Northeast Natural History Conference 2011: Selected Papers 2012 Northeastern Naturalist 19(Special Issue 6):3–12 Notes on the Geology of the Shawangunk Ridge on the Mohonk Preserve and Environs Howard R. Feldman1,*, John A.Smoliga2, and Brian A. Feldman3 Abstract - The Shawangunk Formation, a quartz pebble conglomerate of Middle Silurian age, extends from the lower mid-Hudson Valley through New Jersey and into Pennsylvania. It overlies the Ordovician Martinsburg Formation, which is composed of shales and graywackes. The Martinsburg crops out on the Shawangunk Ridge and is quarried by Mohonk Mountain House in New Paltz, NY, in order to prevent erosion and provide good footing on the trails. The quarry, locally known as the “Shale Bank”, contains a diverse marine fauna of brachiopods, crinoids, bivalves, ostracods, corals, trilobites, and conulariids. In this community, the partition of feeding niches results in a reduced competitive trophic structure and therefore increased community stability. Within the Shawangunk Formation, there are rare “pods”, domelike structures that are filled with a gray matrix of rounded quartz grains supported by a clay matrix. The pods appeared to have formed along cleavage surfaces. A previously unrecognized metal sulfide deposit has been discovered in the conglomerate along Eagle Cliff. This deposit consists of the Fe-sulfide phases pyrite and marcasite, lesser amounts of the Cu-Fe sulfide chalcopyrite, and trace amounts of anglesite (Pb-sulfate). An outcrop of the Middle Devonian Onondaga Limestone in the Port Jervis Trough contains large crinoid columnals, the coral Amplexiphyllum, trilobite fragments, and the brachiopod Levenea subcarinata. The Onondaga in this area is part of a carbonate ramp that was a shallow carbonate shelf in the Helderberg-Coxsackie area, a thick accumulation of shelf-margin bryozoan bafflestone between Leeds and Saugerties, and an even thicker accumulation of sparse to packed biocalcisiltites deposited on a carbonate ramp dipping southward into the Port Jervis area. Introduction The Middle Silurian Shawangunk formation lies unconformably above Ordovician graywackes and shales of the Martinsburg Formation. Near Otisville and Port Jervis, the Shawangunk is overlain by the Bloomsburg Red Beds, which crop out to the west in Pennsylvania and New Jersey (Epstein 1993). The fluvial deposits of the Tuscarora and Shawangunk formations accumulate to the northwest of uplands lifted during the Taconic Orogeny. Thus, the source area lays to the southeast, and the marine basin lays to the west of the Shawangunk/Bloomsburg fluvial plains. The Shawangunk Formation gradually thins from Port Jervis to its disappearance just north of New Paltz, NY. In the early Paleozoic Era, carbonate banks lay along the east coast of proto-North America. Ordovician plate convergence involving proto-North America formed a deep basin into which a thick sequence of muds 1Division of Paleontology (Invertebrates), American Museum of Natural History, New York, NY 10024. 2Consulting Geologist, 23 Rocky Mountain Rd, Roxbury, CT 06783. 3PA Program, Touro College, 27-33 West 23rd Street, New York, NY 10010. *Corresponding author - feldspar4@optonline.net. 4 Northeastern Naturalist Vol. 19, Special Issue 6 and dirty sands accumulated. These deposits were subsequently consolidated to form the Martinsburg Formation. The Martinsburg was deformed during the Taconic mountain-building episode known as the Taconic Orogeny. The trend, or strike, of these rocks in southeastern New York is approximately 20 degrees toward the northeast. The intensity of deformation diminishes to the west. The Taconic Mountain system shed coarse sediment that was transported westward as fluvial conglomerates and sandstones of the Shawangunk Formation over beveled folds of the Martinsburg Formation. To the northwest, erosion of the source area was intense and the climate, based on the mineralogy of the rocks, was warm and at least semiarid (Epstein and Lyttle 1987). The source was composed predominantly of sedimentary and low-grade metamorphic rocks that hosted abundant quartz veins and local occurrences of gneiss and granite. Progressive erosion of the source regions caused the steep, braided streams of the Shawangunk to give way to the lower-gradient meandering streams from which the Bloomsburg Red Beds accumulated. The abundance of vein quartz could explain the abundant conglomerate of the Shawangunk Formation. A thin diamictite (a poorly sorted, noncalcareous, land-derived sedimentary rock that contains a wide range of particle sizes) with exotic pebbles records a brief geologic episode of colluvial deposition that occurred during the Taconic hiatus (Epstein 1989). As the mountains eroded, finer clastic sediments and even carbonates accumulated more or less continuously through the Middle Devonian Period. Clastic influx during the Middle Devonian records another, later mountain-building episode, the Acadian Orogeny. Regional Overview The highest point in the Shawangunk Mountains (698 m [2289 ft]) lies near Sam’s Point (Fig. 1). On a clear day, one can see (from southeast to north) the New York highlands underlain by Precambrian rocks thrust on top of Cambrian and Ordovician carbonates and shales of the Wallkill Valley. West of the highlands are Schunnemunk and Bellvale mountains underlain by conglomerates and sandstones of the Middle Devonian Schunnemunk Conglomerate in the Green Pond Outlier. The rocks of the outlier are in fault and sedimentary contact with the Precambrian. The Shawangunk and Kittatinny mountains, held up by the Shawangunk Formation, trend to the southwest, with Tristates Monument marking the highest elevation in New Jersey at High Point (550 m [1803 ft]). The Shawangunk at Sam’s Point dips very gently to northeast, near the broad crest of the Ellenville Arch. Tough cross-bedding is well exposed and is indicative of current trends ranging between 80SW and 70NW. Glacial striae with chatter marks on the bedding surfaces indicate that the Wisconsinan glacier flowed over the mountains moving 16SW. The lower 24 m (80 ft) of the Shawangunk here consists of medium- to thick-bedded conglomerate with quartz pebbles as much as 5 cm (2 in) long. Channel cuts are common. Nowhere do we see any pebbles from the underlying Martinsburg, a peculiarity that exists throughout New York, New Jersey, and eastern Pennsylvania, and one that eludes a good sedimentologic explanation. The Shawangunk is separated into blocks tens of feet wide that have moved apart along the soft shales of the underlying Martinsburg, probably forced apart by gelifraction, wedging of boulders that fall into the cracks, and block 2012 H.R. Feldman, J.B. Epstein, J. Smoliga, and B.A. Feldman 5 sliding. At the Ice Caves, 0.8 km (0.5 mi) to the east, the joints parallel the cliff face of the mountain (Feldman and Thompson 2008), and cold air trapped in the maze of blocks and snow may persist throughout the summer, hence their name. In Devonian times, there was a shallow carbonate shelf in the Helderberg- Coxsackie area that graded into a shelf-margin bryozoan bafflestone near Leeds and Saugerties. Further southwest toward Port Jervis, the sediments consisted of sparse to packed biocalcisiltites deposited on a carbonate ramp. This ramp occupied the northern margin of a structural basin depocenter, located to the east of and not directly related to the topographic basin of central New York, which was centered in the Tri-states vicinity from the Late Silurian until the early Middle Devonian (Lindemann and Feldman 1987). The “Shale Bank” The Martinsburg Formation (Figs. 2, 3) crops out about 1.6 km (1 mi) from the entrance to Mohonk Mountain House (on Mountain Rest Road). Mohonk workers quarry and crush the shale in order to provide good footing and slow down erosion on the carriage roads. The outcrop consists of 14 m (45 ft) of predominantly dark Figure 1. Locality map of the mid-Hudson Valley. A) Sam’s Point; B) Onondaga Limestone in Wawarsing, NY; C) Mysterious “pods” in Minnewaska State Park; D) The “Shale Bank”; E) Eagle Cliff Road just to the west of Lake Mohonk. 6 Northeastern Naturalist Vol. 19, Special Issue 6 gray shales and siltstones interbedded with fine grained graywacke beds, occasional prominent pyrite layers, and disseminated sphalerite, chalcopyrite, and galena. Oscillation ripples can be observed on some bedding surfaces. Carbonaceous material occurs mostly as fine-grained patches throughout the matrix. The studied exposure illustrates a high degree of strain, mostly manifested by shiny quartz slickensided surfaces, small-scale cross-laminated and parallel-laminated strata, and ripple marks. Crinoid stems, some slightly disarticulated, and free columnals occur on different bedding surfaces, indicating a possible change in current Figure 2. Columnar stratigraphic section of the Martinsburg and Shawangunk formations in the “Shale Bank” at Mohonk. The Martinsburg Formation, estimated to be between 3048–3658 m [10,000–12,000 ft] thick in the mid-Hudson Valley, is highly deformed. Based on field relations, the “Shale Bank” is considered to be near the top of the Martinsburg, but due to repetition of beds in the area, it is difficult to precisely determine exactly how close to the top of the formation it lies. 2012 H.R. Feldman, J.B. Epstein, J. Smoliga, and B.A. Feldman 7 regime. Scattered linear to sinusoidal horizontal burrow structures ranging in diameter from 0.5–3 cm are found on silty beds; some of the burrows are infilled with course quartz grains. Gutter casts are scattered throughout the section. The faunal constituents of this Sowerbyella-Onniella Community (Fig. 4) include brachiopods (93%), crinoids (Ectenocrinus; 3%), bivalves (Deceptrix?) (3%), ostracodes (less than 1%), corals (less than 1%), trilobites (Cryptolithus, Isotelus; less than 1%), conulariids (less than 1%) and unidentified burrowers (less than 1%). Brachiopods are represented by a low-diversity assemblage of Sowerbyella and Onniella with occasional Dalmanella. The fauna can be classified into distinct trophic groups: (1) high-level suspensions feeders (crinoids, corals); (2) low-level suspension feeders (brachiopods, bivalves); (3) animals that collect food from the sediment surface (ostracodes, trilobites); and (4) animals that feed within the sediment (burrowers). This partition of feeding niches leads to a reduced competitive trophic structure and therefore increased community stability. The community appears twice in the section, separated by about 4.9 m (16 ft) and thus appears to be stable. The depositional environment of this fauna may have been in relatively deep water as evidenced by gutter casts, at the bottom of which are shell accumulations, as well as a lack of bioturbation in the sediments. However, the moderately diverse fauna and oscillation ripples indicate a shallower environment. In addition, there are auriculate nuculoid bivalves that were shallow infaunal and deposit-feeding organisms. The sediments were probably not deposited at depths of greater than 2000 m, as suggested by McBride (1962) for the Martinsburg Formation at the Great Valley. Lehman and Pope (1990) Figure 3. The steeply dipping Ordovician Martinsburg Formation in the “Shale Bank” on the Shawangunk Ridge. 8 Northeastern Naturalist Vol. 19, Special Issue 6 noted that Bretsky (1970) divided the Reedsville and Martinsburg formations of the central Appalachians into three paleocommunities, one of which was the Sowerbyella-Onniella Community. This community, according to Bretsky (1970), corresponds to a deep-water depositional environment. The fauna at the “Shale Figure 4. Faunal constituents of the Sowerbyella-Onniella Community in the “Shale Bank.” A) articulated crinoid stem (Ectenocrinus?) indicating little if any current activity; B) slightly disarticulated Ectenocrinus? stem (scale bar = 1 cm); C) an auriculate nuculoid bivalve (Deceptrix?) typical of a shallow water environment (scale bar = 1cm); D) unidentifi ed ostracode adjacent to crinoid stem (scale bar = 1cm); E) glyptocrinid partially buried under shale; F) the brachiopod Sowerbyella sp., ventral valve interior (scale bar = 1 cm); G) Conularia sp., a conulariid common in the Ordovician of the United States; H) interior mold of ventral valve of the brachiopod Dalmanella sp. (scale bar = 1cm). 2012 H.R. Feldman, J.B. Epstein, J. Smoliga, and B.A. Feldman 9 Bank” very closely resembles the Sowerbyella-Onniella communities of Lehman and Pope at Swatara Gap and Bretsky’s community of the central Appalachians, although the Sowerbyella-Onniella Community along the Shawangunk Ridge is much less diverse than the other two and was most likely deposited in shallower water. Bretsky never defined the absolute depth of his communities (Bretsky 1970, Lehman and Pope 1990). “Pods” of Unknown Origin Within the Shawangunk Formation, we note occurrences of what we term “pods” that are domal in cross section with a flat base, the bottoms of which often contain quartz pebbles. The matrix is dark gray and consists of interstitial quartz ranging in size from fine (0.25–0.125 mm) to coarse (1.00–0.50 mm) sand and muscovite mica (Fig. 5). The pods are aligned along bedding and, in many cases, crossbedded foresets, and range in size from 0.2 to 7 cm in diameter (at the base) and 0.2 to 6 cm in height. The rock that hosts the pods is composed of pressure-solution-welded quartz grains consistent with the quartzitic nature of the formation. There is no evidence of stratification. The origin of the pods is under investigation (Feldman et al. 2009), and possibilities include: microbial mounds, sponges, or mud balls. Pods are similar in outline to some thrombolites in that the internal texture is non-laminated; however, there is no indication of microbial activity such as clotting. Though the pods may represent algal mats, their association with braided fluvial deposits renders this interpretation doubtful. It is Figure 5. “Pods” aligned along bedding near the top of the Middle Silurian Shawangunk Formation near Minnewaska State Park. 10 Northeastern Naturalist Vol. 19, Special Issue 6 possible that they are the remains of sponges, since the braided streams drained into a marine basin and the resulting tidal incursions may have supported sponges in an estuarine setting. Most sponges live in a marine environment. Support for this scenario is the occurrence of the trace fossil (ichnofossil) Arthrophycus recently discovered (by H.R. Feldman) on the ridge in the Shawangunk Formation. Rindsberg and Martin (2003) proposed that a rhandophorid trilobite could have been the organism responsible for Arthrophycus burrows in Alabama, and they proposed that Cryptolithus may have been the tracemaker of Arthrophycus. The domed shape of the pods tends to preclude a mud-ball origin. At first glance, the long axes of the pods and gray matrix seem to be aligned along bedding planes, but upon closer inspection, they actually follow cleavage planes and may have a pressure-solution origin. Clearly, further study is necessary to arrive at a definitive conclusion as to the mode of formation of the pods. Sulfide Mineralization Lead-zinc sulfide deposits formed near Ellenville, NY, are well known and were mined from the latter half of the 19th century into the early years of the 20th century (Gray 1961, Ingham 1940). However, a previously unrecognized metal sulfide deposit has been discovered (by J. Smoliga) in the Shawangunk Formation along Eagle Cliff Carriage Way (Fig. 6). This small deposit consists of the Fe-sulfide phases pyrite and marcasite, lesser amounts of the Cu-Fe sulfide chalcopyrite, and trace amounts of anglesite (Pb-sulfate). A larger area of Fe-oxide, hydroxides Figure 6. Sulphide mineral vein, opaque in thin section, along Eagle Cliff Carriage Way in the Shawangunk Formation on the Ridge. 2012 H.R. Feldman, J.B. Epstein, J. Smoliga, and B.A. Feldman 11 and Fe-sulfate minerals, hematite, goethite, and jarosite, which form as weathering products of Fe-sulfides, has also been observed on Eagle Cliff. This finding would suggest that the sulfide deposit may be more extensive than what is seen in surface outcrop. Moreover, jarosite deposited along fracture surfaces has been observed on Sky Top ridge east of Eagle Cliff, suggesting sulfide occurrences in this area. The metal sulfides appear to be associated with faulting and may have been remobilized from sulfides occurring in the underlying Martinsburg Formation. The relationship of the sulfide deposits found at Mohonk to the Ellenville deposits is unknown at this time; further study is underway. Onondaga Limestone An interesting outcrop of the Onondaga Limestone can be found in back of Wendel’s Asphalt on the west side of Route 209 about 0.8 km (0.5 mi) north of Vernoy Kill. The limestone crops out in an abandoned quarry within the Port Jervis Trough and is 15.5 m (51 ft) thick at this locality. The Onondaga passes downward into the underlying Schoharie Formation via a limy grit. Above the transitional zone, which is 0.6 m (2 ft) thick, there is another 7.6 m (25 ft) of the Edgecliff Member consisting of medium-grained to medium-bedded limestone transitional with the lower Moorehouse Member of the Onondaga Formation. The contact between the top of the Edgecliff and the base of the Moorehouse is defined by the presence of large crinoid columnals and the coral Amplexiphllum. This unit of the Moorehouse Member is overlain by a medium-bedded, light-gray limestone unit 2.7 m (9 ft) thick. The topmost unit of the Moorehouse consists of 5.2 m (17 ft) of very fine-grained, massive, medium-gray limestone with discontinuous seams of chert, trilobite fragments, and the brachiopod Levenea subcarinata. At the Eastern New York Correctional Facility, there is an excellent exposure of the Martinsburg-Shawangunk contact, about 6.1 m (20 ft) long. The angular discordance between the two formations is 4°. The Shawangunk dips 22° to the NW in the northwest limb of the Ellenville arch. Mullions are prominent on the basal Shawangunk surface, and a shear fabric can be observed in both the Martinsburg and Shawangunk formations. Summary The geology of the Shawangunk Ridge and environs is varied and often complex. Here we present an overview of a geologically important segment of the Appalachian Orogeny, specifically dealing with aspects of the geology on the Mohonk Preserve in the area around New Paltz, NY. The Martinsburg Formation, which crops out on the Shawangunk Ridge, has yielded a diverse marine fauna of brachiopods, crinoids, bivalves, ostracodes, corals, trilobites, and conulariids. This moderate diversity suggests that feeding niches were partitioned, resulting in a reduced competitive trophic structure and therefore increased community stability. We described a new metal sulfide deposit in the Shawangunk Formation along Eagle Cliff Road on the west side of Lake Mohonk. This deposit consists of the Fe-sulfide phases pyrite and marcasite, lesser amounts of the Cu-Fe sulfide chalcopyrite, and trace amounts of anglesite (Pb-sulfate). The Middle Devonian Onondaga Limestone that crops out on the west side of the Shawangunk Ridge 12 Northeastern Naturalist Vol. 19, Special Issue 6 within the Port Jervis Trough contains large crinoid columnals, the coral Amplexiphyllum, trilobite fragments, and the brachiopod Levenea subcarinata. The Onondaga in this area, near Wawarsing, NY, is part of a carbonate ramp that thickens as it dips toward the south and the Port Jervis region. Acknowledgments We thank John Thompson of the Mohonk Preserve for inviting us to present an oral version of this paper at the Northeast Natural History conference held in Albany, NY in the spring of 2011. The authors gratefully acknowledge John H. Puffer, Rutgers University, and two anonymous reviewers for critically reviewing the manuscript and providing suggestions for improvement. Many thanks to Mena Schemm-Gregory, University of Coimbra, for drafting Figures 1 and 2. Literature Cited Bretsky, P.W. 1970. Ordovician ecology of the central Appalachians. Peabody Museum of Natural History, Yale University, Bulletin 34:1–150. Epstein, J.B. 1989. Regional stratigraphy of Silurian rocks and an enigmatic Ordovician diamictite, southeastern New York. Pp. 3–5, In A.P. Schultz (Ed.). Appalachian Basin Symposium: Program and Extended Abstracts. US Geological Survey Circular Report C 1028. Epstein, J.B. 1993. Stratigraphy of Silurian rocks in Shawangunk Mountain, southeastern New York, including a historical review of nomenclature. US Geological Survey Bulletin 1839:L1–L40. Epstein, J.B., and P.T. Lyttle. 1987. Structure and stratigraphy above, below, and within the Taconic unconformity, southeastern New York. Pp. C1–C78, In R.H. Waines (Ed.). New York State Geological Association, 59th Annual Meeting, Kingston, New York, November 6–8, 1987. 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