Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201 – 213
www.elsevier.com/locate/palaeo
Snake Hill — reconstructing eastern Taconic foreland basin
litho- and biofacies from a giant mélange block in
eastern New York, USA
Adam M. English a,⁎, Ed Landing b , Gordon C. Baird c
a
c
Department of Geological Sciences, 274 Mendenhall Laboratory, The Ohio State University, 125 South Oval Mall,
Columbus, OH 43210, United States
b
New York State Museum, Albany, NY 12230, United States
Department of Geosciences, State University of New York College at Fredonia, Fredonia, NY 14063, United States
Received 24 August 2005; received in revised form 13 April 2006; accepted 28 May 2006
Abstract
Exotic lithofacies and faunas have long been known from Snake Hill, eastern New York, USA. The faunally diverse, sandstonedominated Upper Ordovician succession at Snake Hill sharply contrasts with surrounding tectonized sparsely fossiliferous distal
shale. Re-examination of the Snake Hill section shows that it is a storm- and wave-dominated near-shore facies with a benthic
fauna analogous to that of the younger Lorraine Group (Ashgillian) of central New York, and to that of the upper Martinsburg
Group (upper Caradocian) of eastern Pennsylvania. Orthograptus ruedemanni Chron graptolites indicate that the Snake Hill
succession is older than the surrounding tectonized, deep-water shale (Climacograptus spiniferous Chron). Snake Hill is best
interpreted as a parautochthonous block in mélange originally deposited close to the shoreline of the emergent Taconic accretionary
prism. Because the Snake Hill succession is sandstone-dominated, it is inappropriate to refer mudstone-dominated facies that
underlie the western margin of the Taconic allochthon in the Hudson River valley region to the Snake Hill “Shale,” as has been
done in the past. The thick (ca. 150 m), lithologically distinct succession at Snake Hill is therefore referred to as the “Snake Hill
Formation.” The Snake Hill Formation is the only known example of proximal, near-shore facies deposited on the western side of
the outer Taconic arc, and represents easternmost deposition in the Taconic foreland basin. The Snake Hill Formation is a unique
occurrence, and thus is restricted to its type locality at Snake Hill, New York.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Snake Hill; Taconic; Late Ordovician; Biostratigraphy; Biofacies
1. Introduction
Rudolf Ruedemann (1901, 1912, 1930) described
benthic shelly faunas from a number of localities in the
deformed Upper Ordovician flysch/shale belt of eastcentral New York (e.g., Bosworth and Vollmer, 1981;
⁎ Corresponding author.
E-mail address: english.91@osu.edu (A.M. English).
0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2006.05.014
Kidd et al., 1995). He compared the graptolite component of the fauna with that from the Magog Shale of
southern Québec, and the shelly component with that
from the lower Utica Shale (i.e., Canajoharie Formation)
in the Mohawk Valley. The faunas were also considered
to be coeval with middle Trenton Group limestones
(middle Caradocian) in the Mohawk Valley of central
New York (Fig. 1) and the Black River Valley of westcentral New York. These fossiliferous intervals were
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Fig. 1. Upper Ordovician stratigraphy (modified from Baird and Brett, 2002). Because of covered contacts, the ages of the upper and lower contacts of
the Snake Hill Formation are uncertain.
named the “Snake Hill Shale” (Ruedemann, 1912) after
the most fossiliferous locality at Snake Hill, a small
promontory on the east side of Saratoga Lake (Fig. 2)
where nearly 100 species were reported (Ruedemann,
1912, Plates 3–9; 1930).
Ruedemann (1912. p. 58) originally named the “Snake
Hill Shale” for its middle Caradocian shelly fauna at
Snake Hill and referred other localities in the Capital
District, New York, to this unit based on the presence of
comparable benthic faunas rather than on their lithologies.
Because many of these shell-bearing localities are shaledominated, the designation “Snake Hill” came to be
recognized as appropriate for Upper Ordovician dark
shales on the western margin of the Taconic allochthon in
the Hudson valley region of eastern New York.
“Snake Hill” became a term that evoked the genesis
of synorogenic siliciclastic rocks deposited during the
Late Ordovician Taconic orogeny in easternmost New
York, and was applied to deformed, parautochthonous,
or autochthonous synorogenic siliciclastics west of the
Taconic allochthon with little regard to lithology or
fauna (e.g., Fowler, 1950; Flower, 1964; Fisher, 1977,
1984; Rowley and Kidd, 1981). Consequently, there
have been many various and inconsistent references
over the years to a Snake Hill “shale,” “flysch,”
“Formation,” and “mélange.” For example: Fowler
(1950), Flower (1964), and Fisher (1977, 1984)
mapped Upper Ordovician shale-dominated siliciclastics as Snake Hill shale and flysch that was deposited
after the cessation of Upper Ordovician carbonatedominated deposition in the Hudson River valley (i.e.,
Glen Falls Limestone; Fig. 1). Lehman et al. (1995)
considered the “Snake Hill Formation” to be a unit of
“temporally correlative and downslope-correlative siliciclastic turbidites.”
Significant differences in lithology, biofacies, and age
relations of so-called “Snake Hill” localities in eastern
New York and those at the Snake Hill type locality have
led some reports to question the utility of the term. Berry
(1963) first questioned the regional use of “Snake Hill
Shale” by noting that the designation was inconsistently
applied to three very distinct lithofacies (sandstone,
flysch, and conglomerate units) that appear near the
leading edge of the Taconic overthrust belt. These
included two allochthonous facies: 1) the Austin Glen
Formation (Lower Caradocian turbidites and mudstones
at the top of the Normanskill Group) and 2) undifferentiated, unfossiliferous Normanskill Group siliciclastics
within the overthrust. In addition, “Snake Hill Shale”
was applied to various localities in the parautochthonous
tectonic mélange belt both under and west of the Taconic
overthrust (e.g., Landing, 1988).
Kidd et al. (1995) also questioned the use of “Snake
Hill Shale,” and noted the sharp facies contrasts between
the fossiliferous, sandstone-dominated succession at
Snake Hill and the unfossiliferous, dark gray to black
laminated shales often designated as “Snake Hill Shale”
on geologic maps (e.g., Fisher et al., 1970). Kidd et al.
(1995) recommended that “Snake Hill facies of the
Austin Glen Formation” be applied in eastern New York
to syntectonic siliciclastics with a diverse shelly fauna of
the type seen at Snake Hill.
The Snake Hill biota has long been an anomaly. It is
the only known benthic fauna with abundant mollusks
A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
203
Fig. 2. Locality map of Snake Hill, NY (NYSM Locality 3488). Map inset: Study area 1 — abandoned quarry on the east side of Snake Hill,
equivalent to 35-m–55-m interval (La B into C), very fossiliferous shale below transition to quartz arenite. Study area 2 — shale outcrop of La A,
with sparse graptolites. Study area 3 — abundant float material, easiest collecting due to weathering. Study area 4 — outcrop on top of the hill,
equivalent to 0–5 m. Study area 5 — fossiliferous outcrops in heavily deformed, unmeasured sections. VFZ — Vischer Ferry Shale/Graywacke zone;
MRZ — Mohawk River Central zone (VFZ, MRZ modified from Landing et al., 2003).
in the Upper Ordovician synorogenic siliciclastic rocks
of eastern New York. No investigation at Snake Hill has
been undertaken to explain the geographic restriction of
this fauna or document its stratigraphic distribution.
Indeed, Ruedemann's (1912, 1930) collections through
the Snake Hill succession are stored as a single
“locality” in the New York State Museum Paleontology
Collection (NYSM locality 3488). With exception of
articulated edioasteroids preserved on sandstone bedding surfaces (Ruedemann, 1912; Bell, 1976), no
evidence has been provided as to whether or not
elements of the Snake Hill fauna are in situ or
transported.
At the onset of our work, we felt that the Snake Hill
section would provide an unparalleled look at fossil-rich
facies deposited proximal to the ongoing Taconian
orogeny. In this report, the depositional environments and
biofacies at Snake Hill are evaluated. In addition, the
succession and biota at Snake Hill and other fossiliferous
localities referred by Ruedemann (1912, 1930) in eastern
New York as the “Snake Hill Shale” are related to the
unfolding of the Taconic orogeny.
2. Geologic setting
Snake Hill is a conspicuous peninsula on the east side
of Saratoga Lake (Fig. 2). It is a low, fault-bounded
promontory composed of vertical to steeply south-dipping, locally faulted and folded, medium- to dark-gray
sandstones and interbedded shales (Fig. 3). It rises 110 m
above a drift-mantled lowland developed on extremely
fractured mélange shale with scaly cleavage in which
bedding has been lost. This shale is best exposed just
south of Snake Hill on the east shore of the lake. This shale
is easily weathered, producing the low topography
immediately surrounding Snake Hill. The bedrock to the
north and west consists of strongly folded and faulted,
sparsely fossiliferous black shales and thin sandstone
turbidites, with a uniform dip of 50°E in the Saratoga Lake
region (e.g., Fisher, 1984).
Snake Hill has been mapped as the stratigraphically
highest part of an erosion-isolated, western outlier of the
Taconic allochthon. This outlier has been shown as
ranging down into Lower Ordovician slates (Fisher et al.,
1970; Fisher, 1984), which were originally deposited on
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A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
Fig. 3. Snake Hill succession with corresponding insets (I–III). Explanation: 1 — Shale with scattered, (<10%) lenticular, medium-grained litharenite
beds. 2 — 1:1 ratio of dark gray shale to medium to coarse-grained sublitharenite. 3 — 90% medium- to coarse-grained sublitharenite, 10% shale.
4 — Amalgamated, coarse-grained quartz arenite. 5 — Uni-directional cross-bedding. 6 — Ripple marks. 7 — Hummocky cross-stratification. 8 —
Conglomerates (rounded clasts < 3 cm diameter). 9 — Flute casts. 10 — Ball-and-pillow structures. 11 — Herringbone cross-bedding. 12 —
Significant shell concentrations (>50% shells in matrix). Corresponding insets: I — Example of La A showing a turbidite in shale interbedded with
thin, lenticular sublitharenite beds. II — La B showing 1/1 ratio of lenticular sublitharenite and shale, higher shell concentrations. III — La C showing
amalgamated quartz arenite with no shells.
A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
the west Laurentian continental slope (e.g., Landing,
1988). However, there is no evidence of these Lower
Ordovician slates, or evidence for significant tectonic
displacement of the Snake Hill area in any other studies.
Recent re-mapping of the Saratoga Lake region
shows a NNE-trending belt of Upper Ordovician shales
under the lake and an approximately coeval, thrusted
mélange belt paralleling the east side of the lake (Kidd
et al., 1995; Hayman and Kidd, 2002; Landing et al.,
2003). These shales contain such Climactograptus
spiniferus Chron graptolites as Dicranograptus nicholsoni Hopkinson, 1870; Orthograptus quadrimucronatus
(Hall, 1865); and O. approximatus Nicholson, 1873,
and are Middle Caradocian (Riva, 1974). The presence
of Normalograptus mohawkensis (Ruedemann, 1912;
text Fig. 4I) from the Snake Hill succession places this
unit in the Orthograptus ruedemanni Chron (Fig. 1).
One of these N. mohawkensis specimens was collected
from the base of the measured section (Fig. 3, 0.0 m, 4I),
the other was found in Ruedemann's bulk collections
(NYSM loc. 3488).
This biostratigraphic re-evaluation confirms Fisher's
assertion (In: Ross et al., 1982, p. 44–48, sheet 3,
column 91) that the type “Snake Hill Shale” is a package
of early synorogenic siliciclastics deposited on the east
New York platform. The unique lithology, and older age
of the Snake Hill succession compared with the structurally underlying and surrounding shale, suggests that
Snake Hill is a large block in mélange that lies at the
base of, or within the parautochthonous mélange belt, on
the east side of Saratoga Lake, well removed from the
master thrust located ca. 10 km to the east.
By zonal inference, the Snake Hill succession correlates westward (and basinward) mainly to the Dolgeville
Formation, a rhythmic succession of tabular “ribbon”
limestone beds and shale interbeds, which is exposed in
the eastern and central Mohawk Valley region (Figs. 1
and 6). The Dolgeville, contrasting with near anoxic
facies of the overlying and underlying Utica Shale, records more dysoxic conditions associated with a relative
sea level drop timed with the O. ruedemanni Chron.
3. Lithofacies associations and depositional
environments
The base of the measured section is marked by the
lowest shale occurrence at the north-west corner of
Snake Hill (Fig. 3). Eighty three meters of section were
measured. Above 83 m, large covered intervals and
increasing deformation preclude accurate measurement
of thickness. The total thickness of the Snake Hill section
may actually be on the order of 150 m.
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Synorogenic deposition of the Snake Hill succession
during emergence and erosion of the Taconic accretionary prism is indicated by the presence of sand to pebblesized grains of metamorphosed rock (i.e., green slate)
with white chert and fine-grained limestone. These
lithologies were derived from units of the overthrust (i.e.,
Deep Kill and Mount Merino Formations; see Landing,
1988). Thin-section analysis of a debris flow deposits (at
1.5 m) showed grains of epidote and gneiss that indicate
erosion and probable obduction of high-grade metamorphics and ophiolites in the Taconian orogen (e.g.,
Rowley and Kidd, 1981).
3.1. Lithofacies associations
The Snake Hill succession is divisible into Lithofacies associations (La) A–C (Fig. 3). The succession
coarsens upward from La A into La B and then to La C,
ending with alternations of La B and La C (Fig. 3).
Calcareous shelly fossils preserved primarily as molds
and casts, bioturbation, and reworked mudstone clasts
are pervasive in all three lithofacies associations. Episodically high-energy conditions are indicated by the
alternation of coarse-grained sandstones and lensing
debrites with shale beds (Fig. 3). These debrites with
rounded mudstone clasts up to 3 cm in diameter were
found at 1.5 m, 14.5 m, 27 m, and 46 m and represent
times of highest energy.
Lithofacies association A (Fig. 3, 0–14 m) is
dominated by dark gray to black mudstone with a few
lensing fine-grained, medium gray, sandstone turbidites.
The sandstones contain traces of feldspar (<1%), and
carbonate grains (<5%) with reworked sandstone and
mudstone grains (20–30%) and are best classified as
sublitharenite. This lithofacies association records lowenergy conditions that were periodically interrupted by
high energy turbiditic events. Flute casts and oriented
orthocones found at the base of one of the turbidites (at
1.5 m) show an apparent east–west current flow direction.
However, a 90° fold rotation about strike must be accounted for when considering paleocurrent direction (see
discussion).
Uncompacted moldic specimens of sowerbyellid and
dalmanellid brachiopods and nuculid bivalves are found
throughout La A. There are also localized shell concentrations with larger and rarer taxa. The best example is
found at study area 4 (Fig. 2), and is dominated by the
bivalves Orthodesma? subcarinatum Ruedemann, 1912,
and Solenomya? insperata Ruedemann, 1912. Bioturbation of the sandstones is low (i.e., ichnofabric index 2; see
Droser and Bottjer, 1986), with only a few Teichichnus
and Palaeophycus burrows found in the shale.
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Lithofacies association B (Fig. 3, 14–46 m; 52–83 m)
is composed of an approximate 1/1 ratio of dark grayblack shale to fine-grained, medium gray, sublitharenite.
La B is highly fossiliferous, and has yielded the bulk of
the reported Snake Hill fauna. Delicate skeletons of
many taxa, including articulated trilobites (Fig. 4E) and
edrioasteroids (Fig. 4T) are locally abundant.
Most of the sandstones of La B are somewhat
lenticular hummocky cross-stratified beds that range
from 10 cm to 80 cm in maximum thickness. The hummocks tend to be incomplete with only the planar-bedded swales well preserved. The thicker hummocky
cross-stratified sandstones at Snake Hill are amalgamated. Erosive contacts of the hummocky cross-stratified
beds with underlying dark gray shales are rare, and fairweather wave ripples are infrequently developed on
these hummocky cross-stratified beds.
The prominent hummocky cross-stratified beds of La
B record storm-dominated open-shelf environments in
which the depositional rate of sand was at a minimum
(e.g., Myrow and Hiscott, 1993). The intervening dark
shale intervals record times of low energy. Small scale
(< 2 cm high) wave-rippled surfaces on the hummocky
cross-stratified beds are rare, and it is possible that the
depositional depth was generally greater than that affected by fair-weather waves.
Turbidites with rare complete Bouma Ta–e sequences
are seen at 17 m and 18 m (Fig. 3). The coarsest beds in
La B are mudstone pebble debrites with flattened clasts
up to 3 cm in diameter. These debrites at 14.5 m and 27 m
(Fig. 3) have uni-directionally (easterly) imbricated
pebbles, and represent either grain flows or Ta-turbidites.
Shelly fossils occur throughout the shale and sandstone
with locally dense concentrations. The bioturbation in La
B is moderate (ichnofabric index 3) in the lowest occurrence of this lithofacies (Fig. 3, 14–46 m); Teichichnus and Palaeophycus burrows are preserved in the shale
and on the base of the sandstones.
207
The second occurrence of La B higher in the Snake
Hill succession (Fig. 3, 52–83 m) is different. Although
there is still the 1/1 ratio of shale to sublitharenite, the
hummocky cross-stratified bedding is less lenticular,
and the debrites are absent. In addition, the bioturbation
is more strongly developed in the sandstones (ichnofabric index 4) with large (ca. 2.5 cm × 15 cm) Teichichnus burrows common in the shale. These burrows
are best observed in the unmeasured southwest corner of
Snake Hill (Fig. 2, study area 5). Palaeophycus traces
are also common throughout this lithofacies association.
Lithofacies association C (Fig. 3, 46–52 m) consists
entirely of massive, coarse-grained, subrounded, light
gray quartz arenite. The massive appearance of La C is
produced by the amalgamation of planar-bedded units and
trough cross-bedded dunes that locally show herringbone
cross-bedding (Fig. 3). Individual cross beds reach a
thickness of 0.5 m. Small (<2 cm), rounded mudstone
clasts are common throughout this lithofacies association.
There is a moderate level of bioturbation (ichnofabric
index 3) locally in La C, which features Palaeophycus
traces. Lithologically and structurally analogous quartz
arenites elsewhere have been interpreted to represent a
high-energy, tidally dominated sandwave field (e.g.,
Hiscott, 1982). Thus, La C is considered to represent a
transition into the shallowest environments of the succession. As might be expected, shelly fossils are rare, a
likely consequence of breakage, abrasion, and sorting in
this high-energy facies. Only indeterminable broken
shells of brachiopods were found.
4. Paleontology and paleoecology
The brachiopod genera Sowerbyella Jones, 1946
(Fig. 4H), and Dalmanella Hall and Clarke, 1892
(Fig. 4A), constitute the bulk of the brachiopod fauna
throughout the Snake Hill succession. The remains of
these dominant brachopods occur in a 4/5 ratio, and allow
Fig. 4. Representative Snake Hill Formation taxa. Study area numbers refer to Fig. 2: (A) Steinkerns of the brachiopod Dalmanella testudinaria (Dalman,
1828), ventral valve left, dorsal valves right, NYSM 17228, 17229, and 17230, respectively, study area 5, ×2.4. (B) Nuculid bivalve Clidophorus ventricosus,
NYSM 17237, study area 5, ×2.4. (C) Dalmanella testudinaria, NYSM 17233, study area 1, ×1.4. (D) Steinkern of the brachiopod Rhynchotrema, NYSM
17250, study area 5, ×1.3. (E) Trilobite Cryptolithus tesselatus, NYSM 17246, study area 1, ×2.0. (F) Plate of the “cystoid” Carabocrinus cf. radiatus
superimposed on a trilobite fragment possibly belonging to Isotelus, NYSM 17248, study area 3, ×1.2. (G) Mold of Flexicalymene pygidium, NYSM 17230,
study area 3, ×3.0. (H) Brachiopod Sowerbyella mold, NYSM 17231, study area 3, ×1.5. (I) Graptolite Normalograptus mohawkensis, NYSM 17259, study
area 2, ×9.5. (J) Bivalve Saffordia ulrichi, NYSM 17252, study area 4, ×1.0. (K) Bivalve C. ventricosus, NYSM 17238, study area 1, ×1.8. (L) problematic
Rafinesquinid brachiopod., NYSM 17232, study area 4, ×1.0. (M) Orthoceras hudsonicum, NYSM 17257, study area 1, ×1.4. (N) Heteroconch bivalve
Lyrodesma schucherti, NYSM 17253, study area 4, ×2.2. (O) Nuculid bivalve Ctenodonta subcuneata, NYSM 17235, study area 4, ×1.3. (P)
Modiomorphid bivalves Solenomya? insperata (left, NYSM 17254) and Whiteavesia cumingsi (right, NYSM 17255), study area 4, ×1.6. (Q) Mold of the
bryozoan Batostoma, NYSM 17258, study area 3, ×1.6. (R) Solenomya? insperata, NYSM 17241, study area 4, ×1.6. (S) Modiomorphid bivalve Orthodesma? subcarinatum, NYSM 17236, study area 4, ×2.0. (T) Latex cast of the edrioasteroid Edrioaster saratogensis, NYSM 17251, study area 3, ×2.4. (U)
Large byssate bivalve Ambonychia, NYSM 17240, study area 5, ×0.9. (V) Conularid Conularia trentonensis, NYSM 17256, ×1.2. (W) Mold of articulate
crinoid Cincinnaticrinus varibrachialus, NYSM 17252, study area 3, ×1.3. (X) Brachiopod Trematis ventral valve, NYSM 17254, study area 5, ×0.6. (Y)
Mold of the cyclonemid gastropod Cyclonema cushingi, NYSM 17243, study area 3, ×1.3. (Z) Orthocone Walcottina, NYSM 17244, study area 5, ×1.4.
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comparisons with approximately coeval and younger
brachiopod biofacies that appeared during the Late
Ordovician Taconian orogeny in this part of northeastern
Laurentia.
The presence of abundant Sowerbyella with rarer
Rafinesquinids (Fig. 4L) represent the reappearance, in
eastern New York, of a brachiopod biofacies also characteristic of carbonate platform and upper slope deposits
of the Trenton Group in central New York (e.g., Cisne and
Rabe, 1978). The parautochthonous Snake Hill sandstone–shale sequence and Trenton Group shelf succession to the west are separated by ca. 100 km of coeval
dysoxic to anoxic black mudstone of the Lower Utica
Shale (i.e., Canajoharie Shale). Similarly, the dominance
of Sowerbyella and Dalmanella characterizes the Dalmanella–Sowerbyella Community of Bretsky (1970)
from the younger (Ashgillian) Lorraine Group located
further west in central New York. This community is also
described from the Upper Caradocian Martinsburg Group
of eastern Pennsylvania (Bretsky et al., 1969).
Other rare brachiopod taxa were collected at Snake
Hill. These include single specimens of Rhynchotrema
Hall, 1860 (Fig. 4D); Lingula sp. (Ruedemann, 1912,
Plate 4, Fig. 1); Trematis Sharp, 1848 (Fig. 4X); and a
large problematic rafinesquinid (Fig. 4L).
This study yielded a very diverse bivalve fauna consisting of the following taxa: Ambonychia Hall, 1847
(Fig. 4U); Clidophorus ventricosus Ruedemann, 1912
(Fig. 4B, K); Ctenodonta levata Ruedemann, 1912 (see
Ruedemann, 1912, Plate 6, Fig. 1); C. subcuneata
Ruedemann, 1912 (Fig. 4O); Lyrodesma schucherti
Ruedemann, 1912 (Fig. 4N); Orthodesma? subcarinatum (Fig. 4S); Saffordia ulrichi Ruedemann, 1912
(Fig. 4J); Solenomya? insperata (Fig. 4R, P); and Whiteavesia cumingsi Ruedemann, 1912 (Fig. 4P). These
assignments were made on a provisional basis to allow
comparisons with other eastern New York sections also
last described by Ruedemann (e.g., Green Island, discussed below; Ruedemann, 1912, 1930).
The bulk of the Snake Hill bivalve assemblage is
composed of the nuculids C. ventricosus and Ctenodonta, and the siphonate/siphon-bearing heteroconch L.
schucherti. The rest of the Snake Hill bivalves are
represented only by single specimens or by localized
concentrations. The Snake Hill bivalve assemblage is
comparable to the Nuculites–Colpomya Community of
Bretsky (1970) from the Upper Pulaski Formation of the
Lorraine Group in central and western New York. Although Colpomya Ulrich, 1894 (a nuculid) has not been
found at Snake Hill, both communities are noted for their
abundance of nuculids, and are considered analogous.
This nuculid-rich community overlaps elements of the
Dalmanella–Sowerbyella and Ambonychia–Modiolopsis Communities also described by Bretsky (1970, Fig. 15),
which he considered characteristic of near-shore environments. The small cap-shaped monoplacophoran Archinacella orbiculata (Hall, 1847) (see Ruedemann, 1912, Plate
7, Figs. 1–6) is also fairly common at Snake Hill and in the
Lorraine Group (Bretsky, 1970). Unlike much of the
Lorraine Group, large epifaunal bivalves are scarce at
Snake Hill. The only example is Ambonychia (Fig. 4U),
known from two specimens collected over the course of
several years.
Crinoid columnals and long stem fragments (up to
20 cm) are common throughout the section. Several
articulated specimens of Cincinnaticrinus varibrachialus Warn and Strimple, 1977 (Fig. 4W), were found near
the base of the section (Fig. 2, study area 3). Very rare,
disarticulated plates of the rhombiferan Carabocrinus
cf. radiatus Billings, 1857 (Fig. 4F) were found in float
near the base of the section.
A small, planar sandstone block likely derived from
turbiditic bedding in La A was found with eleven moldic
specimens of the edrioasteroid Edrioaster saratogensis
Ruedemann, 1912 (Fig. 4T) at study area 3 (Fig. 2).
Unfortunately, the substrate to which the edrioasteroids
were attached is not preserved. Because there are no shell
molds under these edrioasteroids or shells of sufficiently
large size to have served as attachment sites elsewhere in
the sublitharenite bed, these edrioasteroids are presumed
to have been attached to a submarine firm ground.
Orthocone cephalopods are present but rare. A large,
15-cm long specimen of Orthoceras cf. amplicameratum
Ruedemann, 1912 (see Ruedemann, 1912, Pl. 8, Fig. 11),
is preserved in the base of the first massive turbidite at
2 m (Fig. 3). Single specimens of the smaller orthocones
Walcottoceras Ulrich and Foerste, 1936 (Fig. 4Z), and
Orthoceras hudsonicum Ruedemann, 1912 (Fig. 4M),
were found at 35 m and 42 m, respectively.
The two archaeogastropods described by Ruedemann
(1912) as Cyclonema cushingi Ruedemann, 1912
(Fig. 4Y) and C. montrealense (Ruedemann, 1912, Pl.
7, Fig. 7) were found in this study. C. cushingi is quite
large, up to 5 cm across, and certainly belongs to the genus
Cyclonema as it has the same type of heavy turbiniform
shell. Cyclonema montrealense, however, does not have
the turbiniform shell characteristic of cyclonemid gastropods, which makes its assignment to Cyclonema questionable. It is typically much smaller and more conical
than C. cushingi. C. cushingi has only been found in float
debris near the base of the section; C. montrealense is
fairly common throughout the section.
Conspicuously absent from the Snake Hill section is
a significant trilobite fauna. The only identified species
A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
is Cryptolithus tesselatus Green, 1832 (Fig. 4E), which
is found sporadically throughout the section. Specimens
of C. tesselatus are almost always found as disarticulated fragments, but articulated specimens do occur
(Fig. 3, Fig. 4E). The only other trilobite species are
represented by a few disarticulated and broken fragments tentatively assigned to Isotelus DeKay, 1824
(Fig. 4F), and Flexicalymene Shirley, 1936 (Fig. 4G).
Cryptolithus is a common trilobite in many muddy,
Late Ordovician siliciclastic environments, including
the Lorraine Group (Bretsky, 1970) and the Martinsburg
Group (e.g., Wright et al., 1977). The unimportance of
trilobites and the great diversity of mollusks at Snake
Hill further suggest that this was a near-shore setting as
offshore trilobite-dominated paleocommunities appear
to be replaced shoreward by mollusk and brachiopodrich communities in the Paleozoic (Westrop et al., 1995;
Westrop and Adrain, 1998).
Bryozoan and coral material is very rare. The only
bryozoan specimen recovered is attributed to the genus
Batostoma Ulrich, 1882 (Fig. 4Q). This specimen was
collected from float debris near the base of the section
(Fig. 2, study area 3). A single rugosan coral was found
in the same study area.
5. Comparative sections
The New York State Museum Paleontological Collection houses large bulk samples collected by Rudolf
Ruedemann (1930) that include similar shelly faunas from
209
a number of localities around the Capital District of
eastern New York. These include the “Delaware and
Hudson (Railroad) Shop” in Watervliet (NYSM locality
3865), localities in Waterford (NYSM locality 2650), and
Green Island (NYSM locality 2651). In addition, there are
a number of very small collections from other localities on
the Hudson River in the Waterford and Green Island areas
(Fig. 5) that were apparently made from float debris.
Of the three major localities listed above, only the
Green Island locality was located (Fig. 5). The graptolites
Amplexograptus Elles and Wood, 1901–1918; Neurograptus Elles and Wood, 1901–1918; O. ruedemanni
Gurley, 1896; and Rectograptus amplexicaulis (Hall,
1847) place Green Island in the O. ruedemanni Chron,
and show a correlation with Snake Hill. The Green Island
locality is an 8 m section of moderately folded, poorly
fossiliferous, unbioturbated black shale with millimeterscale, fine-grained sand laminae. The bedrock dips 50°E.
All of the brachiopod and bivalve fossils are small and
often broken. They were all collected from 5-cm to 15cm thick, normally graded, medium-grained sandy turbidites. The depositional setting is considered to be
dysoxic–anoxic because the shale is pyritiferous and
barren of any trace fossils. Thus, the shelly fossils are
undoubtedly transported, and derived from more oxygenated habitats to the east.
The transported shelly fauna of Green Island contains
many of the same taxa (e.g., sowerbyellids, dalmanellids, nuculids, and Archinacella) as Snake Hill, and was
likely derived from the same biofacies. Ruedemann's
Fig. 5. Green Island locality (NYSM locality 2651), on river bank 200 meters west of abandoned railroad bridge. Watervliet (NYSM locality 3865)
and Cohoes (NYSM locality 2650) localities are inferred from Ruedemann (1930).
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A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
NYSM collections from the other Capital District
localities contain many of the same taxa as well. While
searching for Ruedemann’s Watervliet and Cohoes
localities, the exposed bedrock was examined. It consists
of unfossiliferous, laminated black shale deposited in
strongly dysoxic to anoxic conditions. Because the
Watervliet and Cohoes fossils are also poorly preserved
with no evidence of bioturbaion, and there is no evidence
of proximal lithofacies of the type seen at Snake Hill
(e.g., no sandstone-dominated intervals analogous to
Snake Hill), it is assumed that the faunas of these
localities are allochthonous as well, and that these
deposits are analogous to Green Island.
6. Discussion
A window into the easternmost Taconic foreland
basin depositional environments and biofacies is
supplied by a number of isolated and stratigraphically
short Upper Ordovician sections in east-central New
York. As might be expected, erosion of the emergent
Taconic orogen should have led to filling of the foreland
basin that developed with subduction and loading of the
east margin of Laurentia during collision with the
Taconic arc (e.g., Vollmer and Bosworth, 1984). As in
the arc-continental collision model presented by Rowley
and Kidd (1981, Fig. 4) for development of the Taconic
foreland basin, the Snake Hill Formation preserves a
slice of the shallow basin margin immediately adjacent
to the Taconic orogen, a record that has otherwise been
lost through subduction and erosion.
The Snake Hill Formation consists of proximal sandstone-dominated facies with a diverse, in situ epibenthic
fauna dominated by brachiopods and mollusks with
poorly represented trilobites, and a significant ichnofauna (ichnofabric index 2–4). A depositional history
featuring aggradational and progradational intervals resulted in a lithofacies succession that shoaled-up from
outer shelf shale-dominated deposits (La A) through
massively-bedded quartz arenite tidally-influenced deposits (La C), followed by deepening back to near-shore
amalgamated dark shale and hummocky cross-stratified
sublitharenites with a diverse shelly fauna. The rapid
disappearance of the Snake Hill Formation to the west
and occurrences of debrites and prolific tool marks in the
studied section indicate that the Snake Hill Formation
records a narrow shelf margin that grades rapidly downslope into deeper basinal facies (i.e., Dolgeville Formation and possibly some underlying, bounding strata of
the Utica shale; Figs. 1 and 6). This succession is now
demonstrated by graptolites to be older than the underlying dark grey to black graptoliferous shales of the
Saratoga Lake lowlands and surrounded by intensely
fractured shales. Thus, Snake Hill is herein interpreted as
a giant block in mélange bounded by shear zones or
faults.
Snake Hill is best referred to as parautochthonous
because it is clearly not a part of the Lower Cambrian–
Lower Ordovician slate-dominated Taconic allochthon.
The Upper Ordovician siliciclastics adjacent to the master
thrust have long been regarded as a province distinct from
the Taconic allochthon, that has only been displaced a few
kilometers relative to the 10s of kilometers of the
allochthon (e.g., Fisher, 1977, 1984; Rowley and Kidd,
1981; Landing et al., 2003). The displacement of this
parautochthonous province has been attributed to the
overriding westward movement of the allochthon above it
(e.g., Rowley and Kidd, 1981; see Fig. 6).
The Snake Hill Formation is distinct from the other
units with a Snake Hill-type fauna, such as Green Island
and a number of Ruedemann's (1912, 1930) other
localities. Unlike Snake Hill, the other Ruedemann
localities record comparatively downslope, shale-dominated, minimally oxic to dysoxic facies in which the
shelly fauna appears to be allochthonous with no ichnofauna. Green Island and other Capital District
localities characterized by brachiopods and bivalves
known from Snake Hill thus serve to bridge the sandstone-dominated lithofacies at Snake Hill with coeval
deeper-water deposits of the Dolgeville Formation laid
down further west in the Mohawk Valley (Figs. 1 and 6).
Because the Green Island-type facies is shale-dominated
with a near-absence of sandstone, these deposits should
not be included in the Snake Hill Formation.
Proximal deposits within the Lorraine are similar to
shallower facies of the Snake Hill Formation. Both units
represent proximal siliciclastic deposition associated
with westward Taconian foreland basin progradation.
These lithologic similarities include identical color and
texture of sandstone, and identical moldic preservation
of shelly fossils; relationships that are inherent to units
representing comparable depositional environments. As
discussed above, the Snake Hill fauna is comparable to
the Nuculites–Colpomya Community of Bretsky (1970).
In both regions, this faunal assemblage contains an
abundance of nuculid bivalves, dalmanellid and sowerbyellid brachiopods, sparse trilobites and graptolites, a
dearth of large epifaunal bivalves (i.e., Ambonychia,
Modiolopsis), and presence of the monoplacophoran
Archinacella.
It is important to note that this easternmost record of in
situ shelly faunas at Snake Hill demonstrates an early
occurrence of mollusk-rich communities. These communities tracked and moved westward with the filling of the
A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
211
Fig. 6. Genesis of Snake Hill section. A — Deposition of coarse, neritic (mid- to inner shelf) succession [units La (A)–La (C)] in a syntectonic
progradational setting west of the advancing Taconic allochthon; (G) Green Island-type facies; (T) Trenton shelf. B — Incorporation of Snake Hill
Formation as a fault-bounded slice into the tectonic mélange below the overriding, westward-advancing allochthon. Intense shearing of subjacent
progradational deposits during movement of the allochthon has displaced the Snake Hill succession west of its original site of deposition, and
rendered it parautochthonous.
Taconic foreland basin to be found in the younger Late
Ordovician Lorraine Group of central and western New
York State. The Martinsburg Group (Upper Caradocian) of
Pennsylvania also contains many analogous taxa (see
Wright et al., 1977, Table 1) that lived in similar proximal
depositional sites adjacent to the emergent Taconic orogen.
Flute casts, tool marks, uni-directional ripples, and
oriented fossils show an apparent east–west current
flow direction in La A, and an apparent alternation
between north–south and east–west currents in La B.
However, the enigmatic ESE orientation of the fold axis
at the northwest corner of Snake Hill suggests that the
section had been rotated as fold axes in this region
should roughly parallel the strike of the NNE-trending
Taconic master thrust (“Emmons Line”) as well as that
of subsidiary faults to the west of it. Progressive change
in the orientation of regional fold axes in eastern New
York has been documented by Bosworth and Vollmer
(1981), and Vollmer and Bosworth (1984). These note
that early fold axes west of the allochthon trend NNE.
However, later fold axes adjacent to the Taconic allochthon have an ESE orientation. Therefore, a 90°
clockwise rotation about strike of the section must be
accounted for in future studies involving paleocurrents.
7. Conclusions
The Snake Hill Formation, which displays proximal
neritic facies of the early Late Ordovician (O. ruedemanni
Chron= middle Caradocian) within the parautochthonous
structural belt west of the Taconic Allochthon, records the
earliest known and easternmost shallow shelf facies of the
Taconic synorogenic progradational succession in New
York State. This succession is preserved below the now-
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A.M. English et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 242 (2006) 201–213
eroded master thrust of the Taconic allochthon as a giant
parautochthonous block in mélange adjacent to the master
thrust in the Hudson valley (Fig. 6). The heterolithic sequence at Snake Hill is best regarded as the type section of
a unit designated “Snake Hill Formation,” rather than
Snake Hill “shale”, “flysch,” or “mélange.” The Snake Hill
lithofacies were deposited in the Taconic foreland basin
and cannot be considered a “facies” of the allochthonous
Austin Glen Formation (fide Kidd et al., 1995).
Application of the designation “Snake Hill Formation”
at other localities in the Taconic foreland basin will
continue to remain problematical. Intense tectonic deformation, limited exposures in eastern New York, the
longstanding “Taconic problem” [by which older black
shale–sandstone intervals within the allochthon are
regularly confused with similar younger autochthonous
and parautochthonous lithologies under and immediately
west of the master thrust; e.g., Rickard and Fisher, 1973],
and the rapid westward, downslope transition of proximal
sandstones into more distal basin slope facies that
intergrade into basinal deposits (Dolgeville Formation
and topmost lower Utica Shale succession), all make
recognition of the Snake Hill formation difficult outside of
its type section.
Acknowledgments
We thank C.E. Mitchell of the State University of New
York at Buffalo for identifying the Snake Hill and Green
Island graptolites. L. Van Aller Hernick, T. Beblowski, R.
Barber, and J.B. Skiba of the New York State Museum and
N. Prechel and A. Domst of the State University of New
York at Fredonia assisted with photo preparation. F.
Wolpert lent dental equipment used to prepare the cast for
Fig. 4T. We also thank three anonymous reviewers who
helped to improve this article.
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