GEOSC 465

Field Trip #2:

The Anthracite Region of Pennsylvania

STRUCTURES OF THE APPALACHIAN FORELAND FOLD-THRUST BELT:

SEQUENCE OF STRUCTURAL STAGES OF THE ALLEGHANIAN OROGENY IN THE DEVONIAN THROUGH UPPER CARBONIFEROUS SECTION OF THE ANTHRACITE REGION, APPALACHIAN FORELAND, PENNSYLVANIA

Adapted from:

 Field Trip Guidebook T166: Day 3
28th International Geological Congress

by 

Richard Nickelsen
Department of Geology, Bucknell University, Lewisburg, PA  17837

INTRODUCTION

The eastern end of the Central Appalachian Valley and Ridge contain folded rocks which show superb relations between structure, stratigraphy, and topography, a setting which no doubt spawned the first important work in North American structural geology by Henry Darwin Rogers during 1838-1846.  More recently, the region has been important for early studies of thrusting and thin-skinned deformation (Gwinn, 1964, 1970, Wood, Trexler, and Kehn, 1969, Wood and Bergin, 1970), the kink geometry of fold profiles (Faill, 1969, 1973), prefolding layer-parallel shortening (Nickelsen, 1966, Geiser and Engelder, 1983), spaced cleavage formed by pressure solution (Groshong 1975, Nickelsen 1972) strain disharmonies (Nickelsen, 1986), and the progressive sequence of overprinted structural stages that comprise one orogeny (Nickelsen, 1979).

Objectives during traverse of the eastern end of the Central Appalachian Valley and Ridge are to demonstrate the sequence of structural stages of the Alleghany Orogeny, to show the evidence for clockwise rotation of compression during the sequence, to illustrate an example of strain disharmony, and to examine rocks subject to a higher grade of metamorphism than seen anywhere else on the field trip.  Structures to be observed include joints, spaced cleavage (formed by either dissolution or primary crenulation of originally bed parallel phyllosilicates), wedge (thrust) faults, wrench (strike slip) faults, extensional faults, folds, cleavage halos, and cleavage duplexes.

The sequence of structural stages (I thru VII) is best illustrated at the Bear Valley Strip Mine (Stop 1).  Generally, the sequence includes Pre-Alleghanian jointing, followed by Alleghanian layer-parallel shortening, overprinted by large-scale folding, and layer-parallel extension.  It has been established both by the overprinting of different structural stages at one locality and by the regional distribution of structures associated with the different stages.  On the Appalachian Plateau of New York geologists observe joints formed during Stages I and II and cleavages of Stage III.  Here in the Valley and Ridge portion of the Appalachian foreland these stages are overprinted by faults, folds and layer-parallel extension of Stages IV through VII.

Several different relative ages of Alleghanian deformation were suggested by overprinted joints on the Appalachian Plateau, but within this part of the foreland the sequence will be demonstrated by different trends of the structural stages.

The field trip will pass through the Anthracite Region, the only extensive exposure of Upper Carboniferous (Pennsylvanian) strata within the Valley and Ridge Province of the Appalachian foreland.  This region is an axial depression occupying a recess between the salient/culminations of New England to the north and central Pennsylvania-West Virginia to the south.  Coal is found in three first order synclinoria, the Southern and Northern Anthracite fields and the Middle, which we will visit.  The Anthracite Region has higher deformation temperatures than those found along strike to the southwest.  Temperatures that prevailed during deformation in Alleghanian time are indicated by Conodont Alteration Index Isograds (Epstein and others, 1977), by Fixed Carbon or Vitrinite Reflectance Isograds (Levine, 1983) or by scattered fluid inclusion, illite crystallinity, and mineral phase data.  Temperatures are thought to have approached or exceeded 200°C throughout most of the Anthracite Region but their origin is unknown.  The necessary 6 to 8 km of Late Carboniferous overburden above surface rocks to produce these temperatures could not have been deposited in the time available (Levine, 1983), while, to date, no tectonic evidence of the existence of the proposed Anthracite Nappe has been established.  In addition, no igneous source has been found and the possibility of heating by hot fluids extruded west from the metamorphic Appalachians has not been evaluated.

Figure 1:  Index map and location map of Bear Valley Strip Mine.  Pennsylvania 225, 125, and 61; U.S. 15, and I—80, I-81 are labeled.

STOP 1:   SEQUENCE OF STRUCTURAL STAGES OF THE ALLEGHANY OROGENY AT THE BEAR VALLEY STRIP MINE, SHAMOKIN, PENNSYLVANIA

The Bear Valley Strip Mine is situated along several faulted second order folds on the south limb of the first order Shamokin Synclinorium (Fig. 1).  The local geologic setting is provided by the map of Arndt and others (1973) while the larger anthracite region is described by Wood and others (1969), and Wood and Bergin (1970).  The mine area has been previously described by Nickelsen (1979, 1983).

The Bear Valley Strip Mine offers superb three-dimensional exposures of the structural elements of the northern Valley and Ridge province, and clear views of the geometric relationships, and the sequential overprinting of structures that elucidate the stages and processes of deformation within one orogeny.  All of this is visible within a 320,000 ft2 (30,000 m2) strip mine area that contains exposures of two disharmonically-folded, Pennsylvanian-age cycles of sedimentation in cross section along highwalls and on unique bedding plane surfaces.  Deformation mechanisms and the structural sequence of layer-parallel shortening (Stages II, III, IV) overprinted first by large-scale folding (Stage V) and later by layer-parallel extension (Stage VI) can be fully demonstrated at this locality (Fig. 2).

Figure 2:  Cartoon depicting the sequence of development of structural stages of the Alleghany orogeny:  Stage I.  Orthogonal joint sets form in coal; Stage II.  Several sets of hydraulic extensional joints form in sandstones and shales; Stage III.  Pressure solution and primary crenulation cleavage and small-scale folds form; pressure solution of Stage II joint fillings occurs; Stage IV.  Conjugate wrench and wedge faults deform Stage III cleavage; Stage V.  Large-scale folding of all previous structures occurs; Stage VI.  Extensional joints and faults produce flattening perpendicular to bedding and layer-parallel extension, both parallel and perpendicular to fold hinges. 

The variety of structural elements and deformation mechanisms displayed includes:  Stage I joints in coal that are eastern extensions of the pre-Alleghanian Set I joints in coal observed on the Appalachian Plateau by Nickelsen and Hough (1967, Plate 3); hydraulic extensional joints with quartz-fiber fillings in ironstone and sandstone (Stage II); spaced cleavage in shales and silty shales formed by pressure solution, or grain rotation and sliding, or primary crenulation, accompanied by incipient recrystallization (Stage III), wrench (strike-slip) or wedge (thrust) faults with very obvious slickensides and slickenlines (Stage IV); third (size) order flexural slip folds showing disharmony between adjacent structural lithic units (Stage V); and extensional fault "grabens" or extensional joints formed by buckling or release fracturing (Stage VI).  Elsewhere in the region Stage VII large scale strike-slip faults with horizontal slickenlines cut all other structures.  Finally, it is apparent that ductility contrast ranging from stiff ironstone to sandstone to more ductile shale and coal  controls the relative structural behavior of rock types.

Figure 3 shows the detailed structural geology and topography of the site as well as the location of eight stations (A to H) of particular interest.

Figure 3:  Map showing topography, geology, geologic localities A to H, and figure localities 4, 5, 7, and 8, of the Bear Valley Strip Mine.  Location numbers 0, 1, 2, 3, 4 are at 100 ft (30 m) intervals along crest of Whaleback anticline.

Figure 4:  Conjugate wrench fault systems and a thrust fault between Stations C and D on the south wall, viewed from A.  Least strain axis, IVA, is overprinted by IVB.
STATION A.  Turn right (west) off the entrance road and climb to the crest of the North anticline at A, a flat-topped box fold.  The rocks beneath your feet represent the floor of the coal swamp and have impressions of large lycopsid trees and stigmaria (roots).  From A, the entire mine is visible, including third order folds, fold disharmonies, many faults, and ironstone concretions.  The view of the southwest corner (Fig. 4) shows overprinted Stage IV conjugate wrench fault systems (dihedral angle 35°) and a Stage IV thrust.  To the east the disharmonic third order folding shown in Figure 5 is visible.

Figure 5:  North-south section through location 1 of Figure 3.

STATION B.  Here, 330 ft (100 m) east of A, is an excellent view to the north limb of the Whaleback anticline showing Stage IV thrusts and conjugate wrench faults, the Stage V anticline plunging east, and Stage VI extensional joints and faults (Fig. 6).  The North anticline on which you are standing has a chevron profile, different from that at Station A, because the kink junction axis is inclined to bedding (Faill, 1973, Fig. 20).

Figure 6:  North limb of Whaleback anticline viewed from Station B.  Extensional faults (Stage VI) define "grabens" that are both parallel and transverse to the fold hinge.  The "grabens" overprint Stage IV wrench and thrust faults.  Stations E, G, and H are labeled.

STATION C.  The Stage IV wrench and wedge (thrust) faults of Figure 4 can be studied here.  Note that slickenlines on wrench faults are parallel to the fault-bedding intersection, indicating that they formed prior to folding and were late folded to their present attitude during Stage V.  Slickenlines on wedge faults bisect the dihedral angle between wrench faults and were formed at the same time.  Spaced cleavage (primary crenulation and pressure solution mechanisms) occurs in shales upslope to the southwest of C.  Ironstone concretions are the stiffest member of the sedimentary succession, showing only Stage II joints and surface slickenlines that demonstrate how other strata have shortened and flowed around them.  The contact between concretions and their enclosing rock are shear planes or boundary zones between different structural lithic units.  The sequence of structural stages is established by offset of Stage II joints by Stage III pressure solution cleavage and by drag of Stage III cleavage-bedding intersections against Stage IV wrench faults (Fig. 7A).  All of these structures formed by layer-parallel shortening in horizontal beds prior to folding.

While walking eastward between Stations C and D, you see Stage II joints in ironstone concretions, ductility contrast between ironstone and sandstone or shale, and a major wrench fault that is the west boundary of the thrust slice of Figure 4.

Figure 7(A):  Conjugate wrench faults (Stage IV) viewed on the south wall between Stations C and D cause drag of cleavage-bedding intersections.  Gash veins along the left lateral wrench fault cut preexisting Stage III cleavage.  (B) Drawing to show how thrust faulting followed by flexural-slip folding has caused pull back of hanging wall to expose a fault surface at Station G.  (C) Interpretation of curved slickenlines on a traverse fault at Station H.  Folding to 40  north dip preceded wrench faulting.  The Whaleback anticline and the fault then evolved together during Stage V folding and Stage VI layer-parallel extension.

STATION D.  The east end of the thrust slice illustrated in Figure 4 can be observed.  On the Whaleback anticline to the north, are wrench faults with intricate slickenline patterns that indicate overlap between Stage IV faulting and Stage V folding.  Stage VI strike and transverse extensional faults and "grabens" are well exposed.

STATION E.  The crest of the Whaleback anticline provides the best view of the fold disharmony to the east (Fig. 5), Stage II hydraulic joints on the whaleback (at 0 and 1), and the Stage IV wrench and wedge faults and Stage VI strike joints (of release or buckling origin) on the south wall.  Note that the Stage VI strike joints are not symmetrical with the acute bisectors and slickenlines of Stage IV wrench and wedge faults, indicating that the structural array formed in response to differently oriented strains -- Stage IV layer parallel shortening, versus Stage VI extension due to fold buckling.

STATION F.  The tight hinge of the North syncline is seen here.  Upslope in this multilayered sequence of shale and sandstone are excellent bedding-plane slickenlines as well as spaced cleavages in the shales and shaly sandstones.

STATION G.  Vertical bedding on the north limb of the Whaleback anticline is cut by Stage IV thrust faults and Stage VI extensional faults and "grabens" trending both parallel to and perpendicular to the fold hinge (Fig. 6, above G).  Evidence for the relative age of thrusting and folding is illustrated in Figure 7B and 8.  Thrust faults preceded folding because flexural-slip associated with folding has reversed their slip sense, causing a pull back of the tip line of the thrust and exposing the thrust surface as illustrated in Figure 7B.  Also, thrust faults and their slickenlines as well as the acute bisectors of conjugate wrench faults consistently trend counterclockwise of the third order folds that overprint them (Fig. 8).

About 60 ft (20 m) west of Station G toward Station H is a triangular fault block (photograph in Nickelsen, 1979, Plate 10A) initially defined by Stage IV conjugate wrench faults but later reacting to Stage VI extensional strains to form a "graben."  Overprinting of slickenlines on the faults bounding the block establishes the sequence.  Stage IV slickenlines parallel the bedding-fault intersection, but Stage VI slickenlines are perpendicular to that intersection.

Figure 8:  Equal-area projection showing angular relations of structures after they have been rotated with bedding to prefolding attitude.  Stage III and IV structures trend counterclockwise of the Stage V fold hinge.

STATION H.  Two structural features are of importance here:  a fourth order anticline in the trough of the North syncline that displays the evidence for the sequence of structural Stages IV and V, and a wrench fault on the north limb of the Whaleback anticline that remained active from Stage IV through Stage VI.  The fourth order anticline illustrated by Nickelsen (1979, Pls. 8A and B) has slickenlines on a left lateral wrench fault (Stage IV) that nearly parallel the bedding-fault intersection and have been folded through 50  during Stage V.

The history of the wrench fault on the north limb is interpreted in Figure 7C.  This fault was initiated under compression as a wrench fault, participated in folding (Stage V), and ended movement as an extensional fault, contributing to stretching of the hinge of the Whaleback anticline (Stage VI).

Summary

	At the Bear Valley Strip Mine, a sequence of deformational stages of the Alleghany orogeny can be interpreted from overprinted structures in rocks of Pennsylvanian age.  All of the stages displayed must be Pennsylvanian or younger but no Mesozoic deformation is thought to have occurred here.  Similar sequences of deformation have been recognized elsewhere in the Canadian Rockies (Price, 1967) and the Valley and Ridge province (Perry, 1975).  Rocks of the Bear Valley Strip Mine were deformed at temperatures of 185° to 205°C under an overburden of 3.1 to 5.0 mi (5 to 8 km) (Nickelsen, 1983, p. 137).