Laurentia
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 the Hebridean terrane in northwest Scotland.
During other times in its past, Laurentia has been part of larger continents and supercontinents and 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.
[4] In eastern and central Canada, much of the stable craton is exposed at the surface as the Canadian Shield, an area of Precambrian rock covering over a million square miles.
In the United States, the craton bedrock is covered with sedimentary rocks on the broad interior platform in the Midwest and Great Plains regions and is exposed only in northern Minnesota, Wisconsin, the New York Adirondacks, and the Upper Peninsula of Michigan.
[8] The oldest bedrock, assigned to the Archean Slave, Rae, Hearne, Wyoming, Superior, and Nain Provinces, is located in the northern two thirds of Laurentia.
These then merged with several smaller fragments of Archean crust, including the Wyoming, Medicine Hat, Sask, Marshfield, and Nain blocks.
This long episode of accretion doubled the size of Laurentia but produced craton underlain by relatively weak, hydrous, and fertile (ripe for extraction of magma) mantle lithosphere.
[3] The subduction under the southeast margin of the continent likely caused enrichment of the lithospheric mantle beneath the orogenic belts of the Grenville Province.
This produced the Keweenawan Supergroup, whose flood basalts are rich in copper ore.[19] Laurentia was formed in a tectonically active world.
The Grenville orogen extended along the entire southwest (present southeast) margin of Laurentia, where it had collided with Congo, Amazonia, and Baltica.
[26] By 750 Ma the breakup was mostly complete, and Gondwana (composed of most of today's southern continents) had rotated away from Laurentia, which was left isolated near the equator.
[10] During the early to middle Ordovician, several volcanic arcs collided with Laurentia along what is now the Atlantic coast of North America.
[31][32] Throughout the early Paleozoic, Laurentia was characterized by a tectonically stable interior flooded by the seas, with marginal orogenic belts.
[29] Sedimentary rocks that were deposited on top of the basement complex were formed in a setting of quiet marine and river waters.
[34] The position of the equator during the Late Ordovician epoch (c. 458 – c. 444 Ma) on Laurentia has been determined via extensive shell bed records.
[35] 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.
[49] The breakup of Pangaea began in the Triassic, with rifting along what is now the east coast of the U.S. that produced red beds, arkosic sandstone, and lake shale deposits.
This was accompanied by deposition of evaporite beds that later gave rise to salt domes that are important petroleum reservoirs today.
[50] The regression of the Sundance Sea in the late Jurassic was accompanied by deposition of the Morrison Formation, notable for its vertebrate fossils.
[52] The Western Cordillera continued to suffer tectonic deformation, including the formation of the Basin and Range Province in the middle Cenozoic and the uplift of the Colorado Plateau.
Nine macro scale fluctuations of "global hyper warming", or high intensity greenhouse gas conditions, occurred.
[55] Due to sea level fluctuation, these intervals led to mudstone deposits on Laurentia that act as a record of events.
[57] As indicated by fossilized invertebrates, the western margin of Laurentia was affected by a lasting southward bound cool current.
[57] This opposition suggests that, during Permian global warm period, northern and northwestern Pangea (western Laurentia) remained relatively cool.
