LAURENTIA or the NORTH AMERICAN CRATON is a large continental craton that forms the ancient geological core of the North American continent. Many times in its past, Laurentia has been a separate continent , as it is now in the form of North America, although originally it also included the cratonic areas of Greenland and also the northwestern part of Scotland , known as the Hebridean Terrane . During other times in its past, Laurentia has been part of larger continents and supercontinents and itself consists of many smaller terranes assembled on a network of Early Proterozoic orogenic belts. Small microcontinents and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian craton seen today.
* 1 Interior platform
* 2 Tectonic setting
* 2.1 Volcanism * 2.2 Equatorial Location
* 3 Paleoenvironmental change * 4 Geological history * 5 References * 6 External links
In eastern and central Canada, much of the stable craton is exposed at the surface as the Canadian Shield ; when subsurface extensions are considered, the wider term Laurentian Shield is more common, not least because large parts of the structure extend outside Canada. In the United States, the craton bedrock is covered with sedimentary rocks of the interior platform in the Midwest and Great Plains regions and only in northern Minnesota, Wisconsin, the Adirondacks of New York and the Upper Peninsula of Michigan is the craton bedrock exposed. The sequence of rocks varies from about 1,000 to in excess of 6,100 m (3,500–20,000 ft) in thickness. The cratonic rocks are metamorphic and igneous , while the overlying sedimentary rocks are composed mostly of limestones , sandstones , and shales . These sedimentary rocks were deposited from 650 to 290 million years ago.
The metamorphic and igneous rocks of the "basement complex " of Laurentia were formed 1.5 to 1.0 billion years ago in a tectonically active setting. The younger sedimentary rocks that were deposited on top of this basement complex were formed in a setting of quiet marine and river waters. During much of Mississippian time, the craton was the site of an extensive marine carbonate platform on which mainly limestones and some dolostones and evaporites were deposited. This platform extended either from the present Appalachian Mountains or Mississippi Valley to the present Great Basin . The craton was covered by shallow, warm, tropical epicontinental or epicratonic sea (meaning literally "on the craton") that had maximum depths of only about 60 m (200 ft) at the shelf edge. During Cretaceous times, such a sea, the Western Interior Seaway , ran from the Gulf of Mexico to the Arctic Ocean , dividing North America into eastern and western land masses. Sometimes, land masses or mountain chains rose up on the distant edges of the craton and then eroded down, shedding their sand across the landscape. Subduction of the continent towards the Northwest, that lasted approximately 1.4 to 1.2 billion years, likely caused organic enrichment of the Grenvillian lithospheric mantle. This enrichment is thought to have contributed to the formation of the major supercontinent Rodinia .
The southwestern portion of Laurentia consists of Precambrian basement rocks deformed by continental collisions (violet area of the image above). This area has been subjected to considerable rifting as the Basin and Range Province and has been stretched up to 100% of its original width. The area contains numerous large volcanic eruptions .
The position of the equator during the Late Ordovician Epoch (c. 458 – c. 444 Ma) on Laurentia has been determined via expansive shell bed records. Flooding of the continent that occurred during the Ordovician provided the shallow warm waters for the success of sea life and therefore a spike in the carbonate shells of shellfish. Today the beds are composed of fossilized shells or massive-bedded Thalassinoides facies (MBTF) and loose shells or nonamalgamated brachiopod shell beds (NABS). These beds imply the presence of an equatorial climate belt that was hurricane free which lay inside 10° of the equator at 22.1°S ± 13.5°. This ecological conclusion matches the previous paleomagnetic findings which confirms this equatorial location.
Several climate events occurred in Laurentia during the Phanerozoic Eon. During the late Cambrian through the Ordovician , sea level fluctuated with ice cap melt. Nine macro scale fluctuations of "Global hyper warming", or high intensity greenhouse gas conditions, occurred. Due to sea level fluctuation, these intervals led to mudstone deposits on Laurentia that act as a record of events. The late Ordovician brought a cooling period, although the extent of this cooling is still debated. More than 100 Ma later, in the Permian , an overall warming trend occurred. As indicated by fossilized invertebrates, the western margin of Laurentia was affected by a lasting southward bound cool current. This current contrasted with waters warming in the Texas region. This opposition suggests that, during Permian global warm period, northern and northwestern Pangea (western Laurentia) remained relatively cool.
* Around 4.03 to 3.58 Ga , the oldest intact rock formation on the planet, the Acasta Gneiss , was formed in what is now Northwest Territories (older individual mineral grains are known, but not whole rocks). * Around 2.565 Ga, Arctica formed as an independent continent. * Around 2.72 to 2.45 Ga, Arctica was part of the major supercontinent Kenorland . * Around 2.1 to 1.84 Ga, when Kenorland shattered, the Arctican craton was part of the minor supercontinent Nena along with Baltica and Eastern Antarctica . * Around 1.82 Ga, Laurentia was part of the major supercontinent Columbia . * Around 1.35–1.3 Ga, Laurentia was an independent continent. * Around 1.3 Ga, Laurentia was part of the minor supercontinent Protorodinia . * Around 1.07 Ga, Laurentia was part of the major supercontinent Rodinia . * Around 750 Ma, Laurentia was part of the minor supercontinent Protolaurasia . Laurentia nearly rifted apart. * Around 600 Ma, Laurentia was part of the major supercontinent Pannotia . * In the Cambrian (541 ±0.3 to 485.4 ±1.7 Ma), Laurentia was an independent continent. * In the Ordovician (485.4 ± 1.7 to 443.8 ±1.5 Ma), Laurentia was shrinking and Baltica got bigger. * In the Devonian (419.2 ± 2.8 to 358.9 ±2.5 Ma), Laurentia collided against Baltica, forming the minor supercontinent Euramerica . * In the Permian (298.9 ± 0.8 to 252.17 ±0.4 Ma), all major continents collide against each other for forming the major supercontinent Pangaea . * In the Jurassic (201.3 ± 0.6 to 145 ±4 Ma), Pangaea rifted into two minor supercontinents: Laurasia and Gondwana . Laurentia was part of the minor supercontinent Laurasia. * In the Cretaceous (145 ± 4 to 66 Ma), Laurentia was an independent continent called North America. * In the Neogene (23.03 ± 0.05 Ma until today or ending 2.588 Ma), Laurentia, in the form of North America, crashed into South America , forming the minor supercontinent America.
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