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[[File:M97Ch4FG2.jpg|400px|thumb|{{figure number|2}}A generalized stratigraphic chart of the Marcellus Shale interval in West Virginia, Ohio, Pennsylvania, and New York. Modified from Patchen et al.;<ref>Patchen, D. G., K. L. Avary, and R. B. Erwin, 1985, Correlation of Stratigraphic Units of North America (COSUNA) project-Northern Appalachian Region: American Association of Petroleum Geologists, 1 sheet.</ref> Lash and Engelder;<ref name=L&E2008>Lash, G. G., and T. Engelder, 2008, [http://www.papgrocks.org/lash_p.pdf Marcellus Shale subsurface stratigraphy and thickness trends: Eastern New York to northeastern West Virginia (abs.)]: AAPG Eastern Section Meeting.</ref> and Piotrowski et al.<ref>Piotrowski, R. G., S. A. Krajewski, and L. Heyman, 1977, Stratigraphy and gas occurrence in the Devonian organic-rich shales of Pennsylvania, in G. L. Schott, W. K. Overbey Jr., A. E. Hunt, and C. A. Komar, eds., Preprints, First Eastern Gas Shales Symposium, U.S. Energy Research and Development Administration, Morgantown Energy Research Center, Morgantown, West Virginia, p. 77–94.</ref>]]

[[File:M97Ch4FG2.jpg|400px|thumb|{{figure number|2}}A generalized stratigraphic chart of the Marcellus Shale interval in West Virginia, Ohio, Pennsylvania, and New York. Modified from Patchen et al.;<ref>Patchen, D. G., K. L. Avary, and R. B. Erwin, 1985, Correlation of Stratigraphic Units of North America (COSUNA) project-Northern Appalachian Region: American Association of Petroleum Geologists, 1 sheet.</ref> Lash and Engelder;<ref name=L&E2008>Lash, G. G., and T. Engelder, 2008, [http://www.papgrocks.org/lash_p.pdf Marcellus Shale subsurface stratigraphy and thickness trends: Eastern New York to northeastern West Virginia (abs.)]: AAPG Eastern Section Meeting.</ref> and Piotrowski et al.<ref>Piotrowski, R. G., S. A. Krajewski, and L. Heyman, 1977, Stratigraphy and gas occurrence in the Devonian organic-rich shales of Pennsylvania, in G. L. Schott, W. K. Overbey Jr., A. E. Hunt, and C. A. Komar, eds., Preprints, First Eastern Gas Shales Symposium, U.S. Energy Research and Development Administration, Morgantown Energy Research Center, Morgantown, West Virginia, p. 77–94.</ref>]]

[[:File:M97Ch4FG2.jpg|Figure 2]] shows the regional stratigraphy of the Devonian shales and the Marcellus Shale Formation. The Middle Devonian Marcellus Shale Formation is located within the lower part of the Hamilton Group, which is bounded above by the Middle Devonian Tully Limestone and below by the Lower Devonian Onondaga Limestone (Onesquethaw Group). The Marcellus is divided into two members, the lower Marcellus/Union Springs Shale and the upper Marcellus/Oatka Creek Shale, which are separated by the Cherry Valley/Purcell Limestone. Lash<ref name=Lsh2008>Lash, G. G., 2008, Stratigraphy and fracture history of the Middle and Upper Devonian succession, western New York: Significance to basin evolution and hydrocarbon exploration: Pittsburgh Association of Petroleum Geologists spring field trip guidebook, 88 p.</ref> interprets the Cherry Valley and the Purcell limestones to be equivalent, although other authors, including de Witt et al.<ref>de Witt Jr., W., J. B. Roen, and L. G. Wallace, 1993, Stratigraphy of Devonian black shales and associated rocks in the Appalachian Basin, in J. B. Roen and R. C. Kepferle, 1993, Petroleum geology of the Devonian and Mississippian black shale of eastern North America: U.S. Geological Survey Bulletin, v. 1909, p. M1–M16.</ref> and Werne et al. (2002), show the Cherry Valley and Purcell as separate members.

[[:File:M97Ch4FG2.jpg|Figure 2]] shows the regional stratigraphy of the Devonian shales and the Marcellus Shale Formation. The Middle Devonian Marcellus Shale Formation is located within the lower part of the Hamilton Group, which is bounded above by the Middle Devonian Tully Limestone and below by the Lower Devonian Onondaga Limestone (Onesquethaw Group). The Marcellus is divided into two members, the lower Marcellus/Union Springs Shale and the upper Marcellus/Oatka Creek Shale, which are separated by the Cherry Valley/Purcell Limestone. Lash<ref name=Lsh2008>Lash, G. G., 2008, Stratigraphy and fracture history of the Middle and Upper Devonian succession, western New York: Significance to basin evolution and hydrocarbon exploration: Pittsburgh Association of Petroleum Geologists spring field trip guidebook, 88 p.</ref> interprets the Cherry Valley and the Purcell limestones to be equivalent, although other authors, including de Witt et al.<ref>de Witt Jr., W., J. B. Roen, and L. G. Wallace, 1993, Stratigraphy of Devonian black shales and associated rocks in the Appalachian Basin, in J. B. Roen and R. C. Kepferle, 1993, Petroleum geology of the Devonian and Mississippian black shale of eastern North America: U.S. Geological Survey Bulletin, v. 1909, p. M1–M16.</ref> and Werne et al.,<ref name=Wrnetal>Werne, J. P., B. B. Sageman, T. W. Lyons, and D. J. Hollander, 2002, An integrated assessment of a “type euxinic” deposit: Evidence for multiple controls on black shale deposits in the Middle Devonian Oatka Creek Formation: American Journal of Science, v. 302, p. 110–143, doi:10.2475/ajs.302.2.110.</ref> show the Cherry Valley and Purcell as separate members.

Several unconformities have been identified within the Marcellus Shale by Lash<ref name=Lsh2008 /> in distal areas of the Appalachian Basin in western New York and northwestern Pennsylvania. These include unconformities that are the upper sequence boundaries for Union Springs and Oatka Creek shales. Lash<ref>Lash, G. G., 2009a, [http://www.searchanddiscovery.com/abstracts/html/2009/annual/abstracts/lash.htm The Middle Devonian Marcellus Shale: A record of eustasy and basin dynamics]: AAPG Search and Discovery article 90090.</ref><ref>Lash, G. G., 2009b, Sequence-stratigraphic framework of the Middle Devonian Marcellus Shale (abs.): AAPG 2009 Eastern Section Meeting: http://karl.nrcce.wvu.edu/esaapg/ESAAPG_Meetings/2009/2009_Abstracts.pdf (accessed September 18, 2009).</ref> documents that the entire Union Spring Shale is removed by a regional disconformity in some of these areas. These unconformity surfaces below and above the Marcellus Shale are interpreted to become conformable to the southeast, into the deeper parts of the basin (Hamilton-Smith, 1993; <ref name=M&S2006>Milici, R. C., and C. S. Swezey, 2006, [http://pubs.usgs.gov/of/2006/1237/of2006-1237.pdf Assessment of Appalachian Basin oil and gas resources: Devonian shale–Middle and Upper Paleozoic total petroleum system]: U.S. Geological Survey Open-File Report Series 2006-1237.</ref>; Boyce, 2009). A major Middle Devonian unconformity above the Tully Limestone (Hamilton-Smith 1993) progressively removes stratigraphically older units from east to west. Moving west toward the Cincinnati arch, this unconformity truncates the entire Tully, Hamilton, and progressively older formations.

Several unconformities have been identified within the Marcellus Shale by Lash<ref name=Lsh2008 /> in distal areas of the Appalachian Basin in western New York and northwestern Pennsylvania. These include unconformities that are the upper sequence boundaries for Union Springs and Oatka Creek shales. Lash<ref>Lash, G. G., 2009a, [http://www.searchanddiscovery.com/abstracts/html/2009/annual/abstracts/lash.htm The Middle Devonian Marcellus Shale: A record of eustasy and basin dynamics]: AAPG Search and Discovery article 90090.</ref><ref>Lash, G. G., 2009b, Sequence-stratigraphic framework of the Middle Devonian Marcellus Shale (abs.): AAPG 2009 Eastern Section Meeting: http://karl.nrcce.wvu.edu/esaapg/ESAAPG_Meetings/2009/2009_Abstracts.pdf (accessed September 18, 2009).</ref> documents that the entire Union Spring Shale is removed by a regional disconformity in some of these areas. These unconformity surfaces below and above the Marcellus Shale are interpreted to become conformable to the southeast, into the deeper parts of the basin (Hamilton-Smith, 1993; <ref name=M&S2006>Milici, R. C., and C. S. Swezey, 2006, [http://pubs.usgs.gov/of/2006/1237/of2006-1237.pdf Assessment of Appalachian Basin oil and gas resources: Devonian shale–Middle and Upper Paleozoic total petroleum system]: U.S. Geological Survey Open-File Report Series 2006-1237.</ref>; Boyce, 2009). A major Middle Devonian unconformity above the Tully Limestone (Hamilton-Smith 1993) progressively removes stratigraphically older units from east to west. Moving west toward the Cincinnati arch, this unconformity truncates the entire Tully, Hamilton, and progressively older formations.

The Acadian orogeny was the result of a probable collision between a part of the North American plate and a microcontinent called the Avalonian terrain (Williams and Hatcher, 1982). Ettensohn (1985a, b) linked basin deformation and subsidence to fold-belt orogeny, where a migrating foreland basin (i.e., proximal trough) was created cratonward (westward) of the orogen with a forebulge, the Cincinnati arch, located even farther cratonward to the west. This model suggests that major orogenic highlands (Acadian Highlands) were located to the east of the Marcellus depositional basin from which clastic sediments were derived. These highlands also contributed to deformational loading, providing the accommodation space for accumulating sediments within the subsiding basin.

The Acadian orogeny was the result of a probable collision between a part of the North American plate and a microcontinent called the Avalonian terrain (Williams and Hatcher, 1982). Ettensohn (1985a, b) linked basin deformation and subsidence to fold-belt orogeny, where a migrating foreland basin (i.e., proximal trough) was created cratonward (westward) of the orogen with a forebulge, the Cincinnati arch, located even farther cratonward to the west. This model suggests that major orogenic highlands (Acadian Highlands) were located to the east of the Marcellus depositional basin from which clastic sediments were derived. These highlands also contributed to deformational loading, providing the accommodation space for accumulating sediments within the subsiding basin.

During deposition of the Marcellus Shale, the central Appalachian Basin is interpreted to have been located between 15 and 30degS latitude (Ettensohn, 1992) with an associated dry tropical or savanna-like climate where rainfall was seasonal with extended dry conditions. In addition, the area was likely to have been subjected to significant seasonal storm activity (Woodrow et al., 1973). Reconstructions place the basin in the path of southeasterly trade winds, which would have carried moisture from the Iapetus Ocean westward across the Acadian Highlands located east of the basin (Ettensohn, 1985b). Ettensohn (1985b) proposed that the Acadian Highlands created a rain shadow effect on the western slopes of these highlands that would have contributed to the arid conditions. The arid conditions and prevailing trade winds are likely to have introduced eolian siliciclastics into the Marcellus depositional basin from lands to the east. Werne et al. (2002) reported the presence and enrichment of eolian silt grains in the organic-rich facies of the Oatka Creek and directly related this to a decrease in carbonate and noneolian siliciclastic sediments. In addition, Sageman et al. (2003) reported a direct relationship between increasing eolian silts and increasing total organic carbon in the Marcellus Shale.

During deposition of the Marcellus Shale, the central Appalachian Basin is interpreted to have been located between 15 and 30degS latitude (Ettensohn, 1992) with an associated dry tropical or savanna-like climate where rainfall was seasonal with extended dry conditions. In addition, the area was likely to have been subjected to significant seasonal storm activity (Woodrow et al., 1973). Reconstructions place the basin in the path of southeasterly trade winds, which would have carried moisture from the Iapetus Ocean westward across the Acadian Highlands located east of the basin (Ettensohn, 1985b). Ettensohn (1985b) proposed that the Acadian Highlands created a rain shadow effect on the western slopes of these highlands that would have contributed to the arid conditions. The arid conditions and prevailing trade winds are likely to have introduced eolian siliciclastics into the Marcellus depositional basin from lands to the east. Werne et al.<ref name=Wrnetal /> reported the presence and enrichment of eolian silt grains in the organic-rich facies of the Oatka Creek and directly related this to a decrease in carbonate and noneolian siliciclastic sediments. In addition, Sageman et al. (2003) reported a direct relationship between increasing eolian silts and increasing total organic carbon in the Marcellus Shale.

Based on petrographic analysis of preserved organic matter from the Marcellus in western New York, Sageman et al. (2003) reported that the Marcellus black mudstone contained 100% marine material. The organic-rich facies of the Marcellus are dominated by short alkanes, whereas the reverse is true for the non-organic-rich facies that are dominated by long alkanes (Murphy, 2000). This indicates that terrestrial input of organic material into the basin was dominant during those periods when non-organic-rich muds and carbonate were deposited and that algal marine phytoplankton were dominant during the deposition of the Marcellus organic-rich black mudstones.

Based on petrographic analysis of preserved organic matter from the Marcellus in western New York, Sageman et al. (2003) reported that the Marcellus black mudstone contained 100% marine material. The organic-rich facies of the Marcellus are dominated by short alkanes, whereas the reverse is true for the non-organic-rich facies that are dominated by long alkanes (Murphy, 2000). This indicates that terrestrial input of organic material into the basin was dominant during those periods when non-organic-rich muds and carbonate were deposited and that algal marine phytoplankton were dominant during the deposition of the Marcellus organic-rich black mudstones.

Like other organic-rich shales, the creation, deposition, and preservation of the organic Marcellus sediments was controlled by three factors: (1) primary photosynthetic production, (2) bacterial decomposition, and (3) bulk sedimentation rate (Sageman et al., 2003). The traditional interpretation of deposition of the organic-rich members of the Marcellus Shale is a preservation model of organic enrichment, where a permanently stratified water column with anoxic or euxinic (anoxic-sulfidic) bottom water conditions allowed for the preservation of organic material (Demaison and Moore, 1980). This preservation model is best reflected in the proposed model of a nearly permanent pycnocline by Ettensohn (1992).

Like other organic-rich shales, the creation, deposition, and preservation of the organic Marcellus sediments was controlled by three factors: (1) primary photosynthetic production, (2) bacterial decomposition, and (3) bulk sedimentation rate (Sageman et al., 2003). The traditional interpretation of deposition of the organic-rich members of the Marcellus Shale is a preservation model of organic enrichment, where a permanently stratified water column with anoxic or euxinic (anoxic-sulfidic) bottom water conditions allowed for the preservation of organic material (Demaison and Moore, 1980). This preservation model is best reflected in the proposed model of a nearly permanent pycnocline by Ettensohn (1992).

Recent workers have disputed the original theory of deep-water deposition with consistent anoxia. Werne et al. (2002) and Sageman et al. (2003) proposed that deposition of the Marcellus organic-rich members did not appear to have been beneath a permanently stratified water column. Instead, they proposed that the Marcellus organic-rich units were deposited without a permanent pycnocline and with possible seasonal fluctuations. Macquaker et al. (2009) investigated the Marcellus Shale at bed-scale levels and found a variety of sedimentary structures inconsistent with a continuous deep-water anoxic model, including rip-up clasts and ripple lamina. Based on this work, Macquaker et al. (2009) proposed that the organic-rich mudstones were not deposited in waters that were persistently anoxic, but that instead, the sea floor was occasionally reworked, which would have led to a destruction of a part of the organics. Boyce and Carr (2010) proposed that possible small local microanoxic environments were factors in the deposition of the organic-rich members based on local variations in the black shale units and thin limestones.

Recent workers have disputed the original theory of deep-water deposition with consistent anoxia. Werne et al.<ref name=Wrnetal /> and Sageman et al. (2003) proposed that deposition of the Marcellus organic-rich members did not appear to have been beneath a permanently stratified water column. Instead, they proposed that the Marcellus organic-rich units were deposited without a permanent pycnocline and with possible seasonal fluctuations. Macquaker et al. (2009) investigated the Marcellus Shale at bed-scale levels and found a variety of sedimentary structures inconsistent with a continuous deep-water anoxic model, including rip-up clasts and ripple lamina. Based on this work, Macquaker et al. (2009) proposed that the organic-rich mudstones were not deposited in waters that were persistently anoxic, but that instead, the sea floor was occasionally reworked, which would have led to a destruction of a part of the organics. Boyce and Carr (2010) proposed that possible small local microanoxic environments were factors in the deposition of the organic-rich members based on local variations in the black shale units and thin limestones.

The paleogeographic reconstruction by Ettensohn (1985b), Woodrow and Sevon (1985), and Blakey (2005) shows that the organic-rich deposition occurred in a large, nearly enclosed, three-sided embayment that likely would have served to enhance oceanic organic productivity. [[:File:M97Ch4FG3.jpg|Figure 3]] shows the paleogeographic reconstruction by Blakey (2005) of the Appalachian area about 385 Ma. The arid conditions that were likely present during deposition of the organic-rich facies led to a probable sediment starvation, as evidenced by the decrease in noneolian siliciclastic deposition in the organic-rich facies, preventing dilution of the accumulating organic material.

The paleogeographic reconstruction by Ettensohn (1985b), Woodrow and Sevon (1985), and Blakey (2005) shows that the organic-rich deposition occurred in a large, nearly enclosed, three-sided embayment that likely would have served to enhance oceanic organic productivity. [[:File:M97Ch4FG3.jpg|Figure 3]] shows the paleogeographic reconstruction by Blakey (2005) of the Appalachian area about 385 Ma. The arid conditions that were likely present during deposition of the organic-rich facies led to a probable sediment starvation, as evidenced by the decrease in noneolian siliciclastic deposition in the organic-rich facies, preventing dilution of the accumulating organic material.

*  

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* Weary, D. J., R. T. Ryder, and R. Nyahay, 2000, Thermal maturity patterns (CAI and % Ro) in the Ordovician and Devonian rocks of the Appalachian Basin in New York state: U.S. Geological Survey Open-File Report 2000-496, 39 p.

* Weary, D. J., R. T. Ryder, and R. Nyahay, 2000, Thermal maturity patterns (CAI and % Ro) in the Ordovician and Devonian rocks of the Appalachian Basin in New York state: U.S. Geological Survey Open-File Report 2000-496, 39 p.

* Werne, J. P., B. B. Sageman, T. W. Lyons, and D. J. Hollander, 2002, An integrated assessment of a “type euxinic” deposit: Evidence for multiple controls on black shale deposits in the Middle Devonian Oatka Creek Formation: American Journal of Science, v. 302, p. 110–143, doi:10.2475/ajs.302.2.110.

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* Wheeler, R. L., 1980, Cross-strike structural discontinuities: Possible exploration tool for natural gas in Appalachian overthrust belt: AAPG Bulletin, v. 64, no. 12, p. 2166–2178.

* Wheeler, R. L., 1980, Cross-strike structural discontinuities: Possible exploration tool for natural gas in Appalachian overthrust belt: AAPG Bulletin, v. 64, no. 12, p. 2166–2178.

* Williams, H., and R. D. Hatcher, 1982, Suspect terranes and accretionary history of the Appalachian orogen: Geology, v. 10, p. 530–536, doi:10.1130/0091-7613(1982)102.0.CO;2.

* Williams, H., and R. D. Hatcher, 1982, Suspect terranes and accretionary history of the Appalachian orogen: Geology, v. 10, p. 530–536, doi:10.1130/0091-7613(1982)102.0.CO;2.