Introduction


Physiography and Drainage

All of the stops on this field trip take place in the Appalachian Mountain Section of the Ridge and Valley Province (Figure 1), which consists of long, narrow ridges and broad to narrow valleys exhibiting moderate to very high relief (Sevon, 2000).  Within the field trip area the most prominent ridge trends generally northeast-southwest.  Twists, turns, and offsets in the vicinity of Hollidaysburg make this a complex topographic feature (Figure 2) with various names for the various parts of the mountain.  To the north, between Altoona and Tyrone, it is a gently curving ridge called Brush Mountain, steeply sloped on the eastern flank and more gently on the western flank.  Subsidiary ridges are also common.  These ridges typically are the remnant flanks of breached anticlines and synclines underlain by resistant cherty limestones and sandstones of the Upper Silurian and Lower Devonian Keyser and Oriskany formations.  Although these rocks generally contain more sandstones than the underlying formations, sandstone is not very important in the formation of the mountain itself.  Ridge and Valley Province ridges have fairly steeply dipping slopes, as much as 20o in places. The valleys within the province generally have floors of shales and/or carbonate rocks, both of which are easily eroded in Pennsylvania’s climate.  Some fairly resistant rock layers produce much reduced ridges and knobs within the valleys.   The long continuous valley west of Brush Mountain generally lies at an elevation between 366 and 427 m (1,200 and 1,400 ft) but rises to about 488 to 533 m (1,600 to 1,750 ft) in the foothills of Allegheny Mountain (the Allegheny Front).  This valley goes by many names, depending on where you are.  North of Altoona it is called Bald Eagle Valley; between Altoona and Hollidaysburg it is called Logan Valley; and west and south of Hollidaysburg it goes by the name of Frankstown Valley.  

All of the ridges, escarpments, and folds within the Appalachian Mountain Section have been dissected at intervals by deep valleys or gaps. Known sometimes as water gaps, or wind gaps where no water currently flows, these valleys often indicate the positions of ancient streams that carved down through the Appalachians over millions of years. 

The Allegheny Front forms the major drainage divide in this area. Streams on the western side flow to the Ohio River watershed to the Mississippi River and, eventually, to the Gulf of Mexico. Streams on the eastern side of the mountain flow via the Juniata River watershed to the Susquehanna River, into Chesapeake Bay and the Atlantic Ocean. 

The Juniata River is the major waterway of the Appalachian Mountain Section, draining approximately 8,806 km2 (3,400 mi2) of central Pennsylvania. The main channel of the Juniata River forms to the east, near the village of Petersburg in Huntingdon County, at the confluence of the southern Frankstown Branch, and the northern Little Juniata. The Little Juniata drains 886 km2 (342 mi2) square miles whereas the Frankstown Branch drains 1,026 km2 (396 mi2) (Juniata Clean Water Partnership, 2000). Both of these rivers originate as numerous tributary creeks on the east slope of Allegheny Mountain.  Other tributaries originate on the west slopes of the first set of ridges and within the shale-floored valley between Allegheny Mountain and the ridges. The gradients of the larger streams, such as the Frankstown Branch, are moderate, but smaller streams have much steeper gradients (Butts, 1945). 

Many of the present day streams of Pennsylvania are remnants of ancient streams. The Juniata River, like many of the rivers in the Appalachian Plateau of western Pennsylvania, may have originated in the Early Triassic as a river flowing off the western slopes of the newly formed Alleghanian Mountains, carrying tons of detritus in suspension across a great alluvial plain that stretched all the way across the Midwest and Canada.  Actually, subsurface studies in western Pennsylvania indicate the streams may have had their origins even much earlier than Early Triassic. Upper Devonian and Pennsylvanian sedimentation patterns often lie along some of these streams. Harper (1989) speculated that basement wrench faults beneath the plateau influenced sedimentation and drainage patterns throughout the Phanerozoic. Although they flow eastward now, streams such as the Frankstown Branch of the Juniata River used to flow westward before deep erosion of the mountains and stream piracy by the Susquehanna River changed their direction of flow during the Mesozoic (Sevon, 1993).

General Stratigraphy

Although the strata we will be studying on this field trip are Late Ordovician in age, the route we will take crosses strata that span approximately 205 ma of geologic time, from the Late Cambrian to the Late Pennsylvanian (Figure 3). Butts (1945) estimated the thickness of these strata to be in the neighborhood of 7,010 to 7,620 m (23,000 to 25,000 ft).  

Upper Cambrian (Warrior and Gatesburg – please refer to Figure 3 for reference to formation names) through lower Upper Ordovician (Trenton and Black River) bedrock exposed on the field trip occurs only in the valley of the Little Juniata River between Tyrone and Union Furnace, essentially the last 5 miles before Stop 1. The full Cambrian section exposed in central Pennsylvania consists of approximately 1,065-1,220 m (3,500-4,000 ft) of predominant dolostone with subsidiary sandstone, shale, and limestone strata at varying intervals (Butts, 1945).   Carbonates comprise more than 1,370-1,675 m (4,500-5,500 ft) of Lower, Middle, and Upper Ordovician strata in this area.  About 75% of these carbonates consist of dolostones of varying texture, color, constituents, etc.

The western flanks of Brush Mountain expose Upper Ordovician through Lower Devonian limestones and sandstones, whereas easily eroded shales of the Middle and Upper Devonian form the floor of the Logan Valley west of Altoona and Tyrone. Upper Devonian and Lower Mississippian shales and sandstones underlie the eastern escarpment of Allegheny Mountain, and Lower to Middle Mississippian sandstones form the summit. From there westward the bedrock consists of Middle Mississippian through Upper Pennsylvania mostly terrigenous rocks.

Lower Silurian Tuscarora sandstones derived from erosion of the Taconic highlands to the east gave way to mudrocks and carbonates in the Silurian.  During this transition, the amount of clastic material decreased upward until, by the end of the Silurian, the rocks comprise fossiliferous marine limestones.  The Tuscarora Sandstone acts as the major ridge former in the Ridge and Valley Province.  Although we will not be able to examine it close up, it is very noticeable as nearly ubiquitous boulder fields and talus deposits of white sandstone crowning the upper slopes of the higher ridges. Lower Devonian rocks in central Pennsylvania range from bioclastic shelf carbonates to very coarse-grained sandstones, the result of fluctuating shallow marine depths and increasing clastic input from the east. Sea level continued to fluctuate through the Middle Devonian with carbonates replacing clastics replacing carbonates. By this time, the Acadian orogeny was taking place on the eastern margin of Laurentia.  Faill (1985, 1999) found no evidence of Acadian deformational structures in Pennsylvania west of the Piedmont, but noted that sedimentological changes provide a good record of the event. Numerous K-bentonites within the upper Onondaga and lower Marcellus formations (Tioga ash falls) demonstrate that the Acadian orogeny was under way by that time.  

The encroachment of the Catskill deltaic complex, which began in eastern Pennsylvania in the Middle Devonian, dominated the Late Devonian and Early Mississippian in central and western Pennsylvania. As a result of the Catskill progradation, Upper Devonian rocks consist almost entirely of sandstones, siltstones, and shales, with a few minor limestone beds punctuating the section.  The lower part of the section is dominated by marine shelf mudrocks with a general increase in grain size upward through the section. The rocks also reflect a generally upward-shallowing sequence, from prodeltaic through distributary to continental alluvial deposition. Pennsylvanian rocks in the report area consist of a highly variable sequence of fluvial and deltaic sandstones, shales, siltstones, and claystones, coals, and both marine and nonmarine limestones. With the exception of a few marine limestones, the individual beds generally have limited areal extent, occurring most commonly as lenses and pods.  Thickness for individual units range from a few inches to several tens of feet. Because of this extreme variability only a few of the more extensive and economically important units have been formally named. 

Structural Geology

The Ridge and Valley structural province is the classic example of a folded and faulted foreland mountain system; the structures formed during the Alleghany orogeny (Faill and Nickelsen, 1999). The majority of the fold belt extends 1,127 km (700 mi) along the eastern interior of North America, from Pennsylvania to Alabama. Only a narrow band about 10 miles wide extends from eastern Pennsylvania through New Jersey and New York into the Hudson and St. Lawrence Valleys (Rodgers, 1970).

Although folds as seen at the surface are the most obvious structures, the most important tectonic element actually is a system of southeast-dipping thrust faults that rise through the Paleozoic rock section (Faill and Nickelsen, 1999).  These faults generally run parallel to bedding for many miles, acting as décollement surfaces within certain ductile rock units, before ramping upward through more brittle ones.   Many of these faults are blind – that is, they do not intersect the surface of the earth (Figure 4). Seismic survey data suggest ramping typically occurs above basement normal faults associated with the breakup of the supercontinent Rodinia in the Late Precambrian and Early Cambrian.  During the Alleghanian orogeny, the Upper Ordovician through Permian sedimentary rock section was transported northwestward along a basal Cambrian décollement and deformed in a series of imbricate thrust sheets. These strata are generally deformed in approximate imitation of the basal deformation, but contain folds and faults of their own as well.

Ridge and Valley folds were once thought to be long, continuous folds broken here and there by subsidiary structures resulting from Alleghanian compression alone. However:

“It is now recognized that the folds are but one of a number of stages of a deformation that extended over a period of time, during which the principal stress directions changed orientation. The deformation stages included pretectonic hydraulic jointing followed by tectonic cross-fold extension jointing and layer-parallel shortening, which is expressed as both rock cleavage and conjugate wedge and wrench faulting.  Major flexural-slip folding overprinted all previous structures and led to layer-parallel extension on the steep limbs of folds. The last structures of the Alleghany orogeny were late strike-slip faults, which are sometimes associated with major reverse lineaments, and out-of-sequence high-angle reverse faults.”  (Faill and Nickelsen, 1999, p. 270)

Folds come in all sizes, from anticlinoria (wavelength >16 km [10 mi]) to specimens you can hold in your hand (Nickelsen, 1963). (Synclinoria and smaller-order synclines are basically passive structures lying between the anticlinoria and anticlines.) In the Ridge and Valley, the anticlinoria extend from the Transylvania fault zone of Root and Hoskins (1977), which strikes almost due east-west through Bedford and Breezewood (Figure 2) to the Blue Ridge, to the Susquehanna River or beyond.  The folds within the anticlinoria generally have lengths less than 97 km (60 mi) (Faill and Nickelsen, 1999).  Many of the anticlinoria contain subsidiary en echelon folds that are controlled by local thrust faults occurring within different stratigraphic intervals along the length of the major folds. Smaller (second- and third-order) folds generally stand upright, many with vertical or overturned limbs.  Some are recumbent (Epstein and others, 1974). The fold axes do not exhibit smooth curving along their lengths; rather they consist of small, straight segments that change orientation in increments, from one or two to as many as 20 degrees. Some of these folds are elongate domes, whereas others are long and linear.

Although most of the faults present at the surface in the Ridge and Valley are thrust faults, wrench faults do occur in major zones that cross regional strike. Normal faults typically are rare and small, restricted to vertical and overturned beds in the northwest limbs of the anticlines. 

The Cambrian décollement extends beneath the Ridge and Valley probably as far southeast as the Great Valley, Blue Ridge, and much, if not all, of the Piedmont (the basis of the Eastern Overthrust Belt that had oil and gas explorers interested in the early 1980s). It ramps upward at the Allegheny structural front to the level of the Silurian Salina Group, which forms the principal basal detachment horizon beneath the Appalachian Plateau from West Virginia to New York (Gwinn, 1964; Frey, 1973).   Thrust faults that splay off the basal décollement to form the cores of anticlines tend to be moderately steep and parallel to bedding in the southeastern anticlinal limbs. In contrast, the faults in northwestern anticlinal limbs have low dips and crosscut bedding (e.g. the Nittany anticline – see Gwinn, 1970). 

The Ridge and Valley also contains many transverse fracture zones, some with documented strike-slip components. The Transylvania fault zone of Root and Hoskins (1977) mentioned above is the longest in central Pennsylvania. Most large transverse zones have been described as lineaments or cross-strike structural discontinuities (CSDs) (Kowalik and Gold, 1976; Rodgers and Anderson, 1984; Harper, 1989; Gold, 1999). The most prominent of these is the Tyrone-Mt. Union lineament, which appears to be the longest in the state (running from at least the northern terminus of the Blue Ridge to Lake Erie). It stands out in the Ridge and Valley as the parallel courses of the Little Juniata and Juniata Rivers (Figure 2). In addition, Faill (1987) has described numerous small structures exposed along its trace in the Little Juniata River valley near Birmingham, including slickensided transverse faults, mesoscopic disharmonic folds, and high fracture density.  It also terminates many second- and third-order folds, both in the Ridge and Valley and the Appalachian Plateau.  It apparently has a right-lateral strike-slip component of about 60 km (37.5 mi) in the basement (Lavin and others, 1982, based on evidence from gravity data) (Figure 5), but a down-to-the-southwest normal fault component in the Paleozoic cover (Rodgers and Anderson, 1984). According to Canich and Gold (1985), this zone is still seismically active.

Faill and Nickelsen (1999) divided the Alleghanian deformation in the Ridge and Valley into a several stages in which one or another process was predominant:  1) pre-Alleghanian deformation, which left a regional record of extensional joint sets in coals in the Appalachian Plateau. The presence of several overprinted extensional joint sets implies different stress orientations resulting from epeirogenic movements and/or early Alleghanian layer-parallel shortening (Nickelsen and Hough, 1967); 2) overprinted episodes of layer-parallel shortening, which produced extension joints, spaced cleavage, small-scale folding, and conjugate wrench faulting and thrusting; 3) major folding; 4) late stages of folding in which layer-parallel extension on the steep fold limbs produced joints and both strike and transverse extensional wedges; and 5) strike-slip faulting that cuts all previous structures and appear to be restricted to major gaps located on lineaments.

Hydrothermal Mineral Deposits

Hydrothermal mineralization plays an important role in the emplacement of hydrocarbons in the Trenton-Black River play. Therefore, a few words about hydrothermal minerals occurring within a few miles of the Union Furnace outcrops should be in order.

Blair County is well known for its Colonial Period iron ore mines and furnaces, including Union Furnace.  However, the Birmingham area is even better known for its Mississippi Valley-type lead and zinc deposits. Tectonic deformation of the Upper Cambrian and Lower Ordovician carbonates of Sinking Valley (the breached center of the Nittany anticline) included a high degree of fracturing where mineral-rich fluids deposited sphalerite and galena with calcite, dolomite, barite, pyrite, and minor to trace amounts of hydrozincite, smithsonite, calamine, cerussite, jordanite, and anglesite. Perhaps the best known deposit is the old Keystone mine in Honest Hollow, approximately 1.3 km (0.8 mi) due west of the village of Birmingham.            

The following narrative, from Miller (1924, p. 13-14), explains the history of mining for lead and zinc in Blair County:

“The first lead and zinc mines of Pennsylvania were operated in the Sinking Valley, Blair County, during the Revolutionary War.  The Continental Army being in great need of lead for bullets, a party was sent to investigate some lead deposits said to be in the Wilderness near Frankstown. As a result of the examination General Daniel Roberdeau opened and worked some shallow mines in the southern end of Sinking Valley during 1778 and 1779. . . At one time 1,000 pounds of lead was sold to the State at $6.00 a pound in the depreciated currency of that period. It is not known when the mines closed but probably the operations were short-lived because of the expense of transporting materials for mining and smelting the ore, the maintenance of the laborers in the Wilderness, as it was called, and the guards that were necessary on account of hostile Indians.

“The next period of active mining was in 1795 when John Musser was employed by Robert Morris to drive a drainage tunnel into the side of the hill near Birmingham to connect with a shaft previously sunk. No further information is available concerning this undertaking.

“It seems that there was little if any more work done until 1864 when the Keystone Zinc Company was organized and with abundant capital started operations on a large scale. Most of the work was done in the northern part of the valley near Birmingham but investigations by this company and others were made in a number of places in the southern part of the valley.  New shafts and tunnels were driven and a large reducing plant for the manufacture of zinc oxide was built near Birmingham for the treatment of ores from this and other regions.  In this attempt the principal attention was given to zinc while in the previous operations only lead was sought and the zinc minerals present were regarded as worthless or as part of the gangue. After six years, during which several thousand tons of ore were mined, the company became financially involved and the plant was closed in 1870. Since that time there have been sporadic efforts to discover workable ore beds in various parts of the valley but with indifferent success.  In 1875 some diamond drill boring was done in the southern part of the valley and in 1876 a small quantity of ore from another property was mined and shipped to the Bamford reduction plant near Lancaster.

“The latest known attempt to reopen the mines was in 1901 when a certain company issued a prospectus and endeavored to interest capital in the project. This is said to have been merely a stock-selling scheme.

“At present the mines are in bad condition although the writer was able to go down one of the old shafts near Birmingham in the summer of 1921 and to see something of the old drifts and stopes that are above water level.”

In examining the Keystone mine deposits Moebs and Hoy (1959) determined that they occur in limestones, rather than dolostones, and assigned the host rock to the Black River-Chazy Groups (Loysburg through Linden Hall formations). The mine occurs close to, if not in association with a thrust known as the Honest Hollow fault, and although it is likely the rock is Loysburg-Linden Hall, Smith (1977) stated that he would not rule out the Stonehenge or Warrior as possible host rock.  Other occurrences in the Birmingham are found in the Mines Member of the Gatesburg Formation, and the Stonehenge, Nittany, and Bellefonte formations. 

In discussing the origins of the Birmingham lead and zinc deposits, Smith (1977, p. 99-100) stated that the occurrence of these minerals in the local rocks suggested some post-deformational emplacement:

“In order to mineralize all these hosts, one possibility is that the Sinking Valley thrust was a primary conduit for hydrothermal solutions localized by secondary splays.  However, movement of solutions through a major thrust should be difficult. Problems also arise in thoroughly brecciating the incompetent limestone in the mine and in accounting for mineralization on both sides of the crest of the Sinking Valley fault.  Hydrothermal solution movement toward the crest of the fault from two directions seems unlikely.

 “A. W. Rose (personal commun., 1974) offers an alternative path or primary conduit for hydrothermal solutions.  He believes that steeply dipping, highly fractured Silurian Tuscarora quartzite and Ordovician Bald Eagle (Oswego) sandstone present beneath the Sinking Valley thrust would serve as excellent conduits as they apparently have in the Milesburg Gap and Mapleton areas.  Moebs and Hoy (1959) encountered Reedsville and Juniata Formations in drilling and these as well as the Tuscarora and Bald Eagle Formations are exposed in nearby windows.  According to Rose’s model, metals would be precipitated upon passing through the Sinking Valley thrust and encountering suitable carbonate rocks.  The report of sphalerite, galena, and pyrite in Tuscarora quartzite nearby tends to support this theory.”