7: THE CLASSIC DEVONIAN OF THE CATSKILL FRONT: A FORELAND BASIN RECORD OF ACADIAN OROGENESIS
Charles A. Ver Straeten
New York State Museum
The State Education Department
Albany, NY 12230
cverstra@mail.nysed.gov
INTRODUCTION
Foreland basins sedimentary rocks preserve a record of the evolution of a mountain belt. Indeed, adjacent to ancient, deeply eroded orogens like the Devonian Acadian mountain belt, the foreland basin sedimentary succession is a key source of data on the timing and character of orogenic events and processes long since eroded away.
Making viable interpretations of orogenesis from foreland basin sedimentary rocks is, of course, not simple and straightforward. For example, look at the evolving understanding of Devonian volcanic ash beds ("tephras", "tuffs", "K-bentonites") and their implications for explosive Acadian volcanism, one of the author's primary research interests (Ver Straeten, 2004a, 2008; Ver Straeten et al., 2005).
Prior to the mid-1970s, only a few thin volcanic tephras were known from Devonian strata across eastern North America. Now approximately 100 beds are known from the Devonian of the Appalachian basin. Older perspectives interpreted each bed to represent a single explosive volcanic eruption. Microstratigraphic and geochemical study of the beds, however, indicate that many are multi-event layers, with a history of amalgamation of multiple eruptions, resedimentation on the sea floor, and/or mixing with detrital sediments. Furthermore, it is unknown how many tephra layers, erupted over ca. 60 million years, were not preserved in the sedimentary rock record. The question then arises, does the existing record of foreland basin tephras yield a viable proxy of Acadian volcanism through time, despite biased and incomplete preservation of individual eruptions?
This field trip and paper examine the Devonian (± Upper Silurian) sedimentary rock succession in the Catskill Front of eastern New York, with additional perspectives from across New York and the Appalachian basin. Through a review of the existing literature, and presentation of new data, the author hope to produce a detailed, yet broad perspective of the implications of data from the foreland basin on the Acadian orogeny in the northeastern U.S. It is, of course, a progress report on the subject, a single author's perspective on what is known at this time, and resulting interpretations. Much more and varied work is needed.
This paper and field trip will present an overview of the Acadian orogeny and its foreland basin. It will then focus on a set of sedimentary rock characters that potentially provide regional perspectives on Acadian orogenesis. These include: 1) sedimentation and sea level history; 2) clastic rock composition; 3) volcanic tephra beds; and 4) soft sediment deformation. These four types of information may, and generally indeed do, permit interpretation of the developmental history of the Acadian orogen and the evolution of the Acadian foreland basin.

7: THE CLASSIC DEVONIAN OF THE CATSKILL FRONT: A FORELAND BASIN RECORD OF ACADIAN OROGENESIS

Outcrops of Martinsburg Formation. Interbedded turbidites and shales.

Catskills visible ahead.

Cross over Wallkill River.

Cross over Rondout Creek. Begin climb up onto Devonian bedrock.

Lower Devonian Glenerie Formation.

Lower Devonian New Scotland Formation?

Lower Devonian, upper part of New Scotland to Becraft formations.

New Scotland Formation.

New Scotland to Becraft formations.

Lower Devonian Schoharie Formation

Middle Devonian Onondaga Limestone (Edgecliff Member).

Onondaga Limestone (Nedrow to Moorehouse members).

Cross over Esopus Creek.

Middle Devonian Union Springs Formation (Stony Hollow Member).

Esopus Creek on right.

Cross under Rte. 209, with a classic set of Lower to Middle Devonian road cuts along it.

Cross over Sawkill Creek.

Middle Devonian Mount Marion Formation (East Berne Member).

Mount Marion ahead on left. Type section of Mount Marion Formation.

Cross over Plattekill Creek.

Stony Hollow Member again.

Mount Marion to left.

Note large quarry on north flank of Mount Marion.
East Berne and Otsego members, Mount Marion Formation.

Bypass Saugerties exit.
Most complete section of Onondaga Limestone in eastern New York, visible along Thruway and northbound entrance; Schoharie Formation visible at far end, right side.

Excellent section of Lower Devonian Becraft, Alsen, Port Ewen, and Glenerie fms.

Schoharie Formation (Aquetuck & Saugerties members).

Schoharie Formation.

Extensive limestone quarries
Extensive limestone quarries in Lower Devonian Helderberg Group beyond ridge to right.

Becraft Formation (?).

Schoharie Formation ± uppermost underlying Esopus Formation.

South end of long exposure along abandoned Thruway exit
South end of long exposure along abandoned Thruway exit, ending at Stop 2 of this trip. Strata visible from Thruway include Schoharie and Esopus formations. Complete section of Esopus exposed.

Onondaga to Schoharie formations.

Schoharie exposed on southbound side of Thruway.

Cross over Kaaterskill Creek.

Schoharie Formation on west side, Onondaga Formation on east side.

Esopus Formation (mostly Quarry Hill Member, in middle of formation).

Cross over Catskill Creek.

Becraft to New Scotland formations

New Scotland Formation along exit ramp.

New Scotland Formation on left.

New Scotland Formation on left, Kalkberg Formation on right.

Pull over and park on shoulder.
Cross and continue ahead on Rte. 23b, then walk up exit ramp off of Rte. 23 to prominent angular Taconic unconformity.

STOP 1. Taconic Unconformity and Helderberg Group Limestones, Catskill.
This classic locality has been the subject of many field trips. For this trip, we will visit two road cuts along Rte. 23: a) exit ramp to Leeds/Rte. 23b, off of Rte. 23 westbound; and b) the succession on the north side of Rte. 23 west of Rte. 23b. We will largely focus on various features with implications for Acadian orogenesis, as interpreted from the sedimentary rock record. These include the Taconic unconformity and Manlius-Coeymans contact, carbonate deposition in a back-bulge basin of the foreland basin system, volcanic tephra beds (K-bentonites), and possible Devonian deformation of the "Little Mountains Fold-Thrust Belt" here.
Near the base of the succession, the prominent angular unconformity (Taconic unconformity) places supratidal dolostones and sandstones (Upper Silurian (?) Rondout Formation) over deep water turbidite sandstones and shales (Middle Ordovician Austin Glen Formation). The hiatus represents approximately 30 million years of time, and marks a series of events associated with the Taconian orogeny. This history begins with deep water deposition of synorogenic clastics, overthrusting of the rocks from western Massachusetts into eastern New York, and later uplift of the area due to erosional unroofing and rebound of the Taconian orogen through the latest Ordovician to Late Silurian. The latter resulted in the elevation of eastern New York above sea level throughout most of the Silurian, and restriction of the sea to western ± central New York during most of the Silurian. Transgression of marine waters over eastern New York in the latest Silurian to earliest Devonian is at least in part due to tectonic-induced subsidence and migration of the foreland basin to the east. This was associated with collision and crustal loading during Late Silurian, with the onset of the Acadian orogeny on the far margin of eastern North America.
The limestones, dolostones and minor shales of the Rondout Formation and Helderberg Group along Rte. 23 here comprise a carbonate ramp succession. They were deposited in the back-bulge basin of the greater Acadian foreland basin, at a time when the orogen and the main body of the foreland basin were still far to the east. The strata examined on this trip (Rondout, Manlius, Coeymans, Kalkberg, New Scotland, and Becraft formations) record an overall deepening- to shallowing up succession through supratidal, tidal, shoal, and shallow to deep ramp facies (Rondout to lower New Scotland fms.), followed by a gradational shallowing up to shoal to tidal facies (middle New Scotland to lower Becraft formations). In sequence stratigraphic terms, the succession comprises comprise a single "third order" depositional sequence, and the lowstand base of a second sequence. In total, the Rondout-Helderberg succession represents two major "third order" sea level cycles/depositional sequences.
The Manlius-Coeymans contact in the Catskill area was interpreted by Chadwick (1944) to represent an erosional hiatus. However, the wider distribution and implications of this break has only been recently documented by Ebert and Matteson (2003). Their detailed work through the two formations has documented at least two significant unconformities, including the formational contact. Noting a subtle but documented angularity to the unconformity, Ebert and Matteson (2003) interpret its formation to be associated with the orogen-ward migration of a bulge-like feature, associated with early stages of the Acadian orogen on the distant margin of North America near the SilurianDevonian boundary.
Ancient volcanic airfall tephra beds, altered to clay-rich K-bentonites, occur in the Kalkberg and New Scotland formations along Rte. 23. This cluster of approximately 15 beds, termed the Bald Hill K-bentonites, are widely reported across the Appalachian basin, from eastern New York to Virginia and West Virginia. They preserve a record of explosive, plinian-type volcanism in the Acadian orogen. Additional K-bentonites have been noted in the Manlius Formation at other localities (Ebert and Matteson, 2003; P. Rubin, pers. commun. 2007).
The age of deformation of the strata along Rte. 23 is the subject of debate. Some workers (e.g., Marshak, 1986; Mar shak and Tabor, 1989; Zadins, 1989) interpreted the folding and faulting to be Devonian, at least in part. In contrast, Geiser and Engelder (1983) interpret the structures to have formed later, during the Late Carboniferous-Permian Alleghanian orogeny. It is possible that both orogenic events led to the deformation. At present, the timing of deformation is unclear.

Beginning of another classic cut through the "Little Mountains" fold-thrust belt.

Cross over Catskill Creek.
Downstream the creek passes for approximately 1 mile through the gorge of Austin Glen, descending stratigraphically through Lower Devonian to uppermost Silurian strata of the Esopus through Rondout formations, and into the Ordovician Austin Glen Formation. Classic, beautiful site.

Esopus through basal Onondaga Formations (Stop 1B of Ver Straeten and Brett, 1995).

Onondaga Formation.

Cross over Kaaterskill Creek.
Ordovician Austin Glen Formation exposed in creek bed to left.

Kaaterskill Creek will follow road for some distance.

Onondaga Limestone.
Section between 46.5-.6 exposes Edgecliff, Nedrow and lower Moorehouse members.

STOP 2. Lower Devonian Ramp Carbonates etc.
Lower Devonian Ramp Carbonates and the Initial Acadian Clastic Wedge, the Sprout Brook K-Bentonites, and the Wallbridge Unconformity.
This is another classic Devonian locality, chiefly noted for its prominent structural folds. However, several additional characters of the outcrop provide a record Acadian Lower Devonian activity in the Acadian mountain belt. These include the Wallbridge unconformity; a carbonate-quartz arenite suite of rocks succeeded by the first major influx of Acadian synorogenic sediments; migration of the foreland basin foredeep into the Catskill Front; and a second major cluster of altered volcanic tephra beds. The Glenerie-Esopus contact at this outcrop lies approximately 90 m stratigraphically above the Taconic unconformity, seen at Stop 1.
The units visible at Stop 2, from low to high include the top of the Port Ewen Formation, the Wallbridge unconformity, local chert facies of the Glenerie, and a rare, complete section of the overlying Esopus Formation. At the far end of the outcrop, along the NYS Thruway, lower strata of the Schoharie Formation are visible.
The top of the Port Ewen Formation, a shaly limestone analogous to the New Scotland formation, directly underlies the Wallbridge unconformity here. The Wallbridge, which marks one of the major Phanerozoic sea level lowstands in North America (Sloss, 1963), is of relatively short duration in the Hudson Valley. Deposition across the interval is continuous in the Port Jervis area, where New York, New Jersey, and Pennsylvania meet.
Immediately overlying the Wallbridge unconformity is a conglomeratic lag bed at the base of the Glenerie Formation. The conglomerate in the area of Catskill is largely composed of phosphatic pebbles with scattered milky quartz. To the south, beginning near Kingston, a conglomerate unit of milky quartz wedges in below the Glenerie. This unit, termed the Connelly Conglomerate, is the oldest conglomerate found in the New York Devonian (Table 2). Its quartz composition (>99% milky quartz) contrasts with younger pebbly to conglomeratic strata in the Catskill front, which feature a more diverse composition (Table 2). The conglomeratic base of the Glenerie locally near Catskill is dominated by chert; to the south, it transitions into silica-rich limestones, and to the north to the quartz sand-rich Oriskany Sandstone.
Glenerie-Esopus contact is relatively gradational at Stop 2 and in the eastern New York region. Elsewhere across the central to southern part of the basin, the contact is generally more sharp (Ver Straeten, 2007a). It marks a time of foundering of widespread shallow marine ramp conditions, and progressive subsidence and migration of the foreland basin foredeep into eastern New York, during the onset of the first Acadian Tectophase recognized by Ettensohn (1985). In reality, at least one previous tectonically active to quiescent "tectophase" likely occurred during the Late Silurian to Early Devonian parts of the Acadian orogeny.
The mudstones, shales, siltstones and sandstones of the Esopus Formation mark the first significant influx of Acadian synorogenic clastics into the Catskill Front and the Appalachian basin. Migration of the orogenic front across New England has by the Emsian Stage moved far enough craton-ward for the foredeep segment of the foreland basin system to migrate west of the present day Hudson River. Petrologic data from Rehmer (1976) indicates that fine sandstones to siltstones of the Esopus are rich in quartz (∼33-55%), with a high concentration of matrix (∼19-49%), and minor amounts of fragmentary mica and chlorite, detrital and diagenetic chert, and pyrite.
Detailed work by the author on Emsian-age (upper Lower Devonian) strata of the Esopus and Schoharie formations and equivalent strata across the Appalachian basin (Ver Straeten, 2007a), along with geochronologic age dating (Tucker et al., 1998; Kaufmann, 2006), outline a longer, more complex history to the Emsian stage in the eastern U.S. than has been appreciated. U-Pb dating indicates a duration on the order of 15-18 million years for the Emsian (Tucker et al., 1998; Kaufmann, 2006), and that its global sea level history comprises five major third order cycles (Ver Straeten, in press), not one as previously interpreted by Johnson et al. (1985).
To the author, one of the key stories linking the foreland and orogenic belt is tied a series of thin, tan-colored clay beds in the lower part of the Esopus Formation here (Spawn Hollow Member). These are the Sprout Brook K-bentonites, 15 altered volcanic tephras dated at 408.1 ± 1.5 Ma (Tucker et al., 1998). In contrast with other Devonian clusters of K-bentonites in the Appalachian basin, these are geographically restricted to eastern New York (Ver Straeten, 2004a, b). The age of the Sprout Brook K-bentonites cluster overlaps with dates of numerous volcanic and plutonic rocks in the northern Appalachians, predominantly in Maine (e.g., Bradley et al., 2000; and additional references in main body of this paper). This includes the Katahdin Granite and co-magmatic Traveler Rhyolite of central Maine; the Traveler alone, from only one of many volcanic centers of lower Emsian age, is conservatively estimated to have a volume on the order of the largest Cenozoic-age tuffs in the western U.S. (Rankin and Hon, 1987; Ver Straeten, 2004b). It is plausible that explosive, plinian-type volcanoes in northern New England were the source of the tephra deposited in eastern New York.
As at Stop 1, the age of deformation of the strata here is unknown. An interesting point here is the relationship of deformation to the Sprout Brook K-bentonites. The soft, unlithified clay beds form a sharp rheological contrast with their interbedded thin chert and shale beds. Slippage largely appears to follow the clay beds, which in places appear to have been squeezed through the folds ("like toothpaste"). This is well seen in the uppermost K-bentonite, which varies in thickness from zero to over 1.5 meters along the outcrop over the central anticline. The K-bentonites probably helped concentrate deformation along this zone in the Catskill Front and into the subsurface, the position of a major decollement in the Catskill Front/Little Mountains fold-thrust belt according to Marshak (1986).

Prominent exposures of Lower Devonian Schoharie Formation.

Pass Old Kings Highway on left.
Topmost Schoharie and Edgecliff Member of Middle Devonian Onondaga Limestone exposed along beginning of Road (Stop 3B of Ver Straeten and Brett, 1995).

Folded Nedrow and Moorehouse members
Folded Nedrow and Moorehouse members (Onondaga Limestone) exposed on left. To proceed from here to Stop 3, carefully walk 0.3 miles ahead to east (proximal) side of bridge over Kaaterskill Creek. Walk down steep slope on south (left) side of bridge to exposure along creek.

STOP 3. Middle Devonian Ramp Carbonates etc.
Middle Devonian Ramp Carbonates and the Second Acadian Clastic Wedge, and Tephras.
Note: No hammers or collecting from the Onondaga-Bakoven contact – it is a rare exposure. The creek exposure is on private property - ask permission for access.
Strata exposed at Stop 3 include the uppermost limestones of the Middle Devonian Onondaga Formation and overlying black shales of the Bakoven Member of the Union Springs Formation (lower part of the Marcellus subgroup of Ver Straeten and Brett, 2006). At least one K-bentonite of the Tioga A-G K-bentonite cluster is exposed here. The first part of the outcrop is along Kings Highway and Rte. 23a, where the lower to middle Onondaga Formation is exposed (Edgecliff, Nedrow and Moorehouse members). Walk westward down the hill to the south side of the bridge over Kaaterskill Creek, where the top Onondaga (Seneca Member?) and lower Bakoven Shale are exposed along the creek. This outcrop lies approximately 270 m stratigraphically above the Taconic unconformity, seen at Stop 1.
Above the Esopus Formation at the previous stop, the Schoharie Formation is transitional from extra-basinal clastics to intra-basinal carbonates. This shift culminates in deposition of the Middle Devonian Onondaga Limestone. Onondaga-equivalent carbonate-rich facies occur widely across eastern North America; they mark a shutdown of clastic sedimentation in the northern Appalachian basin, although to the south in deeper parts of the basin (PA through VA-WV), the interval marks more of a decline in % clastic content, and deposition of mixed carbonate-clastic facies.
Base of Onondaga is a lowstand of sea level. In the Schunnemunk outlier north of NYC, and in the area of Palmerton, eastern PA, lower Onondaga ± upper Schoharie strata are represented by shallow marine quartz sandstones conglomerates. Exclusively quartz pebbles also occur scattered through strata at two positions within the Schoharie Formation. Furthermore, rare quartz pebbles have also been found in the same position in the Moorehouse Member and equivalent strata in central New York and central Pennsylvania. Curiously, widespread occurrence of quartz-rich strata are a contrast with the synorogenic sediments of the Esopus Shale below. Conditions that permitted progradation of Esopus extra-basinal clastics no longer existed.
Here at Stop 3 we stand at a major depositional shift, where relatively shallow marine carbonates are succeeded by basinal organic-rich black shales (by some estimates representative of ca. 100-200 m depth). A prominent bone-rich phosphatic lag interval at the formational contact marks a period of sediment starvation across the transition. A similar lag bed occurs along the Onondaga-Bakoven contact into western New York, overlying progressively younger uppermost Onondaga strata in that direction. The top of the Onondaga progressively youngs to the west, indicated but by the progressive upward appearance of the Tioga B, B', C, D, E, and F K-bentonites with the thickening of upper Onondaga (Seneca Member) strata to the west. This younging is associated with earlier foundering and subsidence of the foredeep of the foreland basin in eastern New York; subsequently, the shallow Onondaga ramp progressively subsided to the west, due to loading and uplift during the onset of Acadian Tectophase II of Ettensohn (1985).
The Onondaga to lower Union Springs Formation and equivalent strata across eastern North America are well known for the occurrence of the Tioga K-bentonites. Actually, the so-called "Tioga" interval comprises two major clusters of volcanic tephras (Ver Straeten, 2004a, 2007a). In the northern and central part of the Appalachian basin, an upper cluster of eight beds (Tioga A-G K-bentonites) occurs widely in upper Onondaga and Union Springs-equivalent strata. A lower cluster of up to 32 beds, previously correlated with the Tioga A-G K-bentonites, is found only in middle Onondaga-equivalent strata in the southern part of the Appalachian basin (Virginia and West Virginia; Ver Straeten, 2004a, 2007a). The lower cluster has been dated at 391.4±1.8 Ma (Tucker et al., 1998), the upper one at 390± 0.5 Ma (Roden et al., 1990). They appear to be sourced from volcanic centers in northern Virginia and southeast of the Stroudsburg, PA area, respectively (Dennison and Textoris, 1970, 1978; Ver Straeten, 2004a). At Stop 3, some of the long bedding planes exposed along Rte. 23a may represent thin K-bentonites. Along the creek exposure, a thin centimeter-thick clay bed 35 cm below the contact is a K-bentonite. Another thicker bed, possibly the widely known Tioga B K-bentonite bed from the base of the Seneca Member in New York, is covered just above the base of a small gully closer to the bridge.

Kaaterskill creek visible along road again, on left.

Exposures of upper part of the Mount Marion Formation on left.

STOP 4. Nearshore Marine Clastics, Upper Mount Marion Fm.
This stop examines shallow marine clastics (upper part, Middle Devonian Mount Marion Fm.), not far below the transition into terrestrial strata (Ashokan Fm. and higher strata). Exposed along Rte. 23 are approximately 12.3 m of sandstone-dominated strata. Well defined hummocky cross beds, indicative of storm processes low in the outcrop are replaced above by mega-ripples (dunes), suggesting an overall shallowing up succession. Intraformational conglomerates and soft sediment deformation zones are also visible in the upper part of the outcrop. The strata at Stop 4 are approximately 750 m stratigraphically above the Taconic unconformity at Stop 1.
Two sandstone beds with reworked, intraformational pebbles and brachiopods are visible in the upper part of the outcrop. Along Catskill Creek, approximately 0.8 km to the north-northwest, a conglomerate with abundant macro-crystalline milky quartz and chert, with lesser numbers of sandstone and other clasts, was reported by Wolff (1967) and exposed in the late 1980s. It is presently covered in the creek bed. The author has noted multiple thin, sometimes lensing conglomerate beds in the upper Mount Marion Formation along and in the forest off of Rtes. 28 and 28a northwest of Kingston. They also mark the progradation of gravels into the Catskill Front at approximately 388 Ma, possibly concentrated into beds during the lowstand or basal transgressions of small scale (fifth to sixth order) cycles (e.g., Bergman and Walker, 1987; Smith and Jacobi, 1998).
Compositionally (Table 2), these conglomerates contrast sharply with the only white quartz pebble compositions of conglomerates and scattered pebbles in older strata (Connelly Formation; Schoharie Formation; Kanouse, Palmerton and Onondaga/upper Needmore formations) in New York and Pennsylvania. The relatively high concentration of various-colored cherts in the conglomerates also contrast with relatively chert poor compositions of overlying conglomerates in the Devonian succession. This indicates a relatively significant source of chert in rocks exposed in the Acadian orogen at this time. The upper Mount Marion conglomerates
Smith's (1970) petrographic studies of upper Mount Marion sandstones in the Catskill front found that the sands were composed of a mix of mono- and polycrystalline quartz, chert, metamorphic and sedimentary rock fragments with less abundant chlorite, other micas, and plagioclase feldspars. His reported average compositions for the upper Mount Marion strata ("Solsville" and overlying "Pecksport" equivalents, 5 and 8 samples, respectively) are: Quartz + chert = 50.0 & 37.4%; matrix = 9.1 & 13.2%; rock fragments = 37.6 & 41.1%; and carbonate = 1.6 & 8.3%. The presence of slate and phyllite fragments, along with recycled sandstone and limestone grains indicated erosion of low grade metamorphic and sedimentary rocks in the Acadian orogen by approximately 388 Ma.
On the south side of the outcrop, along Vedder Road, two zones of soft sediment deformation can be seen in the upper part of the outcrops (Figure 15b). These potentially represent "seismite" beds, formed when severe seismic shocks from the Acadian orogen triggered liquefaction of the loosely packed sands, and their subsequent, more condensed repacking, and expulsion of excess water.
The first outcrop west of Five Mile Woods Road, on the south side of Rte. 23, exposes roughly five meters of interbedded sandstone and dark gray mudstones. Two sandstone bodies (ca. 1.5 and 3 m-thick) are not notably cross-bedded or erosively based; small delicate traces in the intervening mudstones may represent small plant root traces. The outcrop may be the lowest terrestrial deposits exposed along Rte. 23. If so, it could represent a thin tongue of terrestrial facies within the upper Mount Marion Formation, as seen near Kingston (Stop 8 of Ver Straeten and Brett, 1995), or deposits low in the overlying Ashokan Formation.
Strata from the lowest red-beds to the top of Slide Mountain, the highest peak of the Catskills, comprise approximately 1.9 km thickness of Middle to Upper Devonian strata (lower Givetian to upper Frasnian stages, Rickard, 1975). They were almost exclusively deposited in fluvial-dominated, terrestrial environments. These strata will be seen at subsequent Stops 5-7, and in road cuts along the route.

Interbedded sandstones and shales on south side of Rte. 23.

Lowest exposure of terrestrial "red-beds"
Lowest exposure of terrestrial "redbeds" of Plattekill Formation along Rte. 23. Lower red and green mudrocks (including paleosols) deposited on floodplain, overlain further along road by channel sandstones. Outcrops for next ∼10 miles (to ∼66.1 miles) are in the Plattekill Formation.

Good exposure of Plattekill Formation along Silver Spur Road to left.

STOP 5. Middle Devonian Fluvial Channel and Floodplain Deposits
This outcrop of the Middle Devonian Plattekill Formation exposes typical terrestrial facies of the Catskill delta complex, as developed in the Catskill front. The strata at Stop 5 are roughly a little less than one kilometer stratigraphically above the Taconic unconformity (Stop 1).
Along the outcrop (ca. 11.6 m-thick), two channel sandstone bodies and two sandy to muddy floodplain deposits, including ancient soils (paleosols) are visible. Fining up pairs of channel sandstones to floodplain deposits in the Catskills are interpreted to represent thousands to tens of thousands of years (Bridge, 2000).
The lower "mudrock" unit (ca. 2.0 m-thick) is characterized by dark gray to gray shales that grade upward into inter bedded thin sandstones and green mudstones. The upper 40 cm have a blocky texture and feature small-scale slickensided surfaces (pedogenic slickensides), associated with soil development.
Lower strata of the overlying sandstone body (ca. 4.2 m-thick) grade laterally between gray sandstones, dark gray shaly sandstones, and intraformational conglomerates, of which at least one is dominated by calcareous nodules, eroded and reworked from erosion of paleosols upstream, Cross-bedded sandstones above feature more than one channel (multi-storied) up through the succession.
The top of the channel is marked by a 0.7 m-thick interval of more tabular sandstone beds, which grade upward into ca. 1.2 m of red mudrock-dominated strata, deposited across a floodplain after migration of the channel. Centimeter scale vertical traces, often green in color represent root traces of Eospermatopteris-type cladoxylopsid trees, similar to the famous tree stumps found near Gilboa, higher in the Catskills. In places, the roots have extensively bioturbated the sediments. In addition, the upper 40-60 centimeters feature multiple "dish/gilgai/pseudo-anticline" soil deformation structures, and pedogenic slickensides occur through the strata. These features form by wetting and drying of expandable smectite (montmorillonite) clay rich, fine-grained sediments, and indicate the development of vertic paleosols.
The upper unit (ca. 3.5 m exposed) is another multi-storied channel sandstone body. Mudrock lenses occur locally along the outcrop, and prominent, down-cutting erosional surfaces are overlain by additional channel deposits. The sandstone bodies are interpreted to be deposited in single channel, sinuous ("meandering") rivers, which migrated across vegetated alluvial plains (Bridge, 2000).
Sandstone petrologic studies by Gale (1985) through the Plattekill to Slide Mountain formations in the Catskills found that composition of the sandstones is relatively consistent. Mono- and polycrystalline quartz, foliated metamorphic rock fragments and sedimentary rock fragments comprise 80% or more of the sandstones (Gale, 1985). Low in the succession (e.g., Plattekill Formation), sandstones contain a greater percentage of foliated metamorphic rock fragments and a correlative lower concentration of quartz. The sandstones are relatively clay poor, and are best defined as lithic to sub-lithic arenites (Gale, 1985). Overall, the sand-size fraction increases in grain-size upward through the succession, as environments change from lowland to transitional lowland-upland alluvial plain environments.
Plattekill Formation sandstones analyzed by Gale (1985; 10 samples) feature concentrations of foliated metamorphic rock fragments between 19-47%, in sharp contrast with the Slide Mountain Formation (3-17%; 5 samples); total macro-crystalline quartz (mono- + polycrystalline) in the Plattekill comprises 25-50 % of the rock, compared with 47-62% in the Slide Mountain Formation. In addition to quartz, metamorphic rock fragments and sedimentary rock fragments (10-23%), lesser amounts of chert (0.4-3.6%), plagioclase and orthoclase feldspars (0.2-1.9%), along with chlorite and micas, illite and kaolinite are found in Plattekill Formation sandstones (Gale, 1985).
Analysis of the clay minerals in the Catskill succession, and marine strata beyond, indicate a dominance of illite with lesser amounts of chlorite and minor kaolinite (e.g., Friend, 1966; Hosterman and Whitlow, 1983). However, vertic paleosols, as relatively well developed in the upper floodplain deposit, form in smectite-rich sediments. These clays, which swell and shrink with wetting and drying, are derived from igneous sources. The abundance of vertic paleosols in the Catskill succession appear to represent an otherwise hidden component of sediments derived from igneous sources. The sources could be derived from weathering of plutonic or volcanic rocks in the source area, or from volcanic ash erupted from explosive volcanic eruption in the Acadian orogenic belt.
At this time, only intraformational conglomerates/breccias are known by the author in the Plattekill Formation (Figure 9d). The clasts in these beds consist of reworked calcareous nodules ("peds") reworked from paleosols, or chips of mud. They commonly occur in the base of channel sandstone deposits.

Exposures of Plattekill Formation are visible along Catskill Creek, 1.4 miles north.
Rte. 32 turns to right (North). More excellent exposures of Plattekill Formation are visible along Catskill Creek, 1.4 miles north. Outcrops include well exposed bedding plane and cross-sectional exposures of fluvial channel sandstones and floodplain mudstones. Also found (some distance upstream of abandoned dam) is a ∼1.5 m-thick interval of carbonate-rich facies, capped by a thin (ca. 20 cm-thick) limestone bed indicative of lacustrine facies in floodplain environments. NOTE: Exposures along the creek are on private property. And pay attention to extensive No Parking signs along Rte. 32 near Catskill Creek.

Exposures of Plattekill Formation along Rte. 23.

Peaks of Acra Point, Burnt Knob, and beyond, Windham High Peak (3524')
Peaks of Acra Point, Burnt Knob, and beyond, Windham High Peak (3524') visible in distance. Cairo-Durham town line. Approximate position of Plattekill-Manorkill ("Moscow") formational boundary, according to Fisher et al., (1970).

Cairo-Durham town line.
Approximate position of Plattekill-Manorkill ("Moscow") formational boundary, according to Fisher et al., (1970).

Good exposures of Manorkill Formation along south side of road
Good exposures of Manorkill Formation along south side of road, near interesting and unusual buildings. Additional exposures of the Manorkill along highway ahead.

STOP 6. Floodplain and Channel Sandstone Deposits, East Windham.
Over 60 m of mudrocks, sandstones and a thin limestone bed in the Manorkill Formation at Stop 6 represent deposition in floodplain, fluvial channel, and lacustrine environments on the subaerial delta plain of the Catskill delta complex. It is possible that a part of the section may represent some brackish water conditions however. This outcrop is roughly 1.25 km stratigraphically above the Taconic unconformity at Stop 1.
Low in the part of the outcrop facing the parking area, another red paleosol zone with green, cm-scale diameter root traces of cladoxylopsid trees is visible, more easily seen than those at the last stop. Additional paleosols along the outcrop show varying development (Mintz et al., 2006). Along the outcrop, floodplain deposits vary between red, green, yellow-tan and dark gray/black mudstones and lesser fine-grained sandstones. Several sandstone bodies occur along the outcrop also, though few are thicker than 1-2 m.
Around the bend and uphill from the parking area, a prominent sloping-to-the-right interval of yellowish-green strata is succeeded by a zone of thin sandstones and dark gray mudrocks, at a distinctly different angle to the underlying beds. A number of isolated soft sediment deformation pillows occur along the outcrop within the lower approximately 1-3 m of the upper unit.
Higher in the section, downhill from the first driveway uphill of the bend, a prominent, thin ledge of limestone sticks out from the outcrop. Fallen slabs of the bed can be seen in the talus. Light gray to brown-gray, varyingly smooth to knobby in appearance, the fauna noted in the bed consists of ostracodes. Mintz et al. (2006) state that the bed appears to have been pedogenically modified and brecciated. It is over and underlain by red to green paleosols.
Apparent lacustrine (lake) and palustrine (wetland) facies, including similar thin limestones, are not often discussed but not unknown in Upper Devonian Catskill magnafacies (DeMicco et al., 1987; Dunagan and Driese, 1999). Similar limestones are documented from Devonian terrestrial facies in Canada, Great Britain and Australia (e.g., Donovan, 1975). They are generally interpreted to have formed toward the center of ponds and lakes, beyond the transport of fine-grained clastics. Carbonate is derived from calcareous, photosynthetic algae (e.g., charophytes). The author has found multiple apparent freshwater limestones in the Plattekill and Manorkill formations along the Catskill Front. Lacustrine and palustrine/wetland environments comprise an interesting and relatively overlooked facies in the Catskill Front, which deserves more attention.
A short distance above the limestone, along the lower part of a driveway, an interval of olive-colored, mudrock-dominated strata above the limestone, best seen along the lowest part of a driveway, features common fish bone material, ostracodes and desiccation cracks. Also found in the interval are Spirophyton trace fossils, which have been interpreted by some to have lived in freshwater setting (Bridge and Gordon, 1985; DeMicco et al., 1987), but by others (Gordon, 1988; Miller, 1991) to indicate brackish water conditions. Miller (1991) proposed that the animals producing Spirophyton lived in ephemeral ponds on the coastal floodplain, with fluctuating fresh- to brackish water salinities, perhaps tied to floods of brackish water that flowed upstream and spread across floodplains during major storms.
As in other fluvial-dominated strata of the Catskills, Manorkill Formation sandstones analyzed by Gale (1985; 3 samples) feature common mono- and polycrystalline quartz (29.6-60.0%), and foliated metamorphic and sedimentary rock fragments (30.4-41.8% and 9.3 to 22.0%, respectively). lesser amounts of chert (0.2-0.7%), plagioclase and orthoclase feldspars (1.6-4.3%), along with chlorite and micas, illite and kaolinite are found in Manorkill sandstones (Gale, 1985). This represents a subtle shift toward increased quartz content from the underlying Plattekill Formation. With the exception of a single bed at or near the base of the Manorkill Formation, conglomerates known to the author at this point are intra-basinal ones, with reworked mud or pedogenic carbonate nodules.

Peak of Burnt Knob and/or Acra Point ahead.

Channel sandstones of Plattekill Formation.
All exposures along Rte. 32 ahead are in the Plattekill Formation.

Area between here and turn at Rte. 23a are called the Kiskatom Flats, which we'll see from above at Stop 7.

Note rise of the highway, ascending up alluvial fan out front of Kaaterskill Clove.

Enter Kaaterskill Clove.

Lower bridge in Kaaterskill Clove.
Cross over Kaaterskill Creek. Excellent exposures of Plattekill Formation upstream, including channel sandstones, floodplain deposits, and at least one more apparent freshwater limestone.

Area of "High Rocks" of Chadwick (1944, Fig. 47).

Middle bridge in Kaaterskill Clove.
Excellent exposures of red mudstone/paleosol facies of Manorkill Formation at and above bridge.

Upper bridge in Kaaterskill Clove.
Excellent exposures of Oneonta Formation upstream, along trail to base of Kaaterskill Falls.

Parking area for trail to Kaaterskill Falls.

Classic exposures of Oneonta Formation along road, with well developed paleosols.

Top of Kaaterskill Clove

STOP 7. Upper Devonian Clastics, North-South Lake
Note: No hammers or collecting at this stop (a Catskill Park campground, run by NYS-DEC).
At this stop, we will walk north along the Catskill escarpment to Artists Rock and Sunset Rock. The rocks exposed comprise lower strata of the Upper Devonian Oneonta Formation, and will include the Twilight Park Conglomerate Member (at Sunset Rock). The latter outcrop is approximately 1.5 km stratigraphically above the Taconic unconformity at Stop 1. Another ∼1.2 km of strata overlie the Twilight Park Conglomerate, to the top of Slide Mountain, the highest peak in the Catskills.
The Escarpment trail going north from the parking area at North Lake slowly rises upward through a major sandstone ledge that caps the Catskill escarpment here. Termed the "Kaaterskill Sandstones" by Chadwick (1944), they form the lower part of the Upper Devonian Oneonta Formation. Sandstones dominate the strata along the trail to Artists Rock. An abandoned trail a short distance beyond Artists Rock exposes red mudstones that lie between major sandstone packages.
Partway along the trail to Artists Rock, shortly after a several-meter rise up through the lower Oneonta sandstones, an odd set of sedimentary structures occur in the sandstones in the trail. At first glance, they appear to be trough cross beds. On closer observation, however, the edges of the troughs are vertical to near vertical, well beyond the angle of repose. The vertical edges are water-escape structures, along the margins of foundered bowls of sandstone. This represents another soft sediment deformation zone, developed in sand-only facies (as found at Stop 4). The author has not examined their correlatability, although previous workers have noted disturbed soft sediments elsewhere in the area, in the lower part of the Oneonta Formation. If the feature is relatively widespread, it could possibly represent a "seismite", triggered by a significant seismic shock during the Acadian orogeny. Without more evidence (e.g., correlatability across a broad area), this is only conjecture.
Mono- and polycrystalline quartz comprise approximately 37.9-59.1% of the rock in petrographically analyzed sandstones of the Oneonta Formation (Gale, 1985). The concentration of foliated metamorphic and sedimentary rock fragments (15.7-33.4% and 8.7 to 21.1%, respectively) have decreased relative to underlying strata, while the concentration of quartz has increased. The concentration of other components (e.g., chert, feldspars, chlorite and micas, etc.) remain about the same as in the underlying Manorkill and Plattekill formations.
There is a rise of approximately 530 m (1750') from the Kiskatom Flats below. In the distance to the east, the Taconic Mountains are visible along the New York-Massachusetts border; high peaks of the Berkshire Mountains (including Mount Greylock) locally project above the Taconics. Farther to the north/left, the Green Mountains are visible in Vermont. These highlands today expose low grade metamorphic rocks (e.g., slates and phyllites in the Taconics; some schist in the higher Berkshires and Greens). However, those rocks would have been deeply buried under younger rocks during the Devonian Acadian orogeny.
At Sunset Rock, up the trail beyond Artists Rock, a 23 m-thick outcrop of the Twilight Park Conglomerate (member of the Oneonta Formation) is very well exposed. The unit marks the first major progradation of clastics into the Catskill front from the Acadian orogen. Bridge and Nickelsen (1985) hypothesized that the progradation was due to increased slope due to tectonic changes, stating that climate did not appear to vary through the interval. In this case, a base-level drop associated with a significant sea level fall was not discussed. Bridge and students generally dismiss eustatic sea level controls over processes active in the terrestrial settings of the Catskill magnafacies. However, Devonian workers (e.g., House and Kirchgasser, 1993; Brett and Baird, 1996; Bartholomew, 2006; Ver Straeten, 2007a, in press) have now established significant eustatic control over sea level changes in New York's marine succession. And that tectonic patterns of flexure are superposed over the record of third order sea level cycles/stratigraphic sequences (Figure 8).
The author proposes a counter hypothesis, that Twilight Park gravels may have prograded basin-ward during one of the major sea level falls near the Middle-Upper Devonian boundary. As there is no tight control over where the Middle-Upper Devonian boundary actually occurs in the Catskill front, that sea level drop could be one of a few near the boundary.
Petrologically, clastic rocks of the Catskill front indicate that rocks exposed in the Acadian orogen consisted of dominantly low grade (up to greenschist) metamorphic and sedimentary rocks. Rare igneous- or high grade metamorphic-derived sediments indicate only very minor exposure of such rocks. A hidden source of more igneous than previously thought seems to be indicated by common vertic paleosols, which form in smectite clay-rich sediments (smectite clays are derived from the weathering of igneous rocks, including volcanic ash).
A number of previous authors hypothesized that Cambrian and Ordovician rocks in the Taconics region were the source of the Devonian Catskill delta sediments. However, beginning in the Late Silurian, the Acadian orogen and the associated foreland basin developed on the far margin of eastern North America. Through the Lower Devonian, massive volumes of Acadian-derived sediments were deposited in the foredeep basin in New England, while the Hudson Valley was positioned in the back bulge basin to craton-ward edge of the foredeep of the foreland basin system. As the orogen progressively migrated craton-ward toward eastern New York, Devonian sediments in that early Acadian foredeep basin (e.g., Littleton Formation of New England) were caught up in the deformational front of the orogen (e.g., wedge-top of the foreland basin system). Some of the Lower Devonian foredeep sediments were metamorphosed; metamorphosed or not, the foreland basin strata were thrust up, eroded and transported into the Catskill front. The clastic wedge, of which 2.7 kilometers thickness is preserved in the Catskills, by the latest Devonian apparently distributed muds as far west as northern Iowa (B. Witzke, pers. commun., 1999). Possibly by the Late Devonian, folding and thrusting of the foreland basin wedge-top may have migrated west of the Hudson River.