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.<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.

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.<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.

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.

[[File:M97Ch4FG4.jpg|400px|thumb|{{figure number|4}}A map showing the primary structural features of the Appalachian Basin. Modified from Shumaker.<ref name=Shmkr1996>Shumaker, R. C., 1996, Structural history of the Appalachian Basin, in J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: Morgantown, West Virginia, West Virginia Geological and Economic Survey Publication V-25, p. 8–10.</ref> Decollement trends are from Colton,<ref name=Cltn>Colton, G. W., 1970, The Valley and Ridge and Appalachian plateau; stratigraphy and sedimentation; the Appalachian Basin; its depositional sequences and their geologic relationships, in G. W. Fisher, F. J. Pettijohn, and J. C. Reed Jr., eds., Studies of Appalachian geology, central and southern: New York, Interscience Publishers, p. 5–47.</ref> Frey,<ref name=Fry>Frey, M. G., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0006/1000/1027.htm Influence of Salina Salt on structures in New York-Pennsylvania part of the Appalachian Plateau]: AAPG Bulletin, v. 57, p. 1027–1037.</ref> and Sanford.<ref name=Snfrd>Sanford, B. V., 1993, St. Lawrence platform: Economic geology, in D. F. Stott and J. D. Aitken, eds., Sedimentary cover of the craton in Canada: Boulder, Colorado, Geological Society of America, The Geology of North America, v. D-1, p. 787–798.</ref> VT = Vermont; CT = Connecticut.]]

[[File:M97Ch4FG4.jpg|400px|thumb|{{figure number|4}}A map showing the primary structural features of the Appalachian Basin. Modified from Shumaker.<ref name=Shmkr1996>Shumaker, R. C., 1996, Structural history of the Appalachian Basin, in J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: Morgantown, West Virginia, West Virginia Geological and Economic Survey Publication V-25, p. 8–10.</ref> Decollement trends are from Colton,<ref name=Cltn>Colton, G. W., 1970, The Valley and Ridge and Appalachian plateau; stratigraphy and sedimentation; the Appalachian Basin; its depositional sequences and their geologic relationships, in G. W. Fisher, F. J. Pettijohn, and J. C. Reed Jr., eds., Studies of Appalachian geology, central and southern: New York, Interscience Publishers, p. 5–47.</ref> Frey,<ref name=Fry>Frey, M. G., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0006/1000/1027.htm Influence of Salina Salt on structures in New York-Pennsylvania part of the Appalachian Plateau]: AAPG Bulletin, v. 57, p. 1027–1037.</ref> and Sanford.<ref name=Snfrd>Sanford, B. V., 1993, St. Lawrence platform: Economic geology, in D. F. Stott and J. D. Aitken, eds., Sedimentary cover of the craton in Canada: Boulder, Colorado, Geological Society of America, The Geology of North America, v. D-1, p. 787–798.</ref> VT = Vermont; CT = Connecticut.]]

The major structural features of the Appalachian Basin, key shale production trends, and structural provinces of the Appalachian Basin are depicted in [[:File:M97Ch4FG4.jpg|Figure 4]]. Key structural elements of the Appalachian Basin from west to east include the Waverly arch and Cincinnati arch to the west, the Cambridge arch and Burning Springs anticline farther eastward, the Rome trough, and the anticlinal fold belts in the Appalachian Plateau and Valley and Ridge province. To date, most economic productive Marcellus wells are located within the Appalachian Plateau physiographic province. This province is marked by generally gentle structures and a lack of intense faulting in the western parts of the province. Structural complexity increases to the east toward the structural front, where high-amplitude, detached, salt-cored anticlines are present trending northeast–southwest. Structural complexity may also occur in the synclines within the eastern parts of this province. The structural front represents the boundary between the plateau and the Valley and Ridge province, where the Devonian section rises quickly to the surface and crops out. The Valley and Ridge province represents the most structurally challenging area in which the Marcellus Shale is present. The area is structurally complex, with high-amplitude detached folds, repeated and overturned beds, and multiple thrust faults.

The major structural features of the Appalachian Basin, key shale production trends, and structural provinces of the Appalachian Basin are depicted in [[:File:M97Ch4FG4.jpg|Figure 4]]. Key structural elements of the Appalachian Basin from west to east include the Waverly arch and Cincinnati arch to the west, the Cambridge arch and Burning Springs anticline farther eastward, the Rome trough, and the anticlinal fold belts in the Appalachian Plateau and Valley and Ridge province. To date, most economic productive Marcellus wells are located within the Appalachian Plateau physiographic province. This province is marked by generally gentle structures and a lack of intense faulting in the western parts of the province. Structural complexity increases to the east toward the structural front, where high-amplitude, detached, salt-cored anticlines are present trending northeast–southwest. Structural complexity may also occur in the synclines within the eastern parts of this province. The structural front represents the boundary between the plateau and the Valley and Ridge province, where the Devonian section rises quickly to the surface and crops out. The Valley and Ridge province represents the most structurally challenging area in which the Marcellus Shale is present. The area is structurally complex, with high-amplitude detached [[fold]]s, repeated and overturned beds, and multiple thrust faults.

===Controls Caused by Basement Faulting===

===Controls Caused by Basement Faulting===