d White, Ron W.; Monteverde, Donald H. (2006-02-01). Karst in the Delaware Water Gap National Recreation Area (pdf). Unearthing New Jersey Vol. 2, No. 1. New Jersey Geological Survey. Retrieved on 2008-06-07.
  3. ^ a b Clark, W. B. (1918.). The geography of Maryland, Maryland Geological Survey, volume = vol. 10. 
  4. ^ a b Mayhood, Kevin (2008-03-11). Low down, rich and stingy. The Columbus Dispatch. Retrieved on 2008-04-04.
  5. ^ a b Alan Bailey. "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.?". Petroleum News. Retrieved on 2008-04-05.
  6. ^ a b HRS Documentation Record, Safety Light Corporation, EPA ID No. PAD987295276 (pdf). Hazard Ranking System. United States Environmental Protection Agency. Retrieved on 2008-04-02.
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