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Structural Style of the Appalachian Plateau Fold Belt, North-Central Pennsylvania

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Structural Style of the Appalachian Plateau Fold Belt, North-Central Pennsylvania Journal of Structural Geology 69 (2014) 284e303 Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Structural style of the Appalachian Plateau fold belt, north-central Pennsylvania Van S. Mount Anadarko Petroleum Corporation, P.O. Box 1330, Houston, TX 77251-1330, USA article info abstract Article history: New seismic and well data from hydrocarbon exploration and development activity associated with the Available online 9 May 2014 Marcellus Formation shale gas play in north-central Pennsylvania provide insight to the structural style of the Appalachian Plateau fold belt in the region north and northwest of the Allegheny structural front Keywords: in Potter, Tioga, Bradford, Sullivan, Lycoming, Clinton and Centre counties. The Plateau fold belt in this Pennsylvania Appalachians area developed over a detachment in Upper Silurian Salina Group evaporites during the Permian Alle- Appalachian Plateau fold belt ghanian Orogeny in response to north-northwest directed shortening. At the Allegheny structural front, a Allegheny structural front deep detachment in Cambrian shales that underlies the Valley and Ridge province to the south- Detachment fold Marcellus Formation southeast, ramps up-section through Cambro-Ordovician carbonates and Lower-Middle Silurian clas- tics to a shallow detachment in Upper Silurian evaporites. At the northeastern plunge of the Nittany Anticline (south and east of Williamsport, PA), only a small amount of slip is interpreted to have been transmitted into the foreland on the shallow Upper Silurian detachment. Instead most slip was consumed in fault-propagation folds immediately north of the Allegheny structural front. The Plateau fold belt, developed above the Upper Silurian evaporites, can be divided into structural domains based on fold characteristics. Domain 1 folds have short wavelengths and low amplitudes. Domain 2 salt-cored anticlines have long wavelengths and large amplitudes. Domain 3 comprises large synclines, located between Domain 2 anticlines. Halite originally beneath Domain 3 synclines is interpreted to have been mobilized, or evacuated, into the cores of adjacent Domain 2 anticlines during folding. Seismic data indicate that the base of the salt detachment underlying Plateau folds is a non-planar, stepped surface. Possible scenarios for the development of the non-planar detachment include: 1) mobilization of halite from an evaporite sequence that contained an originally non-uniform distribution of halite, 2) it is an erosion surface that existed prior to deposition of the evaporite sequence, or 3) it developed in response to buckle folding of the stratigraphic layer overlying the evaporite sequence. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Plateau and 2D seismic reflection data have documented that the broad, subtle, long-wavelength folds in Devonian, Mississippian The Allegheny structural front in north-central Pennsylvania is and Pennsylvanian age strata at the surface are detached from the structurally-defined boundary between the Valley and Ridge deeper strata along an evaporite interval within the Upper Silurian fold belt to the southeast and the Appalachian Plateau fold belt to Salina Group (Gwinn,1964; Prucha,1968; Frey,1973; Wiltschko and the northwest, and approximately coincides with the Allegheny Chapple, 1977; Davis and Engelder, 1985; Shumaker, 2002). Recent front, an area of subdued topography between the Valley and Ridge industry activity in north-central Pennsylvania associated with and Plateau physiographic provinces (Rodgers, 1970; Faill, 1998) exploration and development of hydrocarbons within the Middle (Figs.1e3). The structural style of the Allegheny structural front, the Devonian Marcellus Formation shale, has included acquisition of Valley and Ridge fold belt, and the Appalachian Plateau fold belt high-quality seismic data (both long offset, regional 2D lines and 3D have been studied for over a century (Chamberlain, 1910; Willis and surveys covering large areas) and the drilling of horizontal wells Willis, 1929; Sherrill, 1934; Wedel, 1932). Over the past 50 years, with long lateral reaches e typically over 1 mi (1.6 km). These new deep oil and gas exploration wells drilled on the Appalachian data clarify the structural style of the Appalachian Plateau folds above the Upper Silurian evaporites, and allow the Plateau fold belt to be partitioned into structural domains based on fold character- E-mail address: [email protected]. istics. The results of this study based on new industry data are http://dx.doi.org/10.1016/j.jsg.2014.04.005 0191-8141/Ó 2014 Elsevier Ltd. All rights reserved. V.S. Mount / Journal of Structural Geology 69 (2014) 284e303 285 Lake Ontario 160 km (100 mi) ONT NY R S N B Upper Silurian Lake Salina Group outcrop Erie Anhydrite Finger Lakes (0-250 Thinft, 0-76 Halite m) (250-500 ft, 76-152m) Appalachian Anhydrite Halite Plateau fold belt Thin Halite Halite Thick Halite (>500 ft, 152 m) (0-250 ft, 0-76 m) NY PA (250-500 ft, 76-152m) Lackawana synclinorium W PA NJ NA Figure 3 Figure 2 ASF Dolomite/Shale Valley and Ridge CRA A H Silurian P outcrop PA NW limit of DE PA Triassic rift basins MD Fig. 1. Appalachian Plateau fold belt location map. Dark gray shaded areas are outcrops of Silurian age rocks (from Rickard, 1969 in New York; and Berg et al., 1980 in Pennsylvania). Red dashed lines represent axial traces of surface anticlines (from Engelder and Geiser, 1980 in New York; and Berg et al., 1980 in Pennsylvania). ASF: Allegheny structural front; NA: Nittany Anticlinorium, CRA: Chestnut Ridge Anticline. Cities in Pennsylvania: W: Williamsport; A: Altoona; H: Harrisburg; P: Philadelphia. Cities in New York: B: Buffalo, R: Rochester, S: Syracuse. Bradford County, Pennsylvania is highlighted in yellow. The extent and facies distribution of the main halite bearing unit within the Upper Silurian salt basin is constrained by hundreds of well penetrations (Salina Group “F” facies map from Rickard, 1969). The southeast portion of Rickard’s (1969) map is not tightly constrained by well data and has been modified based on interpretation of seismic data. Notice areal extent of the Plateau fold belt coincides with limit of the halite facies within the underlying Salina Group. A salt isopach map is superimposed on the fold axial trace map in north-central Pennsylvania. The isopach map shows present-day salt thickness greater than 1600 ft (487 m) and illustrates the geometry of the salt cores of the anticlines (key to isopach map shown in Fig. 2). The surface trace of Mississippian strata associated with the Lackawana synclinorium are indicated by black dashed lines. consistent with and expand on earlier interpretations regarding sediments overlying Precambrian basement that thicken toward fold geometry and mechanics of folding within the Plateau fold belt the south-southeast (Nickelsen, 1988; Childs, 1985). The basement by Wiltschko and Chapple (1977). surface dips gently (<1e2) to the south-southeast, from a depth of approximately 6000 ft (w1.8 km) below sea level in the Finger 2. Geologic setting Lakes region of New York, to a depth of approximately 20,000 ft (w6 km) below sea level at the Allegheny structural front in the 2.1. Stratigraphy vicinity of Williamsport in south-central Lycoming County, Penn- sylvania (Figs. 1 and 3)(Patchen et al., 2004; Alexander et al., 2005). The stratigraphic section underlying the Appalachian Plateau in Overlying basement is a thick (approximately 20,000 ft, w6 km) north-central Pennsylvania consists of a wedge of Paleozoic sequence of Cambrian through Middle Silurian age strata 286 V.S. Mount / Journal of Structural Geology 69 (2014) 284e303 NY Tioga Junction PA POTTER TIOGA BRADFORD 32 km (20 mi) Wellsboro 10c PENNSYLVANIAN Granville MISSISSIPPIAN 10a N DEVONIAN 10e SILURIAN Liberty ORDOVICIAN Wilmont Salt Isopach SULLIVAN 488 m (1600 ft) Wallis Run 610 m (2000 ft) Appalachian Plateau LY C O M I N G 732 m (2400 ft) 8 W CLINTON front 854 m (2800 ft) uctural NA str 975 m (3200 ft) CENTRE CR 1097 m (3600 ft) 3 AlleghenyValley and Ridge Fig. 2. Geologic map of north-central Appalachian Plateau (modified from Pennsylvania Department of Conservation and Natural Resources map 2200-MP-DCNR0498, www.dcnr. state.pa.us/topogeo/maps/map7.pdf) with superimposed isopach map of underlying Upper Silurian Salina Group salt (where salt thickness is greater than 1600 ft [487 m]). Thick areas of salt represent cores of large anticlines (yellow areas represent salt thicknesses of up to 3600 ft [1097 m]). Surface outcrop of Mississippian and Pennsylvanian strata define intervening synclines. Red polygon labeled “CR” is Council Run Gas Field in Centre and Clinton counties. “W” indicates the city of Williamsport in south-central Lycoming County. Wellsboro anticline originally identified by Wiltschko and Chapple (1977). Granville (Towanda) and Wilmont anticlines originally identified by Frey (1973). Location of transects in Figs. 3, 8 and 10 are shown. comprising Cambro-Ordovician carbonates and Upper Ordovician/ contrast, the Tonoloway Formation outcrops in the Valley and Ridge Middle Silurian shales, siltstones and sandstones (Fig. 4). The fold belt in Pennsylvania and West Virginia, defining the south- Middle Silurian clastic sequence transitions into an Upper Silurian eastern margin of the Silurian evaporite basin (Fig. 1). The Tono- section that includes rocks deposited in an evaporite basin. The loway Formation is also encountered in wells in the southern part Upper Silurian evaporite basin underlies the Plateau fold belt and of the basin, near the Allegheny structural front. Shales and dolo- controls its areal extent e detachment folding is limited to regions mites of the Syracuse and Bertie formations, facies equivalents to above relatively thick halite. The extent and facies distribution of the Tonoloway Formation crop out along the Mohawk River valley Upper Silurian evaporites in north-central Pennsylvania and and northern Finger Lakes region in western New York and define western New York are summarized in Fig. 1 (for the sequence the northern margin of the Silurian evaporite basin (Fig.
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