TheTaconic orogeny was a mountain building period that ended 440 million years ago (Ma) and affected most of modern-dayNew England. A greatmountain chain formed from easternCanada down through what is now thePiedmont of theeast coast of the United States. As the mountain chain eroded in theSilurian andDevonian periods, sediment spread throughout the present-dayAppalachians and midcontinental North America.[1]
Beginning inCambrian time, about 550 Ma, theIapetus Ocean began to close. 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.[2] In this manner, shallow-water carbonatedeposition that had persisted on thecontinental shelf margin through late Cambrian into earlyOrdovician time, gave way to fine-grainedclastic deposition and deeper water conditions during the middle Ordovician. In this period aconvergent plate boundary developed along the eastern edge of a small island chain. Crustal material beneath the Iapetus Ocean sank into the mantle along asubduction zone with an eastward-dipping orientation.[2]Dewatering of the down-going plate led to hydration of theperidotites in the overlyingmantle wedge, lowering their melting point. This led to partial melting of the peridotites within the mantle wedge producing magma that returned to the surface to form the offshoreTaconic (or Bronson Hill) island arc.
By the Late Ordovician, this island arc had collided with the North American continent. Thesedimentary andigneous rock between the land masses were intenselyfolded andfaulted and were subjected to varying degrees ofmetamorphism. This was the final episode of the Taconic orogeny.[1]Cameron's Line is thesuture zone that is modern-day evidence of the collision of the island arc and the continent.[3] Cameron's Line winds southward out ofNew England into westernConnecticut and passes through southernNew York acrossthe Bronx, following the general trend of theEast River. It extends beneath sedimentary cover onStaten Island and southward beneath thecoastal plain of New Jersey. In general,basement rocks to the west of Cameron's Line are regarded asautochthonous, meaning that they have not been significantly displaced bytectonic processes. The rocks to the west of Cameron's Line includemetamorphosed sedimentary material originally comprising ancient continental slope, rise, and shelf deposits. The rocks to the east of Cameron's Line areallochthonous, which means they have beenshoved westward over autochthonous basement rocks on the order of many tens or even hundreds of kilometers. These rocks were originally deposited as sediments in a deep water basin. Cameron's Line represents the trace of a subduction zone that ceased when the Taconic island arc collided with, and becameaccreted onto, the eastern margin of North America. Many of the rocks east of Cameron's Line were once part of the floor of the Iapetus Ocean.[2]
When the Taconic orogeny subsided during the late Ordovician (about 440 Ma), subduction ended, culminating in the accretion of theIapetus 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 thePiedmont of eastern North America. The expanded continental margin gradually stabilized.Erosion continued to strip away sediments from upland areas. Inland seas covering the midcontinent gradually expanded eastward into theNew York Bight region and became the site of shallow clastic and carbonate deposition. This tectonically quiet period persisted until the late Devonian (about 360 Ma) when the next period of mountain-building began, theAcadian orogeny.[1]
In the southern Appalachians of Alabama, Georgia, and North Carolina, the Taconic orogeny was not associated with collision of anisland arc with ancient North America (Laurentia). Geologists working in these areas have long puzzled over the "missing" arc terrane typical of Taconic-aged rocks in New England and Canada.[4] Instead, the Iapetus margin of this part of Laurentia appears to have faced aback-arc basin during the Ordovician, suggesting that Iapetus oceanic crust was subducted beneath Laurentia—unlike the New England and Canadian segments of the margin, where Laurentia was on the subducting plate.[5][6]
In contrast to the Ordovician geologic history of New England, rocks in Alabama, Georgia, Tennessee, and North Carolina—including those of the Dahlonega gold belt[7] (Georgia and North Carolina), Talladega belt[8][9] (Alabama and Georgia), andeastern Blue Ridge (Georgia, Tennessee, and North Carolina)—are not typical of avolcanic arc in its strictest sense.[10] Instead, these rocks have geochemical and other characteristics typical of back-arc basins, which form behind the volcanic arc on the overriding plate.[5][6] The presence of these early-middle Ordovician (480 - 460 million year old) back-arc basin rocks in direct or faulted contact with rocks of the Laurentian shelf andslope-rise in the southern Appalachians suggests they were built on the margin of Laurentia, beyond the edge of the continental shelf-slope break.[9]
In the southern Appalachians, the Ordovician Laurentian margin probably resembled that seen in the modernSea of Japan, with the continental mainland separated from a volcanic arc by a narrow, "marginal" seaway. Other lines of evidence supporting a back-arc, Sea of Japan-styletectonic model for the Taconic orogeny in the southern Appalachians include mixing of Ordovician andGrenville (ca. 1 billion year old)detritalzircons in metamorphosed sedimentary sequences, and interlayering of metamorphosed Ordovician volcanic rocks with sedimentary rocks derived from the Laurentian margin.[5][6]
It has been suggested that the coevalFamatinian orogeny in westernGondwana (South America) is the "southward" continuation of the Taconic orogeny.[note 1] This has been explained by adding that Laurentia could havecollided with western Gondwana in early Paleozoic times during theclosure of the Iapetus Ocean.[11] According to this view theCuyaniaterrane would be an allochthonous block of Laurentian origin that was left in Gondwana. But such views are challenged since Cuyania is alternatively suggested to have drifted across Iapetus Ocean as amicrocontinent starting in Laurentia and accreting then to Gondwana. A third model claims Cuyania is para-autochthonous and arrived at its current place by strike-slip faults starting not from Laurentia but from Gondwana.[12]
As the Taconic orogeny subsided inearly Silurian time, uplifts and folds in theHudson 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 SilurianShawangunk Conglomerate, a massive,ridge-forming quartzsandstone andconglomerate formation, which restsunconformably on a surface of older gently- to steeply-dipping pre-Silurian agestrata throughout the region. This ridge of Shawangunk Conglomerate extends southward from the Hudson Valley along the eastern front of theCatskills. It forms the impressive caprock ridge of theShawangunk Mountains west ofNew Paltz. To the south and west it becomes the prominent ridge-forming unit that crops out along the crest ofKittatinny Mountain in New Jersey.[2]
Through Silurian time, the deposition of coarsealluvial sediments gave way to shallow marine fine-grained muds, and eventually to clear-watercarbonate sediment accumulation withreefs formed from the accumulation of calcareous algae and the skeletal remains ofcoral,stromatoporoids,brachiopods, and other ancient marine fauna. The episodic eustaticrise 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.[2]
This article incorporatespublic domain material fromValley and Ridge Province.United States Geological Survey. This article incorporatespublic domain material fromThe Highlands Province.United States Geological Survey.