STOP 3:   UNION FURNACE ROADCUT


Leaders:  Christopher D. Laughrey and Jaime Kostelnik, Pennsylvania Geological Survey

David P. Gold and Arnold G. Doden, Pennsylvania State University

Introduction

The best exposed section of the Ordovician carbonates in central Pennsylvania is in the roadcuts along PA 453 between Tyrone and Water Street.  From the Shoenberger Road turn-off (mileage 107.25), the stratigraphic succession from the Nittany (Axemann is missing), Bellefonte, Loysburg, Hatter, Snyder, Linden Hall, Nealmont, Salona and Coburn formations is exposed in almost continuous road-cuts over a distance of 2,107 m (6,912 ft) to the southeast (Figure 39).  To the left (west) of Shoenberger Road is an abandoned quarry in the Mines Member of the Gatesburg Formation, and Stonehenge/Larke limestones and dolostones are exposed in the road-cuts 183 m (200 yd) north of the intersection.     

Although the emphasis of this field trip is on the formations in the Trenton and Black River Groups, we will examine the lithologies from the Coburn Formation to the Loysburg  Formation that are exposed in the Union Furnace roadcut.  We measured 240 m (787 ft) of Trenton and Black River Group rocks at the Union Furnace outcrop along PA 453 (Figure 17). A few more meters of partially to poorly exposed Trenton Group rocks extend above the top of our measured section on the southeast side of the road cut. We also measured 10 m (33 ft) of the Middle Ordovician Loysburg Formation below the Black River Formation in order to demonstrate the continuity of peritidal, low-energy shallowing-upward successions from the Loysburg up into the lower Black River Group.

You will find an abundance of abbreviations, numbers, names, and symbols marked on this outcrop. Numerous workers placed these over the past 17 years. Note the green numbers on the rocks that denote the “mining units” shown in Figures 45, 51, 56, and 59.   Although they were defined on both lithologic and convenient sampling intervals (i.e., thick sections of uniform lithology have been subdivided for sampling purposes), these units do not have stratigraphic significance.  These subdivisions, established by Berkheiser and Cullen-Lollis (1986) (Figure 13), originally went from Unit 04 (Loysburg) to Unit 30 (Nealmont/Salona), and their work portrays the major element chemistry of these units graphically.  The units have been extended into the Antes Formation.  However, only units up to 47 in the Coburn (Trenton Group) are exposed in the roadcut.  They have been carried into the road-cut by D.P. Gold and A.G. Doden from adjacent drill holes (see Plate 2).  The units are discrete packages of strata that are convenient for chemical analyses and physical surface mining (Figure 13). We use the numbered units as a location guide at this outcrop, nothing more. The formation names are abbreviated here too, although in a few instances we question the formation boundaries. Previous workers have also marked the bentonites exposed here. These are designated B0 through B19. These designations are not related to the bentonites in the Salona Formation formally numbered by Kolata and others (1996). 

A vertical section (based on nearby cores) indicates a thickness of 55 m (180 ft) for the Salona Formation and at least 122 m (400 ft) for the Coburn Formation.  A total thickness for the Trenton Group would be on the order of 183 m (600 ft).  The chemistry of this group is characterized by an increasing presence upwards of alumina (0.8 – 4.0%), silica (14-21%), and 1-2% organic (?) “carbon”.  The basal Reedsville beds (Antes Member), despite their appearance as a black shale, are remarkably calcareous (25-37 % CaO, 3.6-4.2% Al2O3, 1-2% K2O, 21-43% SiO2, and 9-11% total carbon).

Structural Geology

The beds in the road-cut strike 060o-070o.  The general dip is 30o southeast, but ranges from 22o to 45o.  A dominant joint set, striking 140o -150o with near vertical dips, has been accentuated by the road-cut excavation.  Apart from some mesoscopic scale folds near the east end, most of the deformation is manifest as faults (mostly mesoscopic scale, steeply dipping transcurrent with a shallow movement direction).  Both left and right lateral senses of movement were recorded from jogged slickenlines (see red symbols on Plate 2).  Movement sense also was determined from the accompanying en echelon tension cracks.  Other faults include east verging normal faults and west verging oblique and dip-slip reverse faults.  The regional faults are inferred from juxtaposed mapping units in the fields to the west.  Except for the regional faults, no significant displacements were noted.  These strata represent the northwestern limb of  the Scotch Valley Syncline (Malik, 1999). 

Stratigraphy, Depositional Environments, and Carbonate Petrology

Loysburg Formation

We will spend only a little time looking at the Loysburg Formation rocks exposed beneath the Black River Group. We want trip participants to see the peritidal low-energy, shallowing-upward successions that make up this formation (Chafetz, 1969), and note that these same cycles continue up into the lowermost Black River rocks.

Field (1919) named the Loysburg Formation for exposures of interbedded limestone and dolomite conformably overlying the Bellefonte Formation (upper Beekmantown Group – see Figure 10) in Bedford County, Pennsylvania. The Loysburg extends throughout the Valley and Ridge province in central Pennsylvania. Its thickness there is highly variable (Kay, 1944a; Faill and others, 1989). Faill and others (1989) report that the upper contact with the Hatter Formation of the Black River Group contact is sharp and conformable, but it is gradational here at Union Furnace along PA 453. The Loysburg is further subdivided into the Milroy (“Tiger Stripe” of Kay, 1944a) and Clover Members (Berg and others, 1983).

The Milroy Member consists of interbedded dolostone and limestone, i.e., ribbon rocks. The characteristic weathered banding of this unit provided Kay (1944) with the name “Tiger Stripe” (Figure 40). The Clover Member constitutes the upper part of the Loysburg, and it consists of thick to very thick-bedded mudstones with minor amounts of wackestone, packstone, and grainstone.

Lithofacies and Depositional Environments – We concur with the interpretations of Chafetz (1969), Berkheiser and Cullen-Lollis (1986), and Gardiner-Kuserk (1988) that the Loysburg Formation is comprised of peritidal carbonate cycles. Chafetz (1969, p. 16) suggested that the Milroy Member was deposited in semi-isolated depositional basins, which were separated from one another by submergent or emergent ridges. The meter scale (or less) cycles exhibit vertical profiles  indicative of low-energy peritidal shallowing-upward successions (Hardie and Shinn, 1986; Pratt and others, 1992).

Cycles begin with lags of intraclastic and/or bioclastic grainstone overlain by wave rippled, burrowed, skeletal wackestones (Figure 41A and B). Selective dolomitization occurred along some laminations, and around some burrows (Figure 41C). These rocks are subtidal deposits. The bioclastic and most  intraclastic lags are  transgressive, and sit on top of an underlying cycle.   Some large, blocky intraclastic lags, however, probably are from the collapse of tidal channel margins,  and represent the base of a tidal channel fill (Figure 41A).

The rippled, burrowed skeletal wackestones are lagoonal sediments. These are overlain by thin, wavy, flaser, and lenticular bedded, variably bioturbated mudstones, with minor lenses and very thin beds of bioclastic and pelletal grainstone; these beds show evidence of periodic exposure such as mudcracks (Figure 42). These are intertidal facies. The ribbon rocks of the Milroy Member (Figure 40) were also deposited in subtidal to intertidal environments (Demicco, 1983).

The low energy shallowing-upward cycles are capped by microbially laminated mudstones, or stromatolites, which are partially to completely dolomitized (Figure 43). The stromatolites sometimes have a fenestral fabric, and may contain very thin interbeds of intraclastic or bioclastic limestone.  The microbially laminated mudstones may or may not show desiccation features. Some stromatolites exhibit large vugs, which might have originated as anhydrite nodules (Figure 43A).  Chert nodules, lenses, and layers also might represent former anhydrite (Folk and Pittman, 1971) (Figure 44). These rocks were deposited in upper intertidal to supratidal environments (see Hardie and Shinn, 1986; Pratt and others, 1992).

 

Hatter Formation (Units 1 to 7) and Lowermost Snyder Formation (Unit 8)

Kay (1944a) named the Hatter Formation for exposures along Hatter Creek north of Roaring Spring in Blair County, Pennsylvania. He designated the exposure along the Conrail Tracks here at Union Furnace as the type section.  Most exposures of the Hatter Formation in central Pennsylvania are very poor. The outcrop here along PA 453 offers the best exposure of the formation in the region; almost all of the formation is exposed here. Faill and others (1989) report that the basal contact with the underlying Loysburg Formation is sharp and conformable, but it is gradational and conformable as marked at the PA 453 outcrop. Faill and others (1989) place the upper contact with the overlying Snyder Formation at the oolite beds at the base of the latter. 

We also discuss Unit 8, which is misplaced in the Snyder Formation here along PA 453, in this section of the guidebook. We suggest that Unit 8 as marked on the outcrop belongs to the Hatter Formation.

Figure 45 shows the details of our measured section through the Hatter Formation, and our interpretation of its sedimentary features.

Lithofacies and Depositional Environments – Berkheiser and Cullen-Lollis (1986) and Gardiner-Kuserk (1988) interpreted the Hatter Formation as tidal flat and lagoonal carbonates that developed on the updip region of a homoclinal ramp. We agree with this interpretation. The lower five meters of the Hatter Formation consist of peritidal, low-energy shallowing upward successions similar to those found in the subjacent Loysburg Formation. These consist of meter-scale or smaller cycles composed of 1) bioclastic/intraclastic packstone or grainstone (subtidal, transgressive lags), 2) burrowed and bioturbated wackestone and mudstone, with minor wavy lamination (subtidal), 3) lenticular, flaser, and wavy laminated mudstone (intertidal), and 4) microbially laminated mudstone (supratidal). The first bentonite (B0) appears within this lower peritidal section of the Hatter Formation.  The cycles of low-energy  shallowing upwards successions end with  a very  thin bed of black, organic-rich, argillaceous mudstone.

The next 20 m (66 ft) of section above the peritidal cycles, the bulk of the Hatter Formation, consists of dark gray, burrowed and   bioturbated,   skeletal   and    pelloidal wackestone and mudstone (Figure 46). These rocks have a number of characteristics typical of middle shelf lagoonal carbonate sediments described by Wilson and Jordan (1983). These characteristics are 1) a normal marine (stenohaline) biota (Figure 47), 2) muddy carbonate rock textures, 3) variable bedding thickness, and 4) extensive burrowing and bioturbation, with minor nodular, wavy, and flaser bedding.  Very thin (centimeter-scale) layers of intraclasts and shell lags occur randomly throughout this lithofacies; these are storm layers (Types A and B1 of Gardiner-Kuserk, 1988) superimposed on a remarkably thick, homogeneous succession of lagoonal carbonate rocks. These sediments were likely deposited in a broad, largely protected lagoon behind the barrier banks of the middle ramp, as shown in Figure 14. Instructive partial modern analogues include the flat embayment plain of Hamelin Basin in Shark Bay, Western Australia (Logan and others, 1970; Harris and Kowalik, 1994), muddy carbonates of the semi-restricted Florida Bay (Enos and Perkins, 1977), Yalahau Lagoon on the northern Yucatan Peninsula (Logan and others, 1969; Harris and Kowalik, 1994), and Khor al Bazm lagoon along the Abu Dhabi coastline in the southern Arabian Gulf (Purser, 1973) (See Figure 48).

The top 6 m (19.6 ft) of the Hatter Formation, and bottommost 1.5 m (5 ft) of the Snyder Formation (as marked on the outcrop) consist of peritidal, high-energy shallowing-upward successions (Figure 45). We recognize three of these successions; they comprise stacked peritidal parasequences separated by thin, dark, faunally- restricted argillaceous mudstones. These mudstones define transgressive flooding surfaces that separate individual parasequences. Lithofacies in these high-energy shallowing-upward successions include 1) wavy, flaser, and lenticular-bedded mudstone/wackestones/packstones (intertidal), 2) low-angle to horizontally bedded, and wave-rippled grainstones (upper intertidal to beach), and 3) coarse skeletal, intraclastic, cross-bedded grainstones (tidal channels) (Figure 49).

The first hardground to occur in the Trenton and Black River Groups at this outcrop is within one of these peritidal high-energy shallowing-upwards cycles near the top of the Hatter Formation, at the base of Unit 7 (Figure 22).

Unit 8 contains the uppermost high-energy peritidal cycle. The entire cycle is 1.5 m (5 ft) thick, and it differs from those beneath it by its apparently high organic content and more restricted biota (Figure 50). This represents more restricted depositional conditions.

Snyder Formation (Units 9 – 16)

Kay (1944a) named the Snyder Formation for outcrop exposures in southeastern Snyder Township, Blair County, Pennsylvania. These exposures are 2 km (1.2 mi.) south-southeast of Tyrone. The type section is along the Conrail tracks here at Union Furnace. Faill and others (1989) described the Snyder Formation as an assemblage of conglomeratic to fine grained limestones containing oolites and numerous fossil fragments. They recognized four types of specific limestone units that were mappable in the field:

1.      Medium to thick bedded conglomeratic calcarenites.
2.      Thin to thick bedded, parallel laminated calcisiltites and calcilutites, with irregular argillaceous partings.
3.      Fucoidal (burrowed) calcisiltites.
4.      Oolite beds.

Faill and others (1989) state that the basal contact of the Snyder Formation with the subjacent Hatter Formation is sharp and conformable, and they place it at the base of the oolitic sequence. This differs from what other workers have marked here at the Union Furnace road cut (Figure 51). Faill and others (1989) consider the top of the conglomeratic beds as the upper contact with the overlying Linden Hall Formation. The upper contact is conformable.

Lithofacies, Depositional Environments, and Diagenetic FeaturesFigure 51 shows our interpretation of the sedimentary successions in the Snyder Formation.  We interpret most of the Snyder Formation at Union Furnace (Units 9–16) as oolitic to skeletal sands and carbonate muds of the barrier bank complex depicted in Figure 14. These barrier bank lithofacies occur within subtidal parasequences that we interpret as stacked shoal complexes (Figure 51). Two types of meter-scale cycles - depositional and diagenetic cycles - occur in the Snyder Formation parasequences; both cycles are common in carbonate strata deposited on subtidal ramps (Jones and Desrochers, 1992). In the lower portion of the formation, the subtidal parasequences consist of oolitic and mixed grainstone-capped, high-energy shoal successions (Lehrmann and Goldhammer, 1999). In the upper portion of the Snyder Formation, the parasequences consist of hardground-bounded, amalgamated grainstone successions (Lehrmann and Goldhammer, 1999). Low-relief (= 1 m [3.3 ft]) bryozoan bioherms and biostromes are common throughout both types of successions.

Grainstone-capped, high-energy shoaling successions:  These successions dominate the Snyder Formation at the Union Furnace outcrop from near the base of Unit 9 to the B1 bentonite, near the base of Unit 11 (Figure 52). The parasequences consist of shallowing upward successions comprised of deep marine offshore carbonates (thin bedded, argillaceous, sparsely fossiliferous, burrowed mudstone), lower shoreface carbonates (burrowed, wave rippled, skeletal wackestones and skeletal/pelloidal packstones), and upper shoreface carbonates (cross bedded, oolitic grainstones or mixed grainstones).

The oolitic grainstones (Figure 52) are composed of fine- to medium-grained ooids (0.3 to 0.5 mm, mean = 0.2 mm) that exhibit a radial fabric. The concentric patterns of original growth lamellae are still visible in most of the ooids. Ooids make up 90 percent of the allochems. Many ooids are partially to completely micritized, and some are partially replaced by chert. Other allochems in this lithofacies include brachiopod (3 percent) and crinoid (1 percent) fragments, trilobite fragments (1 percent), peloids (1%), mollusk (gastropod and pelecypod) fragments (0.5 percent), and mudstone intraclasts (0.5 percent). All of these grains exhibit varying degrees of micritization, and some minor replacement by chert. Other minerals include authigenic quartz (1 percent) and feldspar (1 percent). The quartz has euhedral terminations, and replaces both allochems and cement. The feldspars include microcline and albite, and these minerals also replace allochems and cement. Cements in the oolitic grainstones include isopachous rims of radial-fibrous calcite, meniscus-type cement between some ooids, and pore-filling, non-ferroan equant calcite spar (Figure 52C and D). Compaction has deformed many ooids, and grain-to-grain pressure solution is common in these grainstones.

The mixed grainstones consist of intraclastic/oolitic/peloidal/skeletal grainstones (Figure 53). The intraclasts (up to 10 percent) occur as sporadic basal lags wherever storm beds are superimposed on the overall shoaling upwards successions. Intraclasts consist of mudstone, and oolitic, skeletal, or intraclastic wackestones. Other intraclasts are composed of unique aggregates of meniscus-cemented skeletal grains (small, impunctate brachiopods filled with pelloidal sediment). All of these intraclasts resemble grain aggregates described from the Bahamas and Trucial coast (Bathurst, 1975; Tucker and Wright, 1990). They may be true carbonate lumps, or grapestones, formed through submarine cementation and alteration of microbially bound sediment grains (Gebelein, 1974; Winland and Matthews, 1974, and Fabricius, 1977). Indeed, the presence of the meniscus-type cement in the intraclasts hints of nearby firmground and/or hardground development (Hillgartner and others, 2001).

Other allochems in the mixed grainstones include ooids, peloids, and various skeletal grains. Diagenetic fabrics are nearly identical to those in the oolitic grainstones, i.e., micritization of allochems, isopachous rims of radial-fibrous calcite, and non-ferroan equant calcite spar. Meniscus-type cements also lithify some ooids in mixed grainstones.

The oolitic and mixed grainstone-capped, high-energy shoal successions do occur higher in the Snyder Formation, but become subordinate to the hardground-bounded, amalgamated grainstone successions above Unit 10.

Hardground-bounded, amalgamated grainstone successions:  These unique and spectacular diagenetic successions dominate the upper half of the Snyder Formation (above Unit 10). Some previous workers mistakenly identified the numerous hardgrounds in the Snyder Formation as mud-cracked beds (for example, see Rones, 1969). The cycles consist of closely spaced hardgrounds (sometimes spaced millimeters to centimeters apart), with interlayers of skeletal and pelloidal grainstone, packstone, and/or wackstone (Figure 54). Some hardgrounds were initiated by “event deposition”, i.e. the introduction of a storm-generated tempestite, followed by a period of non-deposition and erosion. The storm generated character of these hardground beds is evident in their normally graded bedding: basal lags of skeletal and/or intraclastic grainstone are overlain by cross bedded or horizontally bedded pelloidal grainstone and skeletal packstone, which is overlain in turn by burrowed and bioturbated wackestone and mudstone. The period of non-deposition is evident in the appearance of burrowing and bioturbation activity, and in the initiation of submarine cementation. Other hardgrounds consist of wackestone and mudstone, and may have been deposited as lower energy sediments between or behind shoal areas.  The hardground-bounded, amalgamated grainstone successions in the Snyder Formation resemble subtidal diagenetic cycles in the Oligo-Miocene Abrakurrie Limestone of southern Australia described by James and Bone (1991). Hardground development occurred during times of relatively lower sea level when the ramp surface was above normal wave base and subject to active water pumping (Figure 20). This was a time of lithification and erosion.  The interlayers of carbonate sediments between hardgrounds reflect sediment accumulation below the depth of wave abrasion during periods of relatively high sea level (Figure 20).

Hardgrounds in the Snyder Formation run the gamut from smooth and rolling types to undercut, pebbly, and reworked types (Figure 54). Brett and Brookfield (1984) suggested that different hardground types tend to be indicative of different sedimentary environments. If Brett and Brookfield’s (1984) interpretations of Ordovician hardgrounds in equivalent strata in southern Ontario are correct, then we may be able to recognize the distal, middle, and proximal segments of carbonate cycles in these rocks, and refine our ability to recognize and interpret these subtidal parasequences.

Bryozoan buildups:  Bryozoan-dominated bioherms and biostromes occur throughout the Snyder Formation (Figure 51). The largest buildup here at Union Furnace (at the base of Unit 13) is up to a meter high and extends across the width of the highway cut. The bioherms are centimeter- to meter-scale trepostome and fistuliporid (order Cystoporata) bryozoan bindstones, bafflestones, cruststones, biocementstones, and globstones (see Cuffey, 1985 for his extension of Dunham’s carbonate rock classification to include bryozoan buildups). These bryozoan reef rocks are subtle. They consist of small, erect globular and branching colonies, encrusting sheets, and soft strands that cement carbonate sediment (Figure 55). Interframe sediments are skeletal and intraclastic wackestones and burrowed mudstones; much of this carbonate mud was most likely produced in situ by fragile, poorly calcified algae (Turmel and Swanson, 1976; Read, 1982). Baffling and cementation inhibited sediment movement and enhanced carbonate sedimentation. These reef rocks developed as open marine bioherms in a lower shoreface environment, possibly on lower shoal flanks, intershoal areas, and/or within the broad tidal exchange channels that crossed the banks. Bryozoan buildups also occur on hardgrounds.

Linden Hall Formation (Units 17 – 23)

Rones (1969) established the Linden Hall Formation to include the Stover and Oak Hall Limestones of Kay (1944a, 1944b).  Faill and others (1989) mapped the Linden Hall and Rodman Formation of Butts (1918) together, undivided.  They described the Linden Hall as interbedded, homogeneous, fossiliferous, and “wormy” limestones (Faill and others, 1989, p. 18).  The latter refers to the fucoidal, or burrowed, mudstones and wackestones that dominate the formation.  The lower contact of the Linden Hall with the Snyder Formation is conformable. Rones (1969) and Ryder and others (1992) placed an unconformity at the top of the Linden Hall at the approximate position of the B6 bentonite. This unconformity is difficult to see at Union Furnace, but we will argue that, although subtle, it is there.

Lithofacies, Depositional Environments, and Diagenetic Features – We interpret the Linden Hall Formation at Union Furnace mostly as packstones, wackestones, and mudstones deposited seaward of the barrier bank complex on a homoclinal ramp (Figure 14). There are subordinate amounts of skeletal and peloidal grainstones that we interpret as shoal flank or bank-fringe sediments, or storm beds.  The rocks of the Linden Hall were deposited in deeper water than the underlying Snyder Formation with bottom conditions above storm wave base, but below normal wave base.  Muddy carbonate textures, normal marine faunas, and extensive burrowing and bioturbation indicate deposition in a mid-ramp environment (Wilson and Jordan, 1983).  Hardgrounds are very common throughout the Linden Hall Formation. 

The Linden Hall Formation deepens upward and consists of meter scale diagenetic cycles of amalgamated packstone/wackestone-mudstone successions capped by hardgrounds (Figure 56).  The hardground surfaces form during periods of low sea level when active seawater pumping facilitates submarine cementation.  The successions of packstone/wackestone and mudstone are deposited during corresponding sea level highs (James and Bone, 1991). 

Hardgrounds: Hardgrounds are the most outstanding feature of the Linden Hall Formation at Union Furnace (Figure 57).  The hardgrounds are most abundant in the lower 5 m (16.4 ft) (Unit 17) and the upper 12.5 m (41 ft) (Units 22, 23, and 24) of the Linden Hall Formation.   Hardgrounds are also observed in the central portion of the section, but they are not as common.  The hardgrounds in the Linden Hall Formation are similar to those observed in the Snyder Formation, except that they occur in muddier carbonates.  Hardground recognition criteria included sharp planar contacts with overlying bed, bored surfaces, and associated intraformational conglomerates.  In thin section pyrite grains were concentrated along the hardground surface.

Amalgamated packstone/wackestone-mudstone successions:  Several relationships are observed between the packstones and wackestones/mudstones of the Linden Hall:  1) distinct storm/bank-fringe packstones and burrowed mudstones separated by dolomitized argillaceous laminae; 2) compactionally deformed packstone/grainstone lenses within a mudstone or most commonly; 3) burrow mottled packstone-wackestone beds with intergranular material ranging from micritic mud to sparry calcite.

The majority of the rocks of the Linden Hall Formation are mud-supported ranging from mudstones to floatstones to wackestones.  Skeletal wackestones dominate and minor mudstones and floatstones occur most commonly in the upper third of the Linden Hall Formation.

Skeletal wackestones range in thickness from 0.5 to 4.5 m (1.6 to 14.8 ft) with the thickest successions occurring in Units 21 and 22.  They are highly burrowed, contain wavy, argillaceous laminae and normal marine fossil assemblages dominated by brachiopods and bryozoans, but also including mollusks, trilobites, echinoderms, and corals.  Skeletal grains as large as 3 mm are observed, but these are rare and most of the grains range from 0.1 to 0.3 mm. Large gastropods are present in Unit 23. Peloids comprise less than 5% of the wackestone, but there are also isolated pockets containing greater than 50% peloids forming peloidal grainstones or packstones. 

Mudstones and floatstones are also burrowed and contain argillaceous laminae, but are more compositionally homogenous than the wackestones.  Floatstones are composed of primarily brachiopod and bryozoan fragments that are less than or equal to 1 cm.  Partial pyritization and chertification of these large skeletal grains are common.  Dolomitic nodules are the only substantial feature of the mudstones.  These nodules range in size from 3 mm to 1.2 cm.  The smaller nodules are spherical or elliptical in shape.  Larger nodules are very irregular and often associated with argillaceous laminae and fractures or stylolites. 

Dolomitization is common along most argillaceous laminae and appears to be associated with stylolites and fractures, and within burrows that have been filled with coarser material.  Stylolitization is more obvious in the muddy units and is often associated with the argillaceous layers. 

Packstones and grainstones, interpreted as storm beds or shoal flank or bank fringe sediments, occur as discontinuous lenses in the first 4 m (13 ft) of the Linden Hall Formation, within Unit 17, but occur as discrete, continuous beds up through the top of Unit 20. 

Significant packstone and grainstone units are absent between Units 21 and 22 and are present again in Unit 23 at the top of the Linden Hall. These are hummocky, planar and cross-stratified intraclastic, skeletal and peloidal packstones.  Burrows are also observed in these coarser units.

There is a thick peloidal grainstone/packstone near the base of Unit 23 composed of approximately 40% peloids and 15% skeletal grains.  The peloids range in size from 0.04 mm to  0.3  mm.  The  larger peloids  are less rounded  than  the  smaller ones and  may  actually  be muddy intraclasts.  The smallest peloids resemble micritic mud, filling intergranular and intragranular spaces in the packstone.  The skeletal grains range from 0.2 mm to 0.8 mm.  Brachiopods are the largest and most common grains, but bryozoans, mollusks, echinoderm, and trilobite fragments are also present.  The skeletal grains are partially micritized and replacement by sparry intragranular cement is common.  The intraclastic packstones and grainstones in the Linden Hall are not as spectacular as those observed in the Snyder Formation, but they also form when hardgrounds are ripped up following lithification. 

Dolomitization is not common in these coarser units and when present it does not penetrate the entire rock, but is confined to burrows and along fracture or stylolites. 

Figure 58 shows selected petrographic features of some of the Linden Hall carbonates. 

As mentioned earlier, Rones (1969) placed a regional unconformity at the top of the Linden Hall Formation. He stated that the formation is, “…progressively arched and truncated southeast and southwestward from Oak Hall” (Oak Hall is approximately 32 km (20 mi) northeast of Union Furnace). Rones (1969) adds that this truncation is accompanied by onlap thinning. This unconformity is not readily apparent at the outcrop, but we think the sudden appearance of a thin, cross-bedded grainstone and thin beds of remarkably fossiliferous grainstones and floatstones in Units 23 and 24 might represent a subtle transgressive lag that is part of a combined sequence boundary/transgressive surface as described by Holland and Patzkowsky (1996) for Ordovician carbonate sequences in the eastern United States.

Nealmont Formation (Units 24 To 27)

Kay (1944b) named the Nealmont Formation for 40 m (131 ft) of gray limestone at Union Furnace conformably underlying the Salona Formation, and unconformably overlying the Linden Hall Formation. The Nealmont Formation has two members, a lower Centre Hall Member and an Upper Rodman Member (Rones, 1969).

Lithofacies, Depositional Environments, and Diagenetic Features – The Nealmont Formation (Figure 59) consists of thick bedded, bioturbated wackestones and mudstones, with subordinate packstone lags and discontinuous skeletal floatstone and pelloidal/skeletal grainstone lenses. Wavy and undulatory, argillaceous and dolomitic laminae and bands grade up to nodular bedding in the upper part of the formation (Figure 60). The nodular bedding defines the Rodman Formation (Faill and others, 1989). The nodular beds are coarser grained and more fossiliferous than the underlying wavy bedded Centre Hall Member. The nodular bedding may be the result of discontinuous sea floor cementation, compaction, and/or pressure solution.

We interpret the Nealmont Formation as relatively deeper water (lower shoreface to outer platform) carbonate sediments deposited on the middle to outer portions of a distally steepened ramp, seaward of the inner ramp shoals (Figure 14B). The change from wavy bedded to nodular bedded limestone reflects the deepening water across the outer ramp. Although below fair weather wave base, the lower Nealmont wackestones and mudstones, along with subordinate lithologies, were affected by storm events. Lenses of skeletal floatstone and packstone, and minor grainstones, attest to brief periods of higher energy. The nodular-bedded limestone of the upper Nealmont Formation reflects deeper water on the outer ramp. Organic matter preservation was higher, as reflected in the darker mudstones and the organic rich micrite matrix (Figure 60C). We have not yet examined the organic petrography and geochemistry of these rocks, but work by Obermajer and others (1999) in Canada suggests that planktonic algal debris was a principal organic substrate for blooming microbes in these seas.

Some of the macrofossils are spectacular in the Nealmont Formation. Note the large macluritids in Unit 25 (Figure 61B). These big gastropods (or paragastropods – untorted, helically-coiled mollusks, if you accept the work of Linsley and Kier, 1984) have hyperstrophic shells with depressed spires and flat bottoms, and are thought to have been sedentary filter feeders in the Middle and Late Ordovician seas (Stearn and Carroll, 1989).

Marine cements lithify the Nealmont rocks (Figure 62). These cements include isopachous rims of radial-fibrous calcite, and pore-filling non-ferroan calcite spar. Some allochems are micritized. Many allochems show evidence of neomorphism and strain recrystalization. In some cases, micrite has recrystallized to microspar and pseudospar. Idiotopic dolomite occurs along stylolites and in more argillaceous limestones. 

Salona Formation (Units 28 To 40)

Field (1919) named the Salona Formation for exposures at Salona in the Nittany Valley of Clinton County, Pennsylvania. The lower contact with the Nealmont Formation, as marked at this outcrop by previous workers, is conformable and gradational. The upper contact with the Coburn Formation is sharp and conformable. Faill and others (1989) place it at the base of the first notably fossiliferous bed that is typical of the Coburn.

Two members are recognized in the Salona Formation by field mappers, the lower New Enterprise Member and the upper Roaring Spring Member. Faill and others (1989) state that the New Enterprise Member consists of interbedded dark gray to grayish black calcisiltite and calcareous shale. They report the beds as nearly nonfossiliferous. Faill and others (1989) distinguish the Roaring Spring Member from the New Enterprise by the appearance of calcarenites exhibiting ripples and crossbedding. Slupik (1999, p. 23.) provided a more tangible description of the transition between these two members. She states that in the upper New Enterprise Member, most beds are medium to very thickly bedded, weathered light gray, and composed of micritic lime mudstone with thin skeletal lags, and argillaceous mudstone interbeds. The Roaring Spring Member is marked by a significant decrease in bed thickness, a notable increase in argillaceous material, and a change to laminated, quartz-rich lime mudstones as the dominant lithology.

Lithofacies, Depositional Environments, and Diagenetic Features – The Salona Formation here at Union Furnace consists of about 60 m (197 ft) of mostly carbonate rock rhythmites (Figure 17). The rhythmites are comprised of repetitive beds of skeletal wackestone/mudstone and fissile, organic-rich mudstone (Figure 63). Both types of lithologies are variably argillaceous. Variable, but significant amounts of quartz silt appear in the upper parts of the formation.

The repetitive successions are 10 to 50 cm thick (Figure 64) and consist of a planar to scoured base overlain by a very sparse bioclastic lag. The fossils are primarily brachiopod, trilobite, crinoid, and ostracod fragments, but some coral, gastropod, pelecypod, bryozoan, and cephalopod fragments also occur in the rocks. Pyrite replaces some fossils.  Skeletal wackestone/mudstone displaying diffuse to distinctly horizontal to low-angle bedding, or hummocky bedding overlies this basal lag. Possible wave ripples may be present; these are very low amplitude, apparently symmetrical ripples with long, short crests (<0.16 cm heights) and long wavelengths (1.6 to 2.5 cm) with multiple periods. These ripples might be antidunes, however, or longitudinal combined current/wave ripples (Reineck and Singh, 1980). Some burrowing is evident in the outcrop exposures; acetate peels reveal extensive bioturbation. The trace fossils Chondrites and Phycodes are common. Each succession is capped with fissile, organic-rich mudstone, argillaceous mudstone, or highly calcareous shale. The beds contain some dolomitic layers.

We interpret the Salona Formation as relatively deep water, slope and basin margin sediments deposited on the far reaches of a distally steepened ramp. The Salona Formation has several features cited by Cook and Mullins (1983) as evidence for deeper water carbonate slope sediments:

  • Dark gray to black mudstones, silty mudstones, and wackestones
  • Insoluble residues composed of organic carbon, pyrite, quartz silt, and clay minerals
  • Planar, parallel, and somewhat continuous bed contacts
  • Preservation of thin bedding and laminations.

Some scoured bases, sparse fossil lags, sedimentary structures, and burrowing suggest tempestite deposition below fair weather wave base, and conceivably below storm wave base, in waters with normal marine circulaton.

Coburn Formation

Field (1919) named the Coburn Formation for exposures at Coburn, in Penns Valley, Centre County, Pennsylvania.  He noted that the lower and middle portions of the Coburn Formation consist of interbedded crystalline, highly fossiliferous limestones and black shaly limestones, and the upper Coburn becomes increasingly argillaceous as it grades into  the Antes Shale. In contrast to the Coburn Formation, the underlying Salona Formation lacks abundantly fossiliferous limestones. The Coburn Formation begins with the Prasopora zone (a hemispherical trepostome bryozoan), which is dramatically exposed here at the Route 454 outcrop (Figure 65A). This contact is sharp and conformable. The upper contact with the Antes Shale is gradational and conformable (Faill and others, 1989). 

Lithofacies, Depositional Environments, and Diagenetic Features – The Coburn Formation here at Union Furnace consists of about 55 m 180.5 ft) of mostly carbonate rock rhythmites (Figure 17). The rhythmites are comprised of repetitive beds of skeletal grainstone and packstone, skeletal floatstone and wackestone, and fissile, organic-rich mudstone or argillaceous shale (Figure 17). Repetitive successions are 10 to 30 cm thick. They typically begin with scour surfaces overlain by a skeletal lag which fines upwards into laminated or cross laminated limestone, then into quartz-rich, mudstone, and finally into argillaceous mudstone and/or calcareous shale (Figure 65B). The skeletal lags usually contain abundant brachiopod, crinoid, trilobite, and bryozoan fragments (Figure 65C and D). Sedimentary structures include horizontal planar and wavy laminations, low-angle cross bedding, hummocky cross stratification, and flame structures. Bioturbation and Chondrites trace fossils are common towards the tops of beds. Our summer college intern, Laura DiCello from the University of Pittsburgh at Johnstown, documented dramatic pebbly hardgrounds and convolute bedding/slump structures in cores of the Coburn Formation recovered  on the Wallace Farm just south of the outcrop (Figure 65E  and F).

The dramatic increase in fossil content of the Coburn Formation, and the increase of wave- and current-induced sedimentary structures indicate that the Coburn Formation reflects a shallowing upwards from the underlying Salona Formation. The hardgrounds attest to periods of lower sea level, marine cementation, and erosion on the sea floor. The slump structures suggest some higher slope and sediment instability on the more proximal region of the distally steepened ramp. Another interpretation is the slump structures represent ancient seismites (Pope and others, 1997; Ettensohn and others, 2002). Slupik (1999) interpreted the Coburn repetitive successions as more proximal (as compared to the distal Salona successions) storm tempestites deposited below fair weather wave base, but above storm wave base. She suggested that the Coburn represents a gradual shallowing from the deep carbonate ramp or slope environment of the the Salona to a middle ramp environment. We tentatively concur for now, but wish to map the regional facies distributions of the Salona and Coburn Formations before advancing a more comprehensive interpretation. A paradox of the Salona and Coburn Formations is the fact that total organic carbon increases upwards from the Salona through the Coburn, and organic matter preservation was significantly higher in the greater energy environment of the Coburn. Our next step is a more detailed investigation of the sedimentology, petrology, and organic geochemistry of the Salona and Coburn Formations here in central Pennsylvania and in the subsurface to the west.

Carbonate cements in the Coburn include isopachous rims of radial-fibrous calcite, and pore-filling non-ferroan calcite spar. Some allochems are micritized.

Reboard the bus and return to the Pittsburgh.