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Interior Plains

Physiographic and geologic region of North America

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Geologic history

A series of tectonic plate collisions in the crust that formed the center of the North American continent laid the groundwork for the modern-day interior plains. Mountain building and erosion around the plains as well as flooding from inland seas provided sediments that make up the rock strata of the interior plains.

Proterozoic Eon (2500 to 539 million years ago)

Between 2.0 and 1.8 billion years ago the Hearne-Rae, Superior, and Wyoming cratons were sutured together to form the North American/Laurentia craton in an event called the Trans-Hudson Orogeny (THO). The tectonic activity that followed along the edges of the four main cratons sparked mountain building in those regions. The formation of the Himalayas is following the same process after the Indian Plate began colliding with the Eurasian Plate about 10 millions years ago.

The interior of Laurentia remained relatively flat and became a basin for eroded sediment from mountains at the beginning of the current time period, the Phanerozoic Eon. The only remaining outcrops from this orogeny in the interior plains are in the Black Hills of South Dakota. The sediments that formed the Black Hills were granite and different types of igneous rocks, which make up the basement of bedrock in central North America. However, much of the Black Hills sediment has been metamorphosed and deformed, so it is uncertain what the conditions were like at the time of their formation.

Paleozoic Era (539 to 252 million years ago)

This period has a large importance in Earth's history as it saw the Cambrian explosion and Permian extinction. When global sea level rose and continents became partially submerged, the oceans had an explosion of complex life, which was the first time an event like this occurred on Earth. However, the center of Laurentia remained above sea level and as the continent moved east towards other supercontinents like Gondwana, the Appalachian Mountains began to form around 400 MYA. This coincided with the formation of Pangea around 300 MYA, when the Appalachians were at their peak height. The central plains of Laurentia were subjected to deposition of eroded sediment from these mountains. The oldest sediments from this period are felsic igneous rocks and granite that have since been metamorphosed, while the younger sediments are made up of sandstone, shale, limestone, and coal. Sediments deposited in the interior plains from this era are currently buried deep beneath the surface where they are difficult to study.

Mesozoic Era (252 to 66 million years ago)

Around 220 MYA, the supercontinent Pangea broke apart, and the North American continent began to move west and isolate itself. For much of this period the interior plains were covered by inland seas. During the Jurassic period, the Sundance Sea formed along the western coast of the North American continent and extended from northern Canada to the interior plains, covering parts of Wyoming, Montana, North Dakota, and South Dakota. Coquina and sandstone layers from marine deposition were deposited on top of rock layers from the Paleozoic Era. During the Cretaceous period, another inland sea called the Western Interior Seaway was formed. This body of water extended from present-day Alaska to the Gulf of Mexico and covered almost all the interior plains west of the current boundary of the Mississippi River. Limestone-shale couplets, as well as carbonate layers, are commonly found in sedimentary deposits from this inland sea. Towards the end of this period, the inland seas began to drain due to uplift from the formation of the Rocky Mountains.

Cenozoic Era (66 million years ago to Present Day)

The Laramide Orogeny event was when the western Cordillera was formed due to the flat-slab subduction of the Farallon Plate under the North American Plate. This created the frontal range of the Rockies from Montana through New Mexico. The outcrops seen at the surface of the Rockies are made up of sandstone, granite, and limestone; as well as metamorphic rocks uplifted from the Proterozoic Period. The interior plains have remained relatively flat during this period and recent sedimentation is from erosion of the newly formed Rocky Mountains as well as continued erosion from Appalachia. In general, Rocky Mountain sediment is deposited on the plains west of the Mississippi River, and Appalachian sediment is deposited to the east of the Mississippi River.

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