Changes

Ettensohn<ref name=Etnsn85a>Ettensohn, F. R., 1985a, The Catskill delta complex and the Acadian orogeny: A model, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 39–49.</ref><ref name=Etnsn85b>Ettensohn, F. R., 1985b, Controls on development of Catskill delta complex basis facies, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 65–77.</ref><ref name=Etnsn1992>Ettensohn, F. R., 1992, Controls on the origin of the Devonian–Mississippian oil and gas shales, east-central United States: Fuel, v. 71, p. 1487–1492, doi:10.1016/0016-2361(92)90223-B.</ref><ref>Ettensohn, F. R., 2004, Modeling the nature and development of major Paleozoic clastic wedges in the Appalachian Basin, U.S.A.: Journal of Geodynamics, v. 37, p. 657–681, doi:10.1016/j.jog.2004.02.009.</ref> assigned the period of deposition of the Marcellus Shale to the second of four tectonically related depositional phases associated with the Devonian Acadian orogeny. Each of Ettensohn's four tectophases of the Acadian orogeny represent regressive episodes that were further subdivided into four stages: (1) the beginning of tectonism and rapid subsidence leading to the accumulation of black shales; (2) deposition of gray shales and siltstones because of impending collision and regression; (3) collision causing widespread uplift and regional disconformities; and (4) widespread accumulation of limestone during a tectonically quiet transgressive period.

Ettensohn<ref name=Etnsn85a>Ettensohn, F. R., 1985a, The Catskill delta complex and the Acadian orogeny: A model, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 39–49.</ref><ref name=Etnsn85b>Ettensohn, F. R., 1985b, Controls on development of Catskill delta complex basis facies, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 65–77.</ref><ref name=Etnsn1992>Ettensohn, F. R., 1992, Controls on the origin of the Devonian–Mississippian oil and gas shales, east-central United States: Fuel, v. 71, p. 1487–1492, doi:10.1016/0016-2361(92)90223-B.</ref><ref>Ettensohn, F. R., 2004, Modeling the nature and development of major Paleozoic clastic wedges in the Appalachian Basin, U.S.A.: Journal of Geodynamics, v. 37, p. 657–681, doi:10.1016/j.jog.2004.02.009.</ref> assigned the period of deposition of the Marcellus Shale to the second of four tectonically related depositional phases associated with the Devonian Acadian orogeny. Each of Ettensohn's four tectophases of the Acadian orogeny represent regressive episodes that were further subdivided into four stages: (1) the beginning of tectonism and rapid subsidence leading to the accumulation of black shales; (2) deposition of gray shales and siltstones because of impending collision and regression; (3) collision causing widespread uplift and regional disconformities; and (4) widespread accumulation of limestone during a tectonically quiet transgressive period.

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<ref name=Etnsn85a /><ref name=Etnsn85b /> 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.<ref>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.</ref> Ettensohn<ref name=Etnsn85a /><ref name=Etnsn85b /> 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<ref name=Etnsn1992 /> 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.<ref name=Etnsn85b /> Ettensohn<ref name=Etnsn85b /> 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.

During deposition of the Marcellus Shale, the central Appalachian Basin is interpreted to have been located between 15 and 30degS latitude<ref name=Etnsn1992 /> 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.<ref>Woodrow, D. L., F. W. Fletcher, and W. F. Ahrnsbrak, 1973, Paleogeography and paleoclimate at the deposition sites of the Devonian Catskill and Old Red Facies: Geological Society of America Bulletin, v. 84, p. 3051–3063, doi:10.1130/0016-7606(1973)842.0.CO;2.</ref> 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.<ref name=Etnsn85b /> Ettensohn<ref name=Etnsn85b /> 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.<ref name=Sgmn>Sageman, B. B., A. E. Murphy, J. P. Werne, C. A. Ver Straeten, D. J. Hollander, and T. W. Lyons, 2003, A tale of shales: The relative role of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian Basin: Chemical Geology, v. 195,  p. 229–273.</ref> 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.<ref name=Sgmn /> 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.<ref>Murphy, A. E., 2000, Physical and biochemical mechanisms of black shale deposition, and their implications for ecological and evolutionary change in the Devonian Appalachian basin: Ph.D. dissertation, Northwestern University, Evanston, Illinois, 363 p.</ref> 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.<ref name=Etnsn1992 />

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.<ref name=Sgmn /> 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.<ref>Demaison, G. J., and G. T. Moore, 1980, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0064/0008/1150/1179.htm Anoxic environments and oil source bed genesis]: AAPG Bulletin, v. 64, p. 1179–1209.</ref> This preservation model is best reflected in the proposed model of a nearly permanent pycnocline by Ettensohn.<ref name=Etnsn1992 />

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<ref>Boyce, M., and T. Carr, 2010, [http://www.searchanddiscovery.com/abstracts/pdf/2010/annual/abstracts/ndx_boyce.pdf Stratigraphy and petrophysics of the Middle Devonian black shale interval in West Virginia and southwest Pennsylvania]: Poster presented at AAPG Denver ACE 2010.</ref> 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.<ref name=Sgmn /> 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.<ref name=Mcquker>Macquaker, J., D. McIlroy, S. J. Davies, and M. A. Keller, 2009, [http://www.searchanddiscovery.com/abstracts/html/2009/annual/abstracts/macquaker.htm ]Not anoxia! How do you preserve organic matter then? (abs.]): AAPG 2009 Annual Convention and Exhibition.</ref> 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.<ref name=Mcquker /> 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<ref>Boyce, M., and T. Carr, 2010, [http://www.searchanddiscovery.com/abstracts/pdf/2010/annual/abstracts/ndx_boyce.pdf Stratigraphy and petrophysics of the Middle Devonian black shale interval in West Virginia and southwest Pennsylvania]: Poster presented at AAPG Denver ACE 2010.</ref> 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,<ref name=Etnsn85b /> Woodrow and Sevon (1985), and Blakey<ref name=Blky /> 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<ref name=Blky /> 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,<ref name=Etnsn85b /> Woodrow and Sevon,<ref>Woodrow, D. L., and W. D. Sevon, eds., 1985, The Catskill delta: Geological Society of America Special Paper 201, 246 p.</ref> and Blakey<ref name=Blky /> 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<ref name=Blky /> 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.

Wrightstone<ref>Wrightstone, G. R., 2010, [http://www.papgrocks.org/Bloomin%20Algae.pdf Bloomin' algae! How paleogeography and algal blooms may have significantly impacted deposition and preservation of the Marcellus Shale]: Pittsburgh Association of Petroleum Geologists.</ref> proposed that the Marcellus organic-rich units were deposited and accumulated in a perfect storm scenario of maximum organic production, noneolian sediment starvation, and maximum preservation. He proposed that algal blooms associated with periodic dust storms led to enhanced production of organics and possible basinwide anoxic events.

Wrightstone<ref>Wrightstone, G. R., 2010, [http://www.papgrocks.org/Bloomin%20Algae.pdf Bloomin' algae! How paleogeography and algal blooms may have significantly impacted deposition and preservation of the Marcellus Shale]: Pittsburgh Association of Petroleum Geologists.</ref> proposed that the Marcellus organic-rich units were deposited and accumulated in a perfect storm scenario of maximum organic production, noneolian sediment starvation, and maximum preservation. He proposed that algal blooms associated with periodic dust storms led to enhanced production of organics and possible basinwide anoxic events.

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