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-building 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.

Aftermath of the Taconic 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.

ee also

* Ebenezer Emmons, the geologist who first described the Taconic system

External links

*http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm (public domain source)
* http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html (public domain source)