- Queens College
- Flushing, NY 11367-1597
- e-mail: pbrock1@nyc.rr.com
INTRODUCTION.
Today
we will see rocks that record critical events in New York City’s long
geologic history. The oldest rocks will be Fordham Gneiss, dating from c. 1.1
Ga (billion years); the youngest, muscovite-bearing granite and pegmatite
with ages of ~380 Ma (million years). For your convenience, the five field
trip stops are shown on the map (Figure 1), and Table 1 and
Figure 4 tie the
stops to specific geologic events.
For
over twenty years we have studied the Manhattan Prong, principally in
Westchester County. Field geology- the mapping of rocks and geologic
structures, together with petrography- has been the springboard of our work,
though we have been supplementing these with electron-microprobe studies of
mineral assemblages, whole-rock geochemical analyses, and radiometric age
determinations. In recent years we have proposed new interpretations of the
metamorphic, stratigraphic, and tectonic histories of the Manhattan Prong
(see, for instance, our abstracts for the Long Island Geologists conferences
in 1998, 1999, and 2001). Here, we provide a brief outline of the geologic
history of the region, which is also summarized on Table 1, “Simplified Geologic
History of the New York City Area”.
BRIEF GEOLOGIC HISTORY
Middle Proterozoic time. The Fordham Gneiss dominantly consists of
metamorphosed igneous rocks, which range from felsic, through mesocratic, to
mafic in composition. Our geochemical studies show that most of these rocks
have compositions indicative of a volcanic-arc origin. Several recent U-Pb
studies of zircons from metaigneous rocks of the Fordham indicate primary
crystallization in the time period 900 Ma to 1.2 Ga years. From these data,
we can envision that ~ one billion years ago an active continental margin
existed here. Next, a major collision took place, triggering the late (Ottawan) phase of the Grenvillian
orogeny; the rocks were deformed, deeply
buried, and underwent high-grade (granulite-facies) metamorphism. In post-Grenville time (after ~ 900 Ma), the crust in the NYC area consisted of
the compositionally varied, medium- to coarse-grained rocks now known
collectively as the Fordham Gneiss (Stop 4).
Late Neoproterozoic time. After a quiet period that lasted >400 million
years, the ancient supercontinent (formed by Grenvillian collisions) began to
split up (Figure 4A). From Quebec to Virginia, rift basins formed and
basaltic rocks erupted at c. 550-570 Ma (Late Neoproterozoic). It was
formerly believed that this rifting event “missed” NYC; Late Neoproterozoic
strata were not recognized in the traditional stratigraphic column (Figure
2). However, over the past 12 years, we have been mapping a complex package
of rock that lies, stratigraphically, over the top of the Fordham basement
and below the Cambro-Ordovician Inwood Marble, and whose age is therefore
constrained as Neoproterozoic. We have informally named this stratigraphic
unit the Ned Mountain formation. The Ned Mountain formation is lithologically
varied, and is divided into several members, illustrated on Figure 3.
Although
the Ned Mountain formation was originally defined on the basis of its
stratigraphic relationships and its particular lithologic characteristics, we
have been testing our interpretation by whole-rock chemical analyses and
zircon age determinations. We have found that all the components of the
Ned Mountain formation are linked together by the special suite of mafic
rocks (amphibolites) that they contain; these amphibolites have many chemical
characteristics in common, which serve to distinguish them from the Fordham
Gneiss and from most other rock units in the region. They share these
characteristics, however, with rift-related Late Neoproterozoic-aged
metabasites found elsewhere in the Appalachians. All of these Late
Neoproterozoic mafic rocks are chemically similar to basalts found on oceanic
islands, such as Iceland. Basalts on these islands are thought to arise from
deep-seated mantle plumes, rather than from shallow-level depleted mantle of
the kind that fuels volcanism at mid-oceanic ridges. We have proposed,
therefore, that a mantle plume prompted rifting along eastern North America
during Late Neoproterozoic time. This plume, we suggest, eventually opened
the Iapetus Ocean, in the same way that the Iceland mantle plume triggered
the opening of the northern Atlantic Ocean.
We
interpret the felsic rocks long known as Yonkers gneiss as part of the Ned
Mountain formation. Yonkers gneiss was previously interpreted as a component
within the Fordham Gneiss (Figure 2). However, our recent work has shown that
Yonkers is everywhere stratigraphically (or structurally) distinct from the
Fordham Gneiss (Figure 3), that it is more regionally extensive than
previously realized (Stop 1),
and that it must have a blanket-like overall geometry, commonly being only a
few hundred feet thick but cropping out for tens of miles along strike. Yonkers
has a concordant, stratiform character, and therefore must be a metavolcanic
(not plutonic) body. Zircons from the Yonkers gneiss in Westchester County
have been dated at 563 Ma (Rankin and others, 1997), compatible with Late
Neoproterozoic rifting elsewhere.
Bimodal
(mafic+felsic) igneous rock suites like the Yonkers/amphibolite association
are characteristic of continental rift zones. Our chemical data also show
that the Yonkers is an A-type granitoid, a rock type strongly associated with
continental rifting, and that it is compositionally distinct from the felsic
components of the Fordham Gneiss and from Paleozoic granites of the Manhattan
Prong.
In the northeastern Manhattan Prong, we find
schists sitting on Fordham Gneiss that strongly resemble the Manhattan
Schist. We assigned these rocks to the Ned Mountain formation (Metawacke
member; Figure 3), and now have a zircon date confirming their Late
Neoproterozoic age (at c.570 Ma). The Manhattan Schist is an allochthon, a
detached sheet of rock (Figure 2), and its age and origin have been unclear. Our
new data show that amphibolites in the Manhattan Schist are chemically
identical to those found in the Ned Mountain formation, confirming that the
two units are related to each other. We interpret the Ned Mountain formation
and Manhattan Schist as age-equivalent, correlative units, and infer that the
Manhattan Schist (Stop 3)
represents a deeper-water facies than most of the Ned Mountain formation (Figure
3).
Amphibolite
composition has also help untangle confusion regarding the “Hartland formation”.
The Hartland is widely interpreted as exotic to ancestral North America,
accreted during the Taconian orogeny in Ordovician time. We agree with this
interpretation, but our geochemical investigation has revealed a problem with
earlier mapping. We have found that the “true” exotic Hartland formation
contains amphibolites having very distinct chemical characteristics, showing
affinity with either volcanic arc basalt or with mid-ocean ridge basalt. “True”
Hartland of this kind crops out from Pelham Bay Park in the Bronx, up north
through eastern Westchester County and into Connecticut. However, west of
this true, “Pelham Bay-type” Hartland, there are rocks which have been called
Hartland, but that contain amphibolites of the Ned Mountain/Manhattan Schist
variety. We call these rocks the “Bronx Zoo-type strata”, and interpret them
as North American strata of Late Neoproterozoic age (Figures 3,
4A).
Cambrian to Early Ordovician time. Late Neoproterozoic
rifting eventually led to the opening of the Iapetus Ocean. New, stable
continental margins developed (Figure 4B). During Cambrian to Early
Ordovician time, a marine incursion transformed much of eastern North America
into shallow-water continental shelf environment. Climate was warm, and a carbonate
bank flourished in the NYC area. Sediments deposited during this period are
now preserved as the Inwood Marble (Stop 2).
Middle to Late Ordovician time. By Middle Ordovician time, the ocean
separating North America from an exotic volcanic-arc terrane had closed. Subduction
occurred eastwards, beneath this arc. This exotic terrane began to ride up
over North American crust, depressing it; in response, a deepening basin
formed along the North American margin. Poorly oxygenated, sulfidic, carbonaceous
(graphitic) sediments were deposited in this trough. These sediments, now
metamorphosed, make up the Walloomsac Schist (Figures 2,
4C). The Walloomsac,
therefore, is the product of the earliest phase of the Taconian orogeny. Only
a little Walloomsac Schist crops out in NYC (Figure 1).
As
convergence continued, the North American margin was fragmented into sets of
westwards-directed thrust slices. Near the top of this structural pile,
sheets of Late Neoproterozoic Manhattan Schist and “Bronx Zoo-type strata”
were emplaced westwards, over the Middle Ordovician Walloomsac Schist
(Figures 2, 3, 4C). Shortly later, our data show that rocks of the Manhattan
Prong (including the young deposits of the Walloomsac) were buried to depths
of at least 40 km, and possibly more.
The
overlying Hartland terrane must account for a large proportion of the 12 kbar
pressure experienced by the Manhattan Prong (Figure 4C). We see only a
fragment of the Hartland that once existed (Figure 1), the remainder long removed
by erosion. The (“Pelham Bay- type”) Hartland formation in NYC consists of
well-bedded quartz-feldspar gneisses and schists (originally turbidite
deposits), amphibolite, and thin marbles (Stop
5). These rocks were deposited in a deep-marine setting in
front of the volcanic arc, possibly in an accretionary prism. These rocks
have not been radiometrically dated, but are interpreted as (?) Ordovician in
age.
During
the later stages of the Taconian orogeny, pressure decreased somewhat
(overburden reduced to some 26 km), but temperature reached an extraordinary
peak. In northern Westchester County, southern Putnam County, and adjacent
Connecticut, we have found evidence of >850 °C, the highest-temperature
regional metamorphism in the Taconian orogen. We have not done electron
microprobe studies of rocks from NYC, but they show petrographic evidence of
an early high-pressure/high temperature phase (K-feldspar + kyanite) followed
by lower pressures and/or higher temperatures (K-feldspar + sillimanite). We
infer that the two adjacent areas experienced comparable metamorphic
histories.
The
last stage of the Taconian orogeny included upright, accordion-style folding.
In the NYC area, these late folds are generally shallow-plunging, so that
lithologic units crop out in long, narrow belts (Figure 1). But the map
pattern is complex, because we are seeing a folded stack of thrust sheets
that have irregular, truncated boundaries against each other.
Devonian time. After the Taconian orogeny, the next event to
leave a distinct imprint on the NYC area occurred at ~ 380 Ma. Around then,
thousands of small, two-mica granitic bodies intruded into NYC metropolitan
region (Stop 5). Mya Mya Than
has dated three of these granite bodies as part of her thesis work. The
granites tend to cluster in groups, and are often associated with shearing. The
granites brought vast amounts of water with them, largely rehydrating the
gneisses and schists of the Manhattan Prong. The best-developed Devonian
mineral assemblages crystallized at ~ 550°C and 20 km depth, a far cry from
the much-higher-grade Taconian event. Devonian metamorphism is pervasive in
much of NYC, and is responsible for the muscovite content of schists (Stops 3,5) and tremolite in the marbles (Stop 2). Taconian-aged assemblages survive
in enclaves.
STOP DESCRIPTIONS
Please note: All stops
will be in NYC parks. Do not hammer outcrops.
Stop 1. “Ravenswood Granodiorite” Queensbridge Park, Queens.
The “Ravenswood Granodiorite” is the only basement unit to crop out on Long
Island, and here beneath the Queensboro Bridge we see one of the largest
exposures. The name “Ravenswood” has been restricted to an entity only a few
miles in length, occurring along the western margin of Queens. However, our
recent studies firmly connect these rocks to a lithologic unit on the
mainland: they are indistinguishable, in fact, from the Yonkers gneiss, a
member of the Late Neoproterozoic Ned Mountain formation (Figure
3).
The
“Ravenswood”, like Yonkers gneiss, is a hornblende-biotite-garnet bearing
plagioclase-K-feldspar-quartz gneiss. The unit is massive and homogeneous,
rock texture is medium-grained, and color is pale pinkish to grayish, with
darkness relating to the abundance of hornblende and biotite. The Geologic
Map of New York State (1970) and various previous workers have considered the
“Ravenswood” to be Lower Paleozoic in age. We tested the identity of the
“Ravenswood” in two ways: by its chemical composition, and by its radiometric
age. We found that the “Ravenswood” is an A-type granitoid, compositionally
identical to the Yonkers gneiss and distinct from every other unit that
occurs in the area. We had zircons from “Ravenswood” dated: they were found
to be 555± 20 Ma, indistinguishable
from the 563±3 Ma that Rankin and
others (1997) obtained from the Yonkers gneiss in Westchester County. We
conclude that “Ravenswood Granodiorite” is not a valid, separate unit; it
consists, in fact, of the Long Island exposures of the Yonkers gneiss.
Stop 2. Inwood Marble at Isham Park, Manhattan Island. The Inwood Marble consists of white to grayish beds of dolomitic and dolomite-calcite marbles. It was deposited, over the Late Neoproterozoic Ned Mountain formation, on a stable continental shelf during Cambrian to Early Ordovician time (Figures 3, 4B). Grey layers are siliceous, and may represent beds of chert. The marbles are rich in magnesium, due to the dolomite content of their protolith, but poor in iron; hence, they contain very pale, Mg-rich phlogopite mica, diopsidic pyroxene, and (standing up on the surface of the outcrop)- white clumps of tremolite. The tremolite is a product of Devonian retrogressive metamorphism, which left a strong imprint in this area. Tight folding can be seen at this outcrop.
Inwood Marble with siliceous layer in center.
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Stop 3. Manhattan Schist, Inwood Hill Park,
Manhattan Island. The Manhattan Schist here is predominantly a massive
quartz-plagioclase-biotite-garnet gneiss, and with varying amounts of
muscovite, sillimanite, staurolite, and kyanite. The late Taconian metamorphic
grade was K-feldspar + sillimanite, but during Devonian retrogression,
muscovite, staurolite, and kyanite grew at the expense of sillimanite +
garnet + K-feldspar + biotite. The outcrop we see by the footpath displays
coarse garnet porphyroblasts in a dark, biotite-rich matrix, and muscovite is
fine-grained and subordinate. This rock was only partially recrystallized
during Devonian time, and largely preserves its Taconian fabric. Elsewhere,
however, retrogression was more complete, and muscovite has grown to dominate
the appearance of the rock. We will not take the time to find Devonian-age
muscovite schist in outcrop, but several rocks placed alongside the footpaths
are representative of the retrograded rocks. These irregular degrees of retrogression
are characteristic of the Devonian metamorphic event.
The Manhattan Schist is an allochthon, emplaced during the Taconian orogeny. We correlate Manhattan Schist with the Late Neoproterozoic Ned Mountain formation (Figure 3, 4A) primarily because of (a) its resemblance to the 570 Ma-year-old Ned Mountain Metawacke member, and (b) the presence of amphibolites chemically indistinguishable from those found in the Ned Mountain formation (and other rift-related Late Neoproterozoic units). Next to a footpath, we can see one of the amphibolite bodies of the Manhattan Schist.
Stop 3. Fordham Gneiss, Van Cortlandt Park, the
Bronx. The Fordham Gneiss comprises the oldest rocks in the NYC area, making
up the already-ancient basement that rifted during Late Neoproterozoic time
(Figure 3, 4A). Here in Van Cortlandt Park, most of the Fordham consists of
felsic plagioclase-quartz-biotite-garnet-hornblende-K-feldspar gneiss. The
gneisses are fairly massive, though layering is locally visible. In addition to
felsic rocks, we will see a small mafic layer, a subordinate component of the
Fordham here. Chemical analyses tell us that Fordham leucocratic gneisses
like these have volcanic-arc affinities; we can be reasonably confident that
gneisses here originated in a continental-arc environment about a billion
years ago. Most of these gneisses are medium-grained, but there are some
fine-grained high-strain zones, probably dating to the Taconian orogeny. Growth
of epidote and blue-green hornblende in these rocks reflects Devonian
retrogressive metamorphism.
Discussion regarding Fordham Gneiss |
|
Fold in lichen covered Fordham Gneiss | |
Folded veins in Fordham Gneiss |
Stop 4. “Pelham Bay-type” Hartland formation, Pelham Bay Park, the
Bronx. Now we have stepped off the old North American continent, onto the
exotic terrane that arrived ~450 Ma ago (Figure 4C). The Hartland formation
here contains a variety of rock types, including quartz-feldspar gneiss,
biotite-sillimanite schist, amphibolite, and marble. Rhythmically bedded
sequences of gneiss and schist occur; these are interpreted as turbidites,
deposits from sediment-laden flows spewed into deep water. On the
northwestern side of North Twin Island, graded bedding is preserved, allowing
us to deduce the direction of stratigraphic tops. Thanks to the excellent
bedding, we can observe Taconian-age isoclinal folding of the turbiditic
sequences on both the Twin Islands, as well as local truncations along shear
zones.
Amphibolites
in the “Pelham Bay-type” Hartland show either volcanic-arc or ocean-floor
chemical affinities. During deformation, the amphibolites were less ductile
than the surrounding schists and gneisses, and many were broken into boudins.
On North Twin Island, we can see that thin white-and-pink banded marbles are
associated with many amphibolite beds. Scapolite is present alongside.
The
metamorphic history of the Hartland formation here parallels that of the
Manhattan Prong. Ordovician and Devonian metamorphism both affected the
Pelham Bay area. The earliest-known assemblage in the pelitic rocks is
K-feldspar + kyanite, which is indicative of both high pressure and high
temperature. This early assemblage was replaced by K-feldspar + sillimanite,
indicating a drop in pressure, rise in temperature- or both. Partial melting
of the schists and gneisses produced the abundant quartz-feldspar leucosomes.
Much later, during Devonian time, retrogressive metamorphism occurred –and
muscovite grew to replace sillimanite, in the same irregular manner as in
Inwood Hill Park. Epidote now present in many Pelham Bay amphibolites
probably was crystallized at the same time.
Here at Pelham Bay Park there are several intrusions belonging to the ~380 Ma-old Devonian granitic suite. They occur as crosscutting dikes, many with straight, planar boundaries, and all unaffected by the tight folding that deformed Taconian-age granitic leucosomes. These Devonian bodies tend to be pegmatitic, and may contain books of muscovite and sprays of tourmaline. A narrow contact metamorphic (or metasomatic) margin is visible around some of these granites. Thousands of similar granites are present in the NYC/Westchester region. These granites derived from very wet magmas; as these granites crystallized, water-rich fluids percolated through the surrounding rocks, inducing the retrograde metamorphism we observe all around.
ACKNOWLEDGEMENTS
Our
thanks go to Grow-Perini-Skanska, joint-venture contractors who funded our
studies of City Water Tunnel 3, and to Charles Merguerian, who mapped the
Queens Tunnel and brought the rocks to our attention. G.N. Hanson took the
photos shown here during the field trip on Oct. 27, 2001.
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and Engineering Geologic Maps of New York County and Parts of Kings and
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