|STRUCTURES OF THE APPALACHIAN FORELAND FOLD-THRUST BELT:
FOLD-THRUST GEOMETRIES OF THE
JUNIATA CULMINATION (State College and environs),
CENTRAL APPALACHIANS OF PENNSYLVANIA
Field Trip Guidebook T166: Day 4
28th International Geological Congress
Department of Geology, Bucknell University, Lewisburg, PA 17837
Department of Geosciences, The Pennsylvania State University, University Park, PA 16802
and adapted from
Field Trips & Experiments in Physical Geology
Terry Engelder and David McConaughy
Department of Geosciences, The Pennsylvania State University, University Park, PA 16802
The Appalachian Mountains contain the record of at least three complex Paleozoic contractional orogenic events (Rodgers, 1970): the Taconic orogeny (Mid- to Late Ordovician), the Acadian orogeny (Mid- to Late Devonian), an the Alleghanian orogeny (Pennsylvanian to Permian time). Subsequent to the Alleghanian orogeny, the range was beveled by erosion, and then in middle Mesozoic time it was subjected to a rifting event which ultimately led to the opening of the Atlantic Ocean. Topographic details for the range reflect structural features created during the Paleozoic orogenies, but the uplift that resulted in the overall elevation that we now see is a more recent event, probably related to rifting.
The structural geology of the Juniata Culmination is, in part, controlled by the mechanical nature of the stratigraphic section (Figure 1). The section may be divided into a Cambro-Ordovician carbonate sheet more than 2 km thick over which is draped a clastic section nearly 4 km thick. The carbonate sheet has detached at the level of the Waynesboro shale below and within the Reedsville shale above. This stiff sheet forms a duplex which gives the Valley and Ridge it present character.
Rocks of the Nittany Valley and its flanks vary in age from Cambrian through Silurian. These are exclusively sedimentary rocks deposited layer by layer in a number of environments. More than two dozen layers were recognized and given names as listed in a stratigraphic column, younger on older (Figure 1). The oldest layers of rock are carbonates, limestones and dolomites, deposited in shallow marine seas at or near equatorial latitudes about 500,000,000 years ago. Deposition of formations such as the Bellefonte Dolomite (Obf) occurred in environments equivalent to the modern Bahama lagoons, east of Miami. More than ten separate formations of carbonate rocks were recognized by geologists. This period of tropical lagoons lasted more than 50,000,000 years after which clay muds started being carried to the region from mountains emerging to the east. The Coburn Limestone (Ocn) is an example of such mud bearing carbonates. Then a thick shale, the Reedsville Shale (Or), was deposited over the entire carbonate bank. The source of sediment for this shale and later sandstones was the tall mountains of the Taconic Highlands to the east. These mountains have since eroded. Marine storms carried sandstones to the deep sea (Bald Eagle Formation, Obe). River muds (Juniata Formation, Oj) are found followed by clean beach sands of pure quartz without any carbonate shell fragments (Tuscarora Formation, St). Younger rocks are found to the southwest in the direction of Huntingdon and Mount Union and also to the northwest of the Nittany Valley on the Appalachian Plateau in the direction of Clearfield.
State College is located within the Valley and Ridge geological province. This is an area of the state that is dominated by a series of folds that have been eroded so that sandstones of several formations, mainly the Bald Eagle, Tuscarora, Pocono, and Pottsville, constitute the ridges. In the vicinity of State College it is the former two that make the ridges. The ridges outline a series of anticlines and synclines. A simple geological map of central Pennsylvania might feature the outcrop pattern of just one formation. For example, Figure 2 shows the outcrop pattern of the Reedsville Formation, an Ordovician shale that was deposited as the first Paleozoic mountains were forming off the east coast of what was then largely a carbonate bank. The geological map of the Reedsville shows a series of anticlines and synclines, particularly to the southeast of State College. The Nittany Mountain syncline is seen to the east of State College and the campus of Penn State (Figure 2).
Nittany Valley is located on the northwestern side of the famous Valley and Ridge Province of Pennsylvania. The Valley and Ridge extends from Harrisburg in the south to its northern boundary, Bald Eagle Ridge. Further north are found the highlands of the Appalachian Plateau. The Valley and Ridge consists of a series of folds which developed during a period of tectonic deformation about 280,000,000 years ago. This period of deformation, called the Alleghanian Orogeny, was one consequence of the collision between two large continental masses called Gondwana and Laurentia. The present continental plates that show the effects of the Alleghanian Orogeny are Africa and North America where the present mountains of northwestern Africa are a mirror image of the Appalachian Mountains on the eastern edge of North America. A simple model for folding of the Valley and Ridge might be that of a rug pushed on a slippery wooden floor. The 'rug' was huge, as much as 6 to 10 km thick and almost 400 km wide including the Appalachian Plateau. There are downfolds or synclines and upfolds or anticlines, side by side. Some folds are larger than others with the largest folds being 10 km across and smallest being no larger than a piece of note paper.
The Alleghanian Orogeny and the push causing the Valley and Ridge came from the southeast, even southeast of Harrisburg. Rocks in that area are more highly deformed and formed a mountain range much like the modern Himalayan Mountains of Asia. The Alleghanian Orogeny is a manifestation of Africa colliding with North America.
Deformation from the Alleghanian Orogeny dies out gradually to the northwest across the Valley and Ridge where major folding ends abruptly at the Alleghany Front, the boundary between the Valley and Ridge and the Appalachian Plateau. Bald Eagle Ridge marks the Allegheny Front. Deformation is found on the Appalachian Plateau but folding is so mild that limbs of sides of anticlines dip just a couple of degrees. For reasons that are not fully understood, the largest upfolds or anticlines of the Valley and Ridge are found at the northwestern boundary. This rule applies all along the Appalachian Mountains from New York and Pennsylvania to Tennessee and Alabama. The Nittany Valley is the location of one of the largest of all these upfolds. The flanks of the Nittany Valley upfold, called the Nittany Anticlinorium, are the previously mentioned Bald Eagle and Tussey Ridges. This anticlinorium is 10 km across (Figure 3).
Smaller folds are superimposed on larger. All this means is that wrinkles can occur on more than one scale and that large wrinkles can carry smaller wrinkles on their surfaces. One of the best examples of superimposed folding is found at Nittany Mountain. Nittany Mountain is a second order syncline (size = 1 km) called the Nittany Mountain syncline. It is superimposed on a first order anticline (size = 10 km), the Nittany Anticlinorium. This superposition is illustrated in Figure 3 where the sandstones of the Bald Eagle and the Tuscarora form a blanket over the several stacked layers of carbonate rocks. These layers of carbonates, stacked like a bent deck of cards, are called horses. The name, horse, is a term used by the 19th century English coal mining industry. In a cross sectional profile on can easily recognize the nose and mane of a horse. Each carbonate layer is more than 2 km thick so that the Nittany Anticlinorium is stacked up as much as 6 km over the sliding base of the Valley and Ridge. The Reedsville Shale, seen curving around the nose of Nittany Mountain, is clearly part of the Nittany Mountain syncline. Just to the southeast of Nittany Mountain is an anticline as indicated by the Reedsville Shale wrapping around toward the southwest.
Because the Nittany Valley is a large upfold, it would seem natural that it should be a mountain rather than a valley. Erosion and weathering have a role in controlling the shape of the valley. The edges of the valley are sandstone ridges which weather slowly whereas the center of the valley consists of carbonates which weather more rapidly. The highest parts of an upfold are always weathered fastest. So, the sandstone with the highest elevation weathered through and permitted weathering the attack the limestones below. Once the carbonates are exposed they weather so rapidly that the topography is inverted which is to say that the structural highs (anticlines) become the topographic lows (synclines). As the Nittany Valley erodes the lowest and last remnants of large upfold of sandstones is found as part of a second order down fold (i.e., syncline), the Nittany Mountain syncline. The Bald Eagle sandstones (Obe) have yet to be eroded and are, in fact, responsible for the ridge which constitutes Nittany Mountain.
In the Juniata Culmination the lowest structural units of the Central Appalachian Valley and Ridge, mainly the Lower Paleozoic carbonate horses, are found in outcrop. Evidence for stacking of carbonate sheets was discovered in the early 20th century with the mapping of the Birmingham window (stop 9). This field trip focuses on the relationship between the carbonate stiff layer and their cover rocks. Stops 8, 9, and 10 will be in the stacked carbonate sheets. Evidence for the passive roof thrust above the carbonate duplex will be examined at stop 3. The remaining outcrops include examples of the structures developed in the cover rocks of the Paleozoic clastic section.
Our objectives for the first part of the field trip in the Kishacoquillas Valley are to demonstrate the characteristics of the Antes-Coburn detachment and the evolution of the northwest limb of the Jacks Mountain Anticlinorium, and to compare the orientation and relative age of tectonic trends in this area with what is seen on the Appalachian Plateau and in the eastern portion of the Pennsylvania Appalachain Valley and Ridge. All structural stages in central Pennsylvania refer to those developed at the Bear Valley Strip Mine (A field trip to be taken later in the semester).
The northwestern limb of the Jacks Mountain anticlinorium has three small folded thrusts or duplexes that originated in horizontal beds as imbricates rising toward the northwestern foreland from the Antes-Coburn detachment (Figure 4). They later were folded by ramping in the underlying Cambro-Ordovician carbonate duplex so that they now in many places dip northwest in the direction of their initial transport. Viewed down plunge to the southwest the Potlicker Flat Thrust is the lowest thrust in the stack, overlain successively by the Bearpen Hollow Thrust and Stone Mountain Duplex. If a forward or foreland directed sequence of thrusting is accepted these faults formed in the following order toward the northeast: 1. Stone Mountain Duplex, 2. Bearpen Hollow Thrust, and 3. Potlicker Flat Thrust. The Stone Mountain Duplex propagated obliquely toward the northwest (transport azimuth 310°-324°) and the strike of its folds and faults trended east of the future axis of rotation. Because of a later rotation around an axis striking 15°-25° parallel to Stone Mountain, the duplex is now viewed on edge and its structures now strike west of the mountain. The structurally advanced Stone Mountain Duplex in the southwest underwent the most structural development as subsequent ramping of folding, and flattening occurred. Both enveloping beds and the duplex were rotated to a dip of 60°, extended, and faulted by a steep out-of-sequence upthrust (Saddler Gap Fault) and by many transverse, right-lateral, strike-slip faults (eg. Jacks Narrow Fault).
Figure 4: Geologic map of the Jacks Mountain anticlinorium.
STOP 1A: SHALE PIT IN THE REEDSVILLE SHALE
(Route 144 just north of Potters Mills)
Shale is a common material used for foundations of roads and driveways throughout Centre County. This shale pit is one of dozens dug into the Reedsville Shale for the purpose of finding ‘cheap’ aggregate for construction. This most common construction aggregate in Centre County are carbonates. However, the carbonates are hard and require a crushing mill to reduce the rock to particle and clast sizes that are useful for construction. Crushing is relatively expensive compared with the shale that is easily broken into construction ready particle sizes merely by the process of digging. Hence, the Reedsville shale has become the ‘poor-man’s’ road aggregate.
This shale pit is characterized by the formation of long, slender pieces of rock called, pencils. Pencils form as a consequence of the intersection of bedding and cleavage. Shale bedding can be closely spaced (≈ several millimeters). Likewise, cleavage can be equally closely spaced. However, the third dimension of the shale has no closely spaced fabric. This characteristic allows the cleavage to into small segments that have a long third dimension.
Stop 1B: The Reedsville-Bald Eagle contact on the north flank of the State Nursery Anticline
(just south of the Handlebar Saloon on Route 322).
Geologists are constantly faced with determining the boundary between two formations. Unusually the boundary is sharp because the two units are composed of a distinctly different lithology. Other times the transition from one formation to the next is gradual. Such is the case at the contact between the Reedsville Shale and the Bald Eagle Formation. The Reedsville shale is exposed in the southern end of this outcrop. The greenish sandstones of the Bald Eagle are also clearly visible within the central portion of the outcrop. Toward the northern end of the outcrop the sandstones become red. However, the transition between the shale and sandstone is characterized by a sandstone-shale layering. At the bottom of the outcrop the layering of sandstone is thin with thick shales separating the thinner sandstone. Moving up section, the sandstone layers (or beds) become thicker and gradually replace the shale beds. By the time the shales have completely disappeared, the unit is clearly distinct from the shale unit in the southern end of the outcrop.
STOP 1C: THE BOY SCOUT CAMP SYNCLINE
(Route 322 at the entrance to the Seven Mountains Boy Scout Camp)
The Juniata Formation is a fluvial sandstone-shale sequence that has been folded into a third order syncline at Stop #1C. Such folding is a consequence of thrusting of the Appalachian Valley and Ridge to the north-northwest. There are three features that are of great interest at Stop #1C: the third order syncline, fault wedging, and cleavage that marks flexural slip folding.
First, by walking to the southwest along Route 322, one can observe a gradual change in dip of the sandstones of the Juniata. On the northeastern end of the outcrop the rocks are dipping distinctly to the southeast. On walking southwest along the outcrop it becomes immediately clear that the rocks change dip so that they appear horizontal in cross section. This behavior was also observed when driving along 322 through the Mount Nittany Syncline. Further the southwest the beds start tilting to the northeast. At this point it should be clear that the student has cross the axis of a syncline.
Second, in response to the thrusting rocks of the Juniata contain many little faults with slickensided surfaces. Within the Juniata Formation the beds of sandstone break up by faulting to act like little wedges overlapping each other. The slip direction is indicated by lineations on the little faults bounding the individual wedges. The wedges reflect a straining process called layer-parallel shortening. In addition to the strain of layer parallel shortening, the Juniata was folded into a simple syncline.
Third, often such folding takes place like the folding of a deck of cards. The layers of sandstone act as the cards with detachment slip taking place within the shale layers. This displacement is largely seen within the shale layers as a cleavage that has refracted in the direction of slip. Cleavage is a planar fabric within the rock. These planar zones are a consequence of pressure solution in which the silica portion of the shale dissolves leaving behind the clay portion of the shale. This process favors the preferential solution of the more soluble minerals in the rock. Cleavage is distinct because the pressure solution process often occurs along discrete planes with undeformed rock between these discrete planes. Slip sandstone beds on the shale layers has caused the cleavage to become refracted. This is most visible on the north-dipping flank of the syncline.
STOP 1D: LONG MOUNTAIN ROADCUT
Pause beside the road, stay in the bus, the outcrop on the right is the southeast dipping (45°) sequence of Ordovician Juniata Formation and Silurian Tuscarora Orthoquartzite and Rose Hill Shale which we will observe again on the other, highly deformed, limb of a 2nd order syncline at Stop 1 (Figure 5). Here the dip is constant and small scale structures are Stage IV wedge and wrench faults of small displacement (Figure 5).
Figure 5: Section CD of the Laurel Creek area, show the Potlicker Flat Thrust (PFT) rising off the Antes-Coburn Detachment (AC).
STOP 1E: LAUREL CREEK RESERVOIR
This long (.2 mi, .3 km) exposure across the vertical beds of the Juniata, Tuscarora and Rose Hill Formations is one of the most structurally complex of the Valley and Ridge Province. Structural complexity results from its position in the footwall of the Potlicker Flat Thrust and its early structural evolution in the steep to overturned limb of an anticline (Figure 5). The sequence of structural development illustrated in Figure 6 includes:
Figure 6: Sequence of stages of deformation at Laurel Creek Reservoir.
1. Stage IV wedging and conjugate kink folding in horizontal to slightly dipping (20°) beds,
2. Major Stage V folding with z-shaped kink folds becoming dominant because they were properly oriented to grow during flexural-slip folding,
3. Stage VI layer-parallel extension seen as wedge-shaped fault blocks cutting beds at large angles. Extensional wedges are bounded by faults dipping 60° NW or horizontally parallel to the inferred Potlicker Flat thrust above the outcrop. The horizontal extension faults parallel to the thrust are better developed. As shown on Figure 5 and 7 the Potlicker Flat thrust formed early in the structural history as an imbricate off the Antes-Coburn detachment. It then contributed to the high strain of this outcrop, was folded as the beds steepened, and finally participated in the Stage VI extension and northwesterly transport.
STOP 2: JOINTS IN THE BALD EAGLE AND JUNIATA FORMATION
Cross-fold joints dominate this south dipping panel of rock which includes the Bald Eagle and Juniate Formations of this stop as well as the Antes Shales of Stop #3. Of particular interest in this outcrop is the spectrum of joint orientations. Unlike outcrops of the Appalachain Plateau which contain well-defined joint sets, this outcrop contains cross-fold joints with a uniform orientation distribution within about a 15° interval. Some of these joints are filled veins whereas others remained unfilled. Associated with this uniform joint distribution are well developed examples of both twist hackles and en echelon veins.
Figure 7: Laurel Creek - Reedsville profile, Section A-B, showing the Potlicker Flat Thrust (PFT) and Antes-Coburn Detachment (AC).
In classic examples, joint surface morphology includes a well developed plumose structure bordered by twist hackles (Woodworth, 1896; Hodgson, 1961; Kulander and Dean, 1985). Twist hackles form during rotation of the crack out of the plane of propagation so the the rupture divides into segments at the top and bottom of a bed (Fig. 4). The plumose structure is, itself, a finer hackle caused by a propagating crack which splits by local twists and tilts. Also called fringe or f-joints (Hodgson, 1961), border or b-planes (Roberts, 1961), twist hackles themselves sometimes display minature plumose patterns which can be traced back to the main joint. Unlike the classic examples, joints in Devonian siltstones of the Appalachian Plateau display well-developed plumose structures but are devoid of twist hackles (Bahat and Engelder, 1984). Evidence is unambiguous in support of the hypothesis that joints in the Appalachian Plateau propagated in a principal stress plane (Engelder, 1982). The regional persistence of joints without twist hackles as in the Appalachian Plateau begs for an analysis of conditions which favor the development of twist hackles.
The explanation for twist hackles most commonly cited in the recent literature is that when a propagating mode-I crack (extension fracture) encounters rotated principal stresses that produce a shear component parallel to the plane of propagation, the rupture front twists or tilts to maintain an orientation normal to the least principal stress (Bankwitz, 1965, 1966; Lawn and Wilshaw, 1975; Kulander and others, 1979; Pollard and others, 1982). Twist hackles are common in bedded sedimentary rocks where the twisting of the rupture front occurs next to bed boundaries. Often, if not always, the twist hackles are paired, top and bottom of beds as thin as a few decimeters. If the above interpretation applies, then two principal stresss axes including s3 have rotated by several degrees about an axis normal to bedding. Apparently this rotation occurs rather abruptly near both the top and bottom of the bed.
Some of the joint surfaces at stop #2 display twist hackles which have a strike within a degree of the parent joint surface. Other twist hackles differ by more than 10° from their parent. As twist hackles grow vertically from their parent joint surfaces, the spacing of the twist hackles increases. This same phenomenon was observed at Taughannock Falls where seccessor joint sets propagated vertically from primary joint sets. In this outcrop en echelon veins are believed to be twist hackles where the parent joint surface is not exposed.
STOP 3: ANTES-COBURN DETACHMENT
This outcrop is a continuous exposure of the 110 m thick detachment zone which contains disharmonic 4th order folds and spaced cleavage. The detachment zone is the only departure from a constant 40° to 50° dip in a 2 to 3 km thick tabular slab that comprises the southeastern limb of the Jacks Mountain anticlinorium (Figure 7, 8). The section exposed is upper Ordovician Trentonian Limestone and Antes and Reedsville shale. There is no evidence at this place or at eight other less well-exposed outcrops of the detachment, that differential slip has occurred between units above and below the detachment. However, large scale section balancing (Herman and Geiser, 1985; Perry, 1978) or rooting of imbricate thrusts seem to demand a detachment or boundary zone and this may be how such zones appear near the northeastern end of large-scale Alleghanian overthrusting.
Figure 8: Drawing of the Antes-Coburn Detachment exposed on the northeast side of Route 322 at Reedsville, PA.
STOP 4: BEARPEN HOLLOW THRUST
The thrust illustrated by Figures 9 and 10 dips northwest less than bedding and consequently cuts up section in a down dip direction. When rotated with bedding until bedding in the hanging wall is horizontal, the thrust dips southeast. This fault formed as a ramp rising off the Antes-Coburn detachment at the trailing branch line (Figure 10). Footwall rocks were dragged against the thrust and rotated during later anticlinal growth until they now are horizontal and overturned. Structural relations shown in Figure 9 will be observed in a shale quarry and during a walk up Bearpen Hollow to the hanging wall.
Figure 10: Bearpen Hollow Thrust rising off the Antes-Coburn detachment (AC) at the trailing branch line (TBL).
We will not visit the Stone Mountain duplex because exposures are on the ridge crest 700 feet above the road. The geologic map (Figure 4) and section (Figure 11) illustrate Stone Mountain, part of which is a duplex, and the Saddler Gap fault, a late, out of sequence upthrust. This structure evolved by imbricate thrusting toward 310°-324° (rightside, Figure 12) followed by rotation of 60° around the 15°-25° strike of Stone Mountain (left side, Figure 12). The duplex is now viewed on edge and its horses strike west of the mountain. This sequence of deformation when correlated with orientation of trends provides a different sense of rotation of tectonic trends than suggested on the Appalachian Plateau to the north of State College and further east in the Valley and Ridge. Figure 13 summarizes the data from the Pennsylvania Valley and Ridge where the the sequence of deformation at localities in northeastern to central Pennsylvania show clockwise rotation. The dominant strike of fold axes in central Pennsylvania between Lock Haven and Tyrone is 50°, corresponding to Lackawanna phase orientations to the northeast. The Stone Mountain duplex apparently formed with strikes of 30° to 48° and transport toward 310°-324°, but was overprinted by the northwest limb of the Jacks Mountain fold and the late, out-of-sequence Saddler Gap Fault on a trend of 15 to 25°. This indicates a counterclockwise rotation of successive phases of the Alleghany Orogeny in the region to the southwest. This later stage folding does not correlate with the Main (II) phase of the Alleghanian Orogeny but rather clearly post-dates it. Evidence for this same sequence of events is found at Stop 6 where later folding rotates both stage I and stage II structures. The trend labelled II 22 on Figure 13 continues for 200 miles (322 km) to central Virginia. The inferred structural development of the Jacks Mountain anticlinorium is illustrated in Figure 14.
Figure 12: Interpretation of the origin of the structural attitudes in the Stone Mountain Duplex by 60° counter- clockwise rotation of a prefolding, northeast-striking, duplex (A) around an AZ 15-25 axis. B is the attitude of the duplex after rotation.
Figure 13: Relative age of tectonic trends in the Pennsylvania Valley and Ridge: Lack = Lackawana phase; Main = Main phase; SWD = South White Deer anticline; BV = Bear Valley; LH = Lock Haven; T = Tyrone; B = Bedford; C = Cumberland
Figure 14: Structural sequence for creation of the Jacks Mountain Anticlinorium. 1.)Ramping from the Waynesboro detachment to the Antes-Coburn detachment and creation of the second order Stone Mountain duplex (Mitra, S., 1986) in upper Ordovician-Silurian rocks. 2.) Northwest rotation of the Stone Mountain duplex by a ramp rising from the Waynesboro detachment, which promotes growth of the Bearpen Hollow and Potlicker Flat imbricates off the Antes-Coburn detachment. 3.) The out-of-sequence, steep, Saddler Gap reverse fault cuts previous structures and is associated with layer parallel extension, steepening, and flattening of the northwest limb of Jacks Mountain anticlinorium.
STOP 5 SMALL FOLDS IN THE CREST OF JACK'S MOUNTAIN ANTICLINE
This roadcut consists of a traverse across the Kishacoquillas Anticline starting near the core in the Reedsville Shale. A traverse to the south will pass through deformed Bald Eagle Sandstones and up into the Juniata Formation. Folds in the Reedsville are characterized by short-kinked limbs and cleavage concentrated in the kinked limb due to shearing. The kinked limb is usually the south limb indicating backthrusting to the south. Other folds appear to be standard LPS folds with symmetrical limbs.
The Bald Eagle is characterized by (Lacazette Abstract). Fault zones cutting the Bald Eagle are characterized by slickelites with a significant coating of chlorite fibers indication that reducing fluid with still moving through the section as it was deformed. In between the major fault zones the Bald Eagle is cut by numerous smaller faults and fracture zones which distort original bedding and reduction spots into odd shapes. Some of the reduction spots have been sheared in a ductile manner. A particularly pretty sheared reduction spot is found in the upper part of the Bald Eagle where a shear zone develops as a series of en echelon fractures all with chlorite fibers. The sigmoidal shape of the en echelon fractures is reflected in the shape of the deformed reduction spot.
The contact with the Juniata Formation is located at the point where the first meter thick shale bed enters the section. Above this point the beds are in large part red with the intensity of faulting and folding decreasing up section.
STOP 6 FRACTURES IN THE DEVONIAN BRALLIER SHALE
The Devonian Brallier Formation of the central Appalachian Valley and Ridge is equivalent to the Genese Group of the Appalachian Plateau. The remarkable aspect of this exposure of Devonian shale and siltstone is its similarity between the development of joints here and in Devonian shales of the Appalachian Plateau. Of all the lithologies in the Valley and Ridge from Cambrian carbonates up through Carboniferous fluvial deposits, none carry joints which more closely resemble those seen on the Appalachian Plateau.
In this outcrop the Brallier dips to the southeast at about 15°. Like the beds of the Genesee Group on the Appalachian Plateau, two sets of cross fold joints cut the Brallier with the shalier beds carrying joints striking about 140° and the siltstone carrying joints striking about 158°. The relative time of propagation of the joint sets may be determined using the joint spacing criterion as was illustrated in the Genesee Group at Taughannock Falls. Toward the north end of the outcrop joints in siltstone beds can be seen propagating upward from joints in shale beds. This criterion suggests that joints in the shale (140°) propagated prior those in the siltstone (158°). Early joints in the Genesee Group at Watkins Glen propagated at 151° versus the late joints which propagated at 163°. The difference between the Appalachian Plateau and the Valley and Ridge is the lithology carrying the early joint: shale in the Valley and Ridge versus siltstone in the Appalachian Plateau. This clockwise rotation of joint propagation is commonly seen throughout the northern edge of the Valley and Ridge.
Aside from the fact that these joints look like those found in the flat-lying Devonian rocks of the Appalachian Plateau, two peices of evidence suggest that the cross-fold joints in the Brallier preceded folding. First, the joints have been rotated to dip between 3° and 6° to the southwest. If the present dip of the Brallier is removed these joints are vertical, presumably the orientation at which they propagated. Secondly, some joints in the shale beds are decorated with slickensides and fibrous calcite indicating a left-lateral shear. This is the type of slip expected for the compression responsible for later folding. Furthermore, the orientation of the calcite fibers indicates that slip direction has a shallower plunge than bedding dip. These are some of the same arguements used by Nickelsen (1979) to demonstrate early jointing at Bear Valley.
This outcrop is located on the north end of the Broadtop syncline, a large fold with a more northerly trend than the folds in the Juniata Culmination. Early jointing indicates compression directions compatible with those of the Juniata Culmination and Appalachian Plateau. However, the major folding event is compatible with the orientation of the Juniata Culmination.
STOP 7: CHARLIE HILL KINK BAND FOLDS IN THE WILLS CREEK SHALE
The Wills Creek Shale is one of the inferred detachment horizons of the stratigraphic column in Pennsylvania, which, at this outcrop, demonstrates the large strains that are commonly found. The unit is the stratigraphic facies equivalent of the Salina salt of the Appalachian Plateau and it is the inferred roof zone of imbricate faulting such as that occurring on the northwestern limb of the Jacks Mountain anticlinorium (Stops 1-5).
Layer parallel shortening (LPS) has produced an array of lithologically segregated structures that were externally rotated during Stage V folding and somewhat modified. Minor structures include small kink band folds in well-bedded limy shales, spaced cleavage in green, mudstones and widely spaced dissolution cleavage in carbonate layers. In the limy shales, Stage III, homogeneous, plane strain LPS produced symmetrical kink bands with both clockwise and anticlockwise rotational senses. During their external rotation on larger flexural slip folds they were either degraded or enhanced, depending upon their rotational sense compared to the flexural slip sense on larger fold limbs (Figure 15).
LPS of carbonate layers was presumably similar in amount but achieved by dissolution. In the green mudstones the mechanism of origin of the spaced cleavage is unknown, but probably includes both dissolution and grain boundary sliding. At the north (right) end of the exposure are extremely tight flexural slip folds and deformed mudcrack polygons.
Figure 15: Rotational sense (clockwise-C, or anticlockwise-A) of small scale, Stage III, LPS kink folds, compared to flexural slip sense (FS) on the limbs of larger Stage V folds at Stop 7. Kink folds are either enhanced or degraded, depending upon whether rotational sense agrees or disagrees (Illustration modified from Faill, 1973).
STOP 8: STIFF LAYER CARBONATE SECTION AT UNION FURNACE
Almost 1000 feet (1600 m) of middle to upper Ordovician carbonates are exposed, including the upper Bellefonte, Loysburg, Hatter, Snyder, Linden Hall, Nealmont, and Salona and Coburn-Trentonian Formations. It is the most complete, best exposure of middle and upper Ordovician carbonates in central Pennsylvania. The regional setting of this stop is shown in Figure 16 a section drawn by Faill (1987). The outcrop is typical of the southeast dipping limbs of major ramp anticlines in the Valley and Ridge in that structures seen consist largely of spaced cleavage, transverse extensional joints, wrench faults with slickenlines that parallel bedding-fault intersections, and transverse extensional grabens. This panel of carbonates would then be one of the major horses in the carbonate duplex of the Juniata Culmination. Summary stereographic projections of bedding, joints, wrench faults and slickenlines on wrench faults are presented in Figure 14, compiled by R. Faill and published in Sevon, 1986. Both right lateral and left lateral wrench faults occur in a conjugate system with an acute dihedral angle of approximately 30°. Slickenlines parallel to bedding-fault intersections indicate that wrench faulting occurred during Stage IV LPS prior to major folding. The transverse extensional grabens with steeply dipping slickenlines are rare here but are a frequently observed feature of the province, denoting axial extension along folds. Spaced cleavage is best seen near the top of the section in Trenton limestones comprising interbeds of shale and argillaceous limestone, but bedding-perpendicular stylolitic cleavage is present locally in all purer limestones of the section. These cleavages originated during Stage III LPS, prior to folding.
Figure 16: Section of the Nittany Anticlinorium showing Stops 8-12 and the location of the Birmingham Window as well as the underlying Cambro-Ordovician carbonate duplex and thrust horizons (Cambrian, Waynesboro, Ordovician Antes-Reedsville, and Silurian Wills Creek-Salina). (Figure copied from Faill, 1987.)
STOP 9: THE BIRMINGHAM WINDOW
Three areas of exposure will be visited: A. Roadcuts along Route 453 which show the footwall of overturned, tectonized Silurian Tuscarora Formation and Ordovician Juniata and Bald Eagle Formations, B. The Sinking Valley Fault zone comprising cleaved and folded upper Ordovician Antes Shale, and C. The Conrail Railroad cut showing Cambrian Gatesburg Formation of the hanging wall, thrust over Ordovician limestones. The footwall at Station A is a polygonized, thinned section which extends from Route 453 up the road towards Birmingham. Station B begins south of the Birmingham road in carbonized, cleaved, and microfolded Antes Shale containing a "floating" block of tectonized Ordovician (?) limestone. This zone is the Sinking Valley Fault which passes above Station A and beneath Ordovician (?) limestones exposed in a cliff above the shales. Presumably the same Ordovician (?) limestones occur at the Conrail railroad cut (Station C) south of the Little Juniata River. The rocks at Station C are folded, faulted, and overthrust by the Cambrian Gatesburg (?) Formation, which occurs above the Birmingham Thrust Fault, well exposed at the south end of Station C. The Birmingham Thrust Fault is presumably an imbricate off the main Sinking Valley Fault. Stations A, B and C thus are in sequence from the Ordovician-Silurian footwall, through the fault zone, and into the Cambrian hanging wall of the Birmingham Window.
The exposure of the Sinking Valley Fault zone at Station B presents some evidence of transport directions and the sequence of stress orientations in this area of the Appalachians. A slip line orientation was determined from the separation direction between the array of clockwise and anticlockwise minor folds in the Antes shale, using the method of Hansen (1971). Transport in the direction 155-335 along the Sinking Valley Fault is indicated (Figure 17). This transport direction is north of the perpendicular to strike and has been plotted as the black arrow near T (Tyrone) on Figure 10. This transport direction is similar to that of the Stone Mountain duplex of the Jacks Mountain anticlinorium (Stop A5).
Structural relations between Stations A and B north of the river and Station C south of the river may be complicated by the Tyrone-Mount Union lineament, which extends along the fairly straight course of the Little Juniata River between these stations (Kowalik and Gold, 1974 and Canich and Gold, 1985).
The regional setting of this stop is shown in Figure 16.
Figure 17: Stereographic projection of hinges of clockwise and anticlockwise folds in the Antes shale of Sinking Valley Fault zone at Stop 9, Station B. The slip line and inferred transport direction are parallel to the separation line between clockwise and anticlockwise folds (AZ 155-335) (Hansen, 1971).
STOP 10: VERTICAL TO OVERTURNED ORDOVICIAN DOLOMITES OF THE HANGING WALL OF THE SINKING VALLEY FAULT AND THE NORTHWEST LIMB OF THE NITTANY ANTICLINORIUM
This exposure shows structures typical of the northwest limbs of 1st order anticlines in the Valley and Ridge province of Pennsylvania, including steep to overturned dips, intrabed wedging, and medium scale hanging wall ramps. A sequence of deformational stages was essential for forming the structures of this outcrop.
Intrabed wedging and the hanging wall ramp formed during Stage IV LPS in horizontal beds. Beds of the hanging wall ramp which dipped steeply northwest after their initial thrusting in Stage IV, now are horizontal and overturned after having been rotated another 90° by Stage V folding. Asymmetrical parasitic folds in the overturned beds could have formed either during Stage IV (and later been rotated to present attitude) or during late stage flattening of the nearly horizontal, overturned beds. The regional setting is shown in Fig. 16.
THIS ROAD CUT IS A DANGEROUS PLACE BECAUSE THE ROAD THROUGH THIS CUT HAS VERY NARROW SHOULDERS. IT IS IMPORTANT TO STAY AT THE EDGE OF THE ROAD.
STOP 11: VERTICAL JUNIATA FORMATION ON THE NORTHWEST LIMB OF THE NITTARY ANTICLINORIUM
The steeply dipping Juniata Formation contains several faults or parasitic folds showing down-to-the-northwest slip or fold asymmetry. These structures are interpreted as prefolding, Stage IV thrust faults that originally climbed section toward the northwest. They were later rotated 90° to their present attitude during Stage V large scale folding. It is possible but unlikely that these structures are steep, late-in-the-sequence, northwest-dipping kink bands that either developed after beds were vertical or followed earlier zones of weakness. If the latter interpretations is adopted, this is the only outcrop in the Pennsylvania Valley and Ridge province where such structures have been recognized in this attitude. The preferred interpretation is in agreement with other structural sequences demonstrated on this trip.
STOP 12: UPPER REEDSVILLE SHALE AND BALD EAGLE SANDSTONE
The steeply dipping beds here show a variety of structures that can be placed in sequence by their mutual relations. Early structures are wedges, small scale duplexes that show transport and propagation down dip to the northwest, and kink folds. All probably originated when bedding was horizontal. Wrench faults formed at different times during the structural sequence. Some formed early in horizontal beds as indicated by their slickenlines which parallel fault-bedding intersections. Other slickenlines cross kink bands with no change in orientation, showing that they are later than kinking. Still other wrench faults have curving slickenlines or overprinted slickenlines of different plunges, suggesting that they rotated through the stress field with the enveloping bedding while they were forming. The structural features of this outcrop on the northwest limb of the 1st order anticlinorium stand in sharp contrast to the array of structures seen at Stop 8, on the southeast limb of the structure. This outcrop is different because of the abundance of wedging and duplexing as well as the greater complexity of wrench fault slickenline orientations. These differences suggest that the geographic position of the northwest limb was selected early in the evolutionary history of the anticlinorium.