12: THE SHAWANGUNK AND MARTINSBURG FORMATIONS REVISITED: SEDIMENTOLOGY, STRATIGRAPHY, MINERALOGY, GEOCHEMISTRY, STRUCTURE AND PALEONTOLOGY
Howard R. Feldman
Division of Paleontology (Invertebrates)
Museum of Natural History
Street at Central Park West
New York, NY 10024
Jack B. Epstein
Geologist Emeritus, MS926A
US Geological Survey
12201 Sunrise Valley Drive
Reston, VA 20192
John A. Smoliga
Senior Principal Scientist
Solid-State Characterization
Analytical Sciences,
Boehringer Ingelheim Pharmaceuticals, Inc.
INTRODUCTION
In southeastern New York Middle Silurian Shawangunk Formation (Figure 1), containing gray conglomerate, sandstone and shale, lies unconformably above the Ordovician Martinsburg Formation, consisting of shales and graywackes. In southwestern New York, near the Port Jervis area, The Shawangunk Formation is overlain by the Bloomsburg Red Beds, the same stratigraphic sequence that occurs in Pennsylvania and New Jersey to the southwest. The Shawangunk Formation thins gradually from Port Jervis to its pinchout near Hidden Valley and Binnewater, New York. Two tongues of the upper part of the Shawangunk are: the Ellenville Tongue that extends from the Ellenville-Accord area to its feather edge just southwest of the New York-New Jersey border and, the High View Tongue that is restricted to the Wurtsboro area (Epstein and Lyttle, 1987; Epstein, 1993).
Early in the Paleozoic carbonate banks lay along the east coast of the ancient North American continent. During the Ordovician plate convergence commenced in the closing of the Iapetus Sea and a deep basin developed into which thick muds and dirty sands were deposited. These were later lithified into the Martinsburg Formation. Eventually, with continued compression, these sediments were folded and faulted during the complex deformation of the Taconic Orogeny. The trend of these folds in southeastern New York is approximately N20E. As one proceeds westward across the Wallkill Valley these structures become less intense. Subsequent to the Taconic Orogeny mountains rose to the east and coarse sediments were transported westward and deposited as the conglomerates and sandstones of the Shawangunk Formation across the beveled folds of the Martinsburg. Deposition occurred on a plain of alluviation and in a marine basin 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. The source was composed predominately of sedimentary and low-grade metamorphic rocks with exceptionally abundant quartz veins and small local areas of gneiss and granite. As the source highlands were eroded, the steep braided streams of the Shawangunk gave way to more gentle-gradient streams of the Bloomsburg Red Beds.

12: THE SHAWANGUNK AND MARTINSBURG FORMATIONS REVISITED: SEDIMENTOLOGY, STRATIGRAPHY, MINERALOGY, GEOCHEMISTRY, STRUCTURE AND PALEONTOLOGY

Leave parking lot at SUNY New Paltz.
This locality is on blacktop of Holocene age, conformably overlying fill of unknown age brought in by contractors.

Cross bridge over Wallkill River.

Continue straight up steep hill.
Several sharp curves will signal approach to the top of hill. Note outcrops of Martinsburg Formation on right, consisting of thin-bedded shale, minor siltstone, and very rusty weathering with fine-grained graywacke. Cleavage is poorly developed.

Stop at gatehouse and check in.

STOP 1. We Will Be Spending About 2 Hours Here Examining Various Outcrops on the Mohonk Mountain House Grounds.
Please, no hammers, only cameras! The Shawangunk (pronounced Shongum) Formation (Figure 1) was deposited during the Silurian Period, approximately 438-418 million years ago. At that time a shallow sea covered the southeastern part of New York State into which drained rivers and streams. The bottoms of these streams were layered with pieces of abraded quartz. Constant motion of the water eroded the chunks of quartz into sub-rounded 'pebbles' that were eventually "glued" together by silica-rich cement carried by percolating ground water. The name of the sedimentary rock formed by this process is conglomerate (Figure 2). The exact source of the quartz that makes up the conglomerate is not clear. It may have formed by braided streams or alluvial fan deposition. A braided stream divides into an interlacing network of several small branching and reuniting shallow channels. An alluvial fan is a low outspread flat to gently sloping mass of rock material shaped like an open fan, deposited by a stream at the place where it issues from a narrow mountain valley upon a plain (Jackson, 1997). The Shawangunk Mountains (Figure 3) extend to the southwest into New Jersey where they are called the Kittatinny Mountains. The Shawangunk Formation ranges in thickness from 733 m (2,200 ft) near Ellenville, to the southwest, to <1 m (about 1 ft) near Hidden Valley, to the northeast, finally 'pinching out' and disappearing. Stratigraphically below the Shawangunk lies the Middle to Late Ordovician age (489-439 million years old) Martinsburg Formation, consisting mostly of shale, up to a thickness of almost 3333 m (10,000 ft).
Glacial Geology
Note the beautiful glacially paved surface with striations and chattermarks. Chattermarks are small, closely spaced curved scars or cracks made by chipping a brittle bedrock surface, in this case the conglomerate, by rock fragments carried in the base of a glacier. Each chattermark is roughly transverse to the direction of ice movement and its 'horns' point in the direction the glacier moved. Compare the direction of the striations here (about 10 degrees to the northeast) with the direction of the striations at the top of the crevice (about 20 degrees to the northwest). As the glacier moved down through the valley a small lobe moved around the ridge changing direction slightly resulting in a change in strike.
Mineralization within the Shawangunk Formation
The sulfide mineralization observed within the Shawangunk Formation along Eagle cliff consists of predominately marcasite and pyrite with small amounts of chalcopyrite (Figure 4). The sulfides are disseminated within the conglomerate. Emplacement appears to be associated with fault structures. SEM/EDS analysis of the sulfides also shows trace amounts of lead and zinc. Gossan in the form of goethite, hematite and jarosite has formed in places as a result of sulfide weathering. In some areas the water soluble sulfate minerals melanterite Fe(SO4)·7H2O) and rozenite Fe(SO4)·4H2O) are observed as a white to yellowish encrustation on the surface of sulfide containing exposures, when weather conditions permit (Figure 5). One from Eagle Cliff (valley side) sample contained two hydrated Fe-sulfate phases: melanterite Fe(SO4)·7H2O) and rozenite Fe(SO4)·4H2O). Rozenite is a lower hydrate containing 4 moles of water, as compared to melanterite which contains 7 moles of water. Rozenite can be formed in one of two ways: 1. Dehydration of melanterite or, 2. during lower relative humidity conditions; at room temperature rozenite is the stable phase below 70-80% RH. Examination by polarized light microscopy of the sample shows euhedral (well formed) crystals of rozenite encased in melanterite. This relationship indicates that the rozenite crystallized first, followed by the melanterite. Additionally, no alteration/dehydration textures were observed in the melanterite. From this we conclude that the rozenite crystallized during lower RH conditions, followed by melanterite which crystallized during increased RH conditions (i.e., above 70-80% RH depending on ambient temperature).
The Sky Lakes
Looking toward the southwest observe the ridge forming the "backbone" of the Shawangunks. Continuing along the ridge, located at its highest point about 30km (10 mi) away, is Sam's Point and the Ice Caves, so called because of the cold air trapped at the bottom of open vertical joints. Within the deep recesses of the caves one can find ice that lasts all summer long. Along the top of the Shawangunk Ridge are located four "Sky Lakes" in addition to Mohonk Lake (Figure 6), that are so-called because they receive their water supply solely from rainwater. These lakes are Minnewaska (closest to Mohonk), Awosting, Mud Pond and Maratanza. The lakes most likely overlie faults that weakened the bedrock, which was then scooped up by the passing glaciers. Interestingly, Mohonk Lake (Figure 6) is the only one of the five sky lakes that has greenish water; the others have clear blue water. The reason for this is that the acidic pH of the water of the four sky lakes (4.0) precludes algae and fish from living in them. However, Mohonk Lake's water is buffered by the alkaline rocks of the underlying Martinsburg Formation that floors the southeastern portion of the lake and has a pH of 7-7.5. This raises the pH level such that the conditions are more conducive to life. Between the Shawangunks and the Catskill Front is a broad valley known as the Port Jervis trough. The trough was formed by erosion and glacial scouring of the relatively weak sediments beneath the Catskill Formation. The floor of the valley is fairly flat due to the deposition of glacial debris (till) and lake sediments. Looking toward the southeast one can see the Hudson Valley (the Hudson River is about 18km [6 mi] to the east). Continue to the Huntington Lookout summer cottage. The view here is spectacular. Toward the southwest we see the Shawangunk Ridge and the Trapps (famous among technical rock climbers). To the northwest one can see the Catskill Mountains, also known as the Catskill Front, rising in the distance to a height of 700 m (2,100 ft) above the Hudson River. The highest peak in the Catskill Mountains is Slide Mountain which rises 1,282 m (3,846 ft) above sea level. These mountains began to rise in New England and the Canadian Maritime Provinces during the Devonian Period (due to a collision of tectonic plates known as the Acadian Orogeny). Throughout the Middle and Late Devonian Period the mountains were eroded by streams and rivers that flowed toward the west, carrying vast quantities of mud and sand that were deposited on the floor of a great inland sea as the rivers lost velocity. Closer to the ancient shoreline the heavier particles settled out first, leaving mostly sands and some shale, whereas farther from the shoreline the deposits consisted of mainly shales with some sand and, farthest from the shoreline we find only shale. Thus, as the rivers and streams lost velocity, the heaviest particles (larger sand-size grains) settled out first and the smallest, finer particles settled out last forming the fine-grained shales.

Contact between the Shawangunk and Martinsburg
On right observe the (covered) contact between the Shawangunk and Martinsburg at Woodland Bridge. Note the relatively shallow dip of the shales typical of the proximity to the contact with the Shawangunk.

STOP 2. Shale Pit.
In this shale pit (Figure 7) the Martinsburg contains fault-related structures interpreted to be of two different ages. A mélange, or broken formation, is part of a mélange zone that is Taconic in age that has been carried westward over the Silurian Shawangunk in the hanging wall of the younger Kleine Kill thrust. Most of the small faults and slickensided surfaces that are common in this locality are probably related to this younger thrust fault. The trace of the Kleine Kill thrust nearby has a strike of N15E and is interpreted to be Alleghanian in age. The fault zones in the Martinsburg which clearly do not cut Silurian and younger rocks generally have: (1) a trend more northerly than the northeast-trending folds and faults in the Silurian rock, (2) a diagnostic scaly cleavage, (3) tightly folded "floating" knockers of graywacke whose axes plunge predominantly to the northeast with variable azimuth, and (4) very little, if any, vein quartz. The faults that definitely cut both the Ordovician Martinsburg and the Silurian Shawangunk, and which we interpret to be Alleghanian in age, generally have: (1) a slightly more easterly strike, (2) well-developed "pencils" in the shale formed by the intersection of bedding and cleavage, and (3) vein quartz parallel to bedding and/or cleavage that commonly contains shale fragments.
The exposure here consists of predominantly dark gray shales and siltstones interbedded with fine grained graywacke beds, occasional prominent pyrite layers and disseminated sphalerite, chalcopyrite and galena. Oscillation ripples (Figure 8) occur on some bedding surfaces. Carbonaceous material occurs mostly as fine-grained patches throughout the matrix. The studied section is tectonically stressed with shiny quartz slickensided surfaces, parallel cross-laminated strata and ripple marks. Crinoid stems, some disarticulated, and free columnals occur on different bedding surfaces, indicating a possible change in current regime. Scattered linear to sinusoidal horizontal burrow structures ranging in diameter from .5-3 cm are found on the silty beds. Some of the burrows are infilled with coarse quartz grains. The faunal constituents include brachiopods (93%), crinoids (Ectenocrinus; 3%), bivalves (3%), ostracodes (<1%), corals (<1%), trilobites (Cryptolithus, <1%) (Figure ), conulariids (<1%) and unidentified burrowers (<1%). The brachiopods are represented by a low diversity assemblage of dalmanellids and what may be a new species of Sowerbyella. 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.
Mineralization within the Martinsburg Formation
Within the Martinsburg Formation at the Mohonk shale pit sulfide mineralization has also been observed. The sulfides include pyrite, chalcopyrite, galena and sphalerite. The sulfides are found associated with calcite veins and as disseminations throughout the shale. Carbonaceous material has also been observed in thin section (Figure 9).

Observe cliffs of the extremely resistant Shawangunk Fm
As we progress in a southerly direction on Butterville Road observe cliffs of the extremely resistant Shawangunk Formation on the right. The lowland and hills in the foreground are underlain by the shales and graywackes of the Martinsburg Formation, striking toward the northeast. Note Sky Top (tower) from which one can see six states (New York, New Jersey, Pennsylvania, Connecticut, Vermont, and Massachusetts). Sky Top sits atop a rock scramble and steep climb through the "Crevice" and ending with an egress called the "Lemon Squeeze." This is the signature hike of Mohonk Mountain House that is adjacent to the 6,400 acre Mohonk Preserve.

Overturned Martinsburg Formation on right
On the right note the overturned shales and thin graywackes of the Martinsburg Formation. Here there are several narrow fault zones with vein quartz.

Ahead view the "Trapps"
On the right note the steeply dipping and overturned Martinsburg shales and thin graywackes.
Ahead view the "Trapps" a world famous site for rock technical climbers located on the Mohonk Preserve. The Mohonk Preserve, founded in 1963, was established to protect the northern Shawangunk Ridge. Its mission is to protect the ecology of the area and to provide for public environmental education and recreation.

Steeply dipping and overturned Martinsburg on right.
Steeply dipping and overturned Martinsburg shales and thin graywackes on right. View of the Shawangunk cliffs at the Trapps straight ahead.

Note Martinsburg with graywackes on right.
The bedding appears to be right-side-up however, the poorly developed cleavage in the shales dips steeply to the west suggesting that both bedding and cleavage have been rotated by faulting.

We will be climbing to the top of the Shawangunk Mountains for the next mile or so.

Visitor's Center, Mohonk Preserve on right.
Shortly we will be making a sharp left turn with the Martinsburg (on the right) and shale interbedded with thin-bedded (up to 5 in), cross-bedded to planar laminated siltstone, and minor fine-grained sandstone. Soft-sediment slump folds are common in the siltstones. Cleavage is absent or very poorly developed, except near tight folds and narrow fault zones. Most of these faults and folds do not affect the overlying Shawangunk Formation and must be Taconic in age. They trend from N5E to N20E, whereas structures in the overlying Shawangunk trend in a more easterly direction.

On the left is a scenic overview of the Wallkill Valley.

The contact between the Shawangunk and Martinsburg
The contact between the Shawangunk and Martinsburg formations is exposed at two spots approximately 400 feet southwest of the road at the base of the cliffs. Fairly regular and linear mullions can be observed in the basal Shawangunk at the contact.

Conglomerates and quartzites of the Shawangunk
As we pass under the Trapps Bridge note the conglomerates and quartzites of the Shawangunk dipping to the northwest (31NW).

Cross Coxing Kill.
As we cross Coxing Kill note that the Coxing Kill Valley coincides with the trough of a broad, open syncline that plunges gently to the northeast. This is also a favorite site for skinny dipping. For the next 0.5 mile we will begin to cross a broad open anticline that exposes a window of Martinsburg that is about 2 miles long. To the right note an outcrop of basal Shawangunk with unconformably underlying shales of the Martinsburg exposed about 40 feet away. Dips in both units are very gentle and the angle of unconformity is as little as 2⁰. The divergence in strike, however, is as much as 38⁰, with the strike in the Martinsburg being more northerly. The Martinsburg here is in the broad open-fold Taconic tectonic zone (zone 3 of Epstein and Lyttle, 1987).

Peters Kill parking area on right.

Entrance to Minnewaska State Park on the left.

Trailhead leading to Lake Awosting on left.

Cross Sanders Kill.

Scenic overview of the Rondout Valley
Turn into small parking area with scenic overview of the Rondout Valley underlain by Upper Silurian through Middle Devonian rocks that are buried by a variety of glacial sediments. In the distance note the Middle and Upper Devonian clastics forming the Catskill Mountains. There is a thrust fault with nice slickensides across the road from the parking area. BE CAREFUL OF ONCOMING CARS!

STOP 3. Mysterious 'Pods'.
Walk back along NY 44/55 (north side of road) and observe the mysterious 'pods' of Feldman et al., (2009) that follow the beds (N20E; 38NW) in the upper part of the Shawangunk Formation. The pods (Figures 10, 11) are often conical with flat bottoms and associated with quartz pebbles. Note that the quartzite is recrystallized. The matrix filling the pods is often dark gray and seems to follow the bedding and/or crossbedding. WHAT ARE THEY? The pods range from 0.2 to 7 cm across (at the base) and 0.2 to 6 cm in height. Internally, the 'pods' are mostly non-recrystallized quartz grains with interstitial mica and possibly clays (Figure 12). In contrast, the areas outside the 'pods' consist of pressure solution welded quartz grains consistent with the quartzitic nature of the formation. The pods are restricted to a relatively narrow stratigraphic interval and show no evidence of internal bedding. The pods may be: (1) microbial mounds/algal mats, (2) sponges, (3) mud balls or SOMETHING ELSE! They are similar in general outline to some thrombolites in that the internal texture is non-laminated but there is no indication of microbial activity such as clotting. The possibility that they are sponges or algal mats is somewhat doubtful because the braided stream depositional environment was one of relatively fast-moving fresh water draining mountains to the southeast; in addition, most sponges live in a marine environment. The conical shape does not favor a mud ball origin. Hint: note that the gray matrix follows the cleavage. PLEASE DO NOT COLLECT SAMPLES; ACTIVE RESEARCH SITE!)