Bedrock Geology of New York City: More than m.y. of geologic history Bedrock Geology of New York City: More than m.y. of geologic history

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Zircons from the Yonkers gneiss in Westchester County have been dated at Ma Rankin and others,compatible with Late Neoproterozoic rifting elsewhere. Grey layers are siliceous, and may represent beds of chert. The Manhattan Schist is an allochthon, a detached sheet of rock Figure 2and its age and origin have been unclear.

Climate was warm, and a carbonate bank flourished in the NYC area. Devonian metamorphism is pervasive in much Dating queens ny NYC, and is responsible for the muscovite content of schists Stops 3,5 and tremolite in the marbles Stop 2.

Here in Van Cortlandt Park, most of the Fordham consists of felsic plagioclase-quartz-biotite-garnet-hornblende-K-feldspar gneiss.

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These sediments, now metamorphosed, make up the Walloomsac Schist Figures 24C. Next to a footpath, we can see one of the amphibolite bodies of the Manhattan Schist. In the northeastern Manhattan Prong, we find schists sitting on Fordham Gneiss that strongly resemble the Manhattan Schist. 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.

Rhythmically bedded sequences of gneiss and schist occur; these are interpreted as turbidites, deposits from sediment-laden flows spewed into deep water.

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We have informally named this stratigraphic unit the Ned Mountain formation. By Middle Ordovician time, the ocean separating North America from an exotic volcanic-arc terrane had closed. Support for Late Neoproterozoic Iapetan rifting: Inwood Marble with siliceous layer in center.

We see only a fragment of the Hartland that once existed Figure 1the remainder long removed by erosion. For your convenience, the five field trip stops are shown on the map Figure 1and Table 1 and Figure 4 tie the stops to specific geologic events.

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. This rock was only partially recrystallized during Devonian time, and largely preserves its Taconian fabric.

Around then, thousands of small, two-mica granitic bodies intruded into NYC metropolitan region Stop 5. Subduction occurred eastwards, beneath this arc.

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Click on image for larger image. The gneisses are fairly massive, though layering is locally visible. The Walloomsac, therefore, is the product of the earliest phase of the Taconian orogeny.

Several recent U-Pb Dating queens ny of zircons from metaigneous rocks of the Fordham indicate primary crystallization in the time period Ma to 1.

During deformation, the amphibolites were less ductile than the surrounding schists and gneisses, and many were broken into boudins.

Geological Society of America Abstracts with Programs, v. 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.

These rocks have not been radiometrically dated, but are interpreted Arlington tx speed dating On the northwestern side of North Twin Island, graded bedding is preserved, allowing us to deduce the direction of stratigraphic tops.

The Manhattan Schist here is predominantly a massive quartz-plagioclase-biotite-garnet gneiss, and with varying amounts of muscovite, sillimanite, staurolite, and kyanite. The oldest rocks will be Fordham Gneiss, dating from c.

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These irregular degrees of retrogression are characteristic of the Devonian metamorphic event.