Laurentia


Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. 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.
The craton is named after the Laurentian Shield, through the Laurentian Mountains, which received their name from the Saint Lawrence River, named after Lawrence of Rome.

Interior platform

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 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. The sequence of rocks varies from about 1,000 m to in excess of 6,100 m in thickness. The cratonic rocks are metamorphic or igneous with the overlying sedimentary layers composed mostly of limestones, sandstones, and shales. These sedimentary rocks were largely deposited from 650 to 290 million years ago.
The oldest bedrock, the Archean provinces Slave, Rae, Hearne, Wyoming, Superior, and Nain, are located in the northern two thirds of Laurentia. During the Early Proterozoic they were reactivated and covered by sediments, most of which has now been eroded away.

Tectonic setting

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 dolomites and evaporites were deposited. This platform extended from either the present Appalachian Mountains or Mississippi Valley to the present Great Basin. The craton was covered by shallow, warm, tropical epicontinental or epicratonic sea that had maximum depths of only about 60 m 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 enrichment of the lithospheric mantle beneath the orogenic belts of the Grenville Province. This enrichment is thought to have contributed to the formation of the major supercontinent Rodinia.

Volcanism

The southwestern portion of Laurentia consists of Precambrian basement rocks deformed by continental collisions. 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.

Equatorial location

The position of the equator during the Late Ordovician epoch 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 and loose shells or nonamalgamated brachiopod shell beds. 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.

Paleoenvironmental change

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 million years 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 remained relatively cool.

Geological history