New England is a region in the North Eastern United States consisting of the states Rhode Island, Connecticut, Massachusetts, New Hampshire, Vermont, and Maine. Most of New England consists geologically of volcanic island arcs that accreted onto the eastern edge of the Laurentian Craton in prehistoric times. Much of the bedrock found in New England is heavily metamorphosed due to the numerous mountain building events that occurred in the region. These events culminated in the formation of Pangaea; the coastline as it exists today was created by rifting during the Jurassic and Cretaceous periods. The most recent rock layers are glacial conglomerates.
Chronology
Overview The bedrock geology of New England was heavily influenced by various tectonic events that have occurred since the Paleozoic Era including the accretion of land masses that formed various continental terranes to the Mesozoic rifting of the Hartford Basin.
Archean and Proterozoic eons In the Archean Eon Western Massachusetts and Vermont were the eastern edge of Laurentia (now the Canadian Shield). Laurentia is believed to have originated at the end of the Hadean, making it one of the oldest regions with continental crust, as evidenced by the discovery of Acasta Gneiss in Canada. At the end of the Hadean massive eruptions of felsic lava became cool enough to form a permanent crust. The felsic nature of Laurentia allowed it to float over the denser ocean basins that surrounded it, so it was not submerged under the then-forming oceans. During the Archean Eon the surface of New England was a coastal desert covered by silica-rich sediments with outcroppings of granite bedrock.
Paleozoic Era Starting in the Paleozoic Era the supercontinent Pannotia began to break up, forming smaller continents including Laurentia (North America and Greenland), Gondwana, Baltica, and Siberia. At the same time sea levels were rising, which resulted in the flooding of most of the continents with shallow epicontinental seas. This was followed by three periods of extensive orogeny. Much of the geology in New England is based on formation of the Appalachian Mountains through a series of Paleozoic accretion episodes to the terminal collision between Laurentia (proto–North America) and Gondwana (proto–Africa–South America) at ca. 300 Ma.
Cambrian Period
Ordovician Period During the Middle Ordovician period of the Taconic orogeny, volcanic island arcs collided with the eastern coast of North America, causing extensive metamorphism, faulting, and uplift. The result of these processes were the Taconic Mountains, located along the border of New York and New England. After the Taconic orogeny, the Humber seaway was on its way to closure; at the same time, the Dashwoods microcontinent accreted to Laurentia. This then led to the accretion of oceanic terranes including the Bay of Islands in Newfoundland and Thetford Mines Ophiolites in Quebec. Additionally, the Late Ordovician-Early Silurian mélange was present, which consisted of blueschists and deepwater deposits. These blueschists and deepwater deposits indicate that subduction continued during this period of time.
Silurian Period Closure of the Tetagouche-Exploits back-arc in the Early Silurian (430 Ma) accreted the bulk of Ganderia to Laurentia. This event is known as the Salinic Orogeny and was responsible for most of the bedrock that is found in New Brunswick, Newfoundland, and Maine. Examples include the Rangeley sequence found in the Presidential Range that consists of Silurian and Devonian Turbidite sequences, Early Silurian Rangeley Formation, the Middle to Late Silurian Perry Mountain, Smalls Falls, and Madrid Formations.
Devonian Period The Acadian orogeny took place during the middle to late Devonian. Following the subduction of the Iapetus Ocean floor, the microcontinent Avalonia slammed into eastern North America, which caused another period of metamorphism, faulting, and mountain building. Evidence for these events can be found within the Littleton Formation on the summits of the Presidential Range in northern New Hampshire and southern Maine. The Littleton Formation was deposited in the Early Devonian, approximately 409 million years ago with a Gander and/or Avalon Terrane source. The lower part of this formation is found proximal to the Bronson Hill Island Arc and consists of basaltic and rhyolitic volcanic rocks along with low grade metamorphic shale while the upper part of this formation consists of the youngest rocks of the Presidential Range. This area has experienced extensive deformation which is expressed as pre-metamorphic faults, various folds, thrust faults, and doming. Evidence for igneous activity include the early-Devonian (408 Ma) diorites, early to mid-Devonian (390–400 Ma) granites, and the Carboniferous (360–350 Ma) granites.
Carboniferous and Permian periods The Alleghenian Orogeny occurred during the late Paleozoic and was the result of the collision of Africa with North America during the formation of Pangea. The collision events produced tremendous occurrences of thrust faults, folding, and metamorphism. As a result of this collision event a huge mountain belt formed that ran up the east coast of North America into Canada and Baltica, which had collided with Greenland and northern North America. Rocks were newly deformed and folded from the coast to as far as the Allegheny Plateau and the Adirondack Mountains of New York. Although the modern-day Appalachian Mountains have been heavily weathered over time, they were thought to once rival the Himalayas in size.
Mesozoic Era
Triassic Period
Jurassic Period
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