Taconic orogeny
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Illustration of the Taconic orogeny |
The
Taconic orogeny was a great mountain building period that perhaps had the greatest overall effect on the geologic structure of basement rocks within the
New York Bight region. The effects of this
orogeny are most apparent throughout
New England, but the sediments derived from mountainous areas formed in the northeast can be traced throughout the
Appalachians and midcontinental North America.
Beginning in
Cambrian time, about 550 million years ago, the
Iapetus Ocean began to grow progressively narrower. The weight of accumulating sediments, in addition to compressional forces in the crust, forced the eastern edge of the North American continent to fold gradually downward. In this manner, shallow-water carbonate deposition that had persisted on the
continental shelf margin through Late Cambrian into Early
Ordovician time, gave way to fine-grained clastic deposition and deeper water conditions during the Middle Ordovician. Sometime during this period a
convergent plate boundary developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean sank into the mantle along a
subduction zone with an eastward-dipping orientation. Partial melting of the down-going plate produced magma that returned to the surface to form the offshore Taconic island arc. By the Late Ordovician, this island arc had collided with the North American continent. The sedimentary and igneous rock between the land masses were intensely folded and faulted, and were subjected to varying degrees of intense metamorphism. This was the final episode of the long-lasting mountain-buliding period referred to as the Taconic Orogeny.
When the Taconic Orogeny subsided in the New York Bight region during Late Ordovician time (about 440 million years ago), subduction ended, culminating in the accretion of the
Iapetus Terrane onto the eastern margin of the continent. This resulted in the formation of a great mountain range throughout New England and eastern Canada, and perhaps to a lesser degree, southward along the region that is now the Piedmont of eastern North America. The newly expanded continental margin gradually stabilized. Erosion continued to strip away sediments from upland areas. Inland seas covering the Midcontinent gradually expanded eastward into the New York Bight region and became the site of shallow clastic and carbonate deposition. This tectonically-quiet period persisted until the Late
Devonian time (about 360 million years ago) when the next period of mountain-building began, the
Acadian orogeny.
As the Taconic Orogeny subsided in early
Silurian time, uplifts and folds in the
Hudson Valley region were beveled by
erosion. Upon this surface sediments began to accumulate, derived from remaining uplifts in the New England region. The evidence for this is the Silurian Shawangunk Conglomerate, a massive, ridge-forming quartz
sandstone and
conglomerate formation, which rests
unconformably on a surface of older gently- to steeply-dipping pre-Silurian age
strata throughout the region. This ridge of Shawangunk Conglomerate extends southward from the Hudson Valley along the eastern front of the
Catskills. It forms the impressive caprock ridge of the
Shawangunk Mountains west of
New Paltz (town), New York. To the south and west it becomes the prominent ridge-forming unit that crops out along the crest of
Kittatinny Mountain in New Jersey.
Through Silurian time, the deposition of coarse
alluvial sediments gave way to shallow
marine fine-grained muds, and eventually to clear-water
carbonate sediment accumulation with
reefs formed from the accumulation of calcareous algae and the skeletal remains of
coral,
stromatoporoids,
brachiopods, and other ancient marine fauna. The episodic eustatic rise and fall of sea level caused depositional environments to change or to shift laterally. As a result, the preserved faunal remains, and the character and composition of the sedimentary layers deposited in any particular location varied through time. The textural or compositional variations of the strata, as well as the changing fossil fauna preserved, are used to define the numerous sedimentary formations of Silurian through Devonian age preserved throughout the region.
*
Ebenezer Emmons, the geologist who first described the Taconic system
*http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm (public domain source)
* http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html (public domain source)
