Geology of North Dakota

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North Dakota is underlain by Precambrian crystalline basement rock, although these rocks are less well understood than in neighboring states. In the Proterozoic, a mountain range known as the Western Dakota Mobile Belt formed between two billion and 1.8 billion years ago in connection with the Trans-Hudson orogeny, stretching north into Manitoba and Saskatchewan before eroding almost entirely 1.5 billion years ago, shedding sediments now found in the Montana Belt Supergroup. Commonly glacial erratic boulders are Precambrian rocks transported to the region by ice sheets.^[1]

In the Cambrian at the beginning of the Paleozoic, North Dakota was located at the equator. Extensive deposition of sedimentary rocks began 515 million years ago, when the Canadian Shield and North Dakota flooded during a marine transgression forming the Sauk Sequence. The sequence contains only one major unit—the Deadwood Formation. The area of the Nesson Anticline and some hills eroded out of the rugged Precambrian landscape likely remained above the water surface. The sequence begins with sandstone eroded from Precambrian rocks and ascends through limestone and shale, before returning to sandstone when sea levels dropped again. Conodont fossils are common in the Deadwood Formation.

During a dryland period, for 15 to 20 million years the land surface was eroded and no rocks from 485 to 470 million years ago are found in the rock record. A renewed marine transgression began the Tippecanoe Sequence, including Winnipeg Group sandstone and shale, overlain by Red River, Stony Mountain, Stonewall and Interlake Formation carbonates. In total, the Tippecanoe Sequence is 2000 feet thick.

During the Silurian, the Williston Basin subsided becoming a defined feature. Percolating groundwater eroded limestone into caves and hollows typical of a karst topography during a mid-Paleozoic dry period, before a return to a shallow sea in the Ordovician.

The region was above sea level for 40 million years during the Devonian as streams eroded the land surface until sea levels rose 356 million years ago. A weathered paleosol (known as the Ashern Formation) at the top of the Interlake Formation indicates that the Devonian and Mississippian Kaskaskia Sequence eroded the carbonates of the Interlake Formation. Winnipegosis Formation limestone and dolomite formed on top of the Ashern Formation, ascending to sandstone and shale in the Souris River and Dawson Bay formations. Periodically, the area dried out, resulting in weathered paleosols and the red and green siltstone layers of the Three Forks Formation which cover the Birdbear Formation carbonates. The collision of North America and Europe as Pangea began to form kicked off the Caledonian orogeny and realigned the Williston Basin opening to the sea in the west rather than the north.

During the maximum extent of the sea, the Lodgepole and Mission Canyon formations took shape. A dry period precipitated Charles Formation evaporites, followed by Big Snowy Group carbonates, sand and shale. The ancestral Rocky Mountains began to rise around this time, bringing the Otter Formation shales and then draining the sea and uplifting the land.

After a 10 million year dry period, shallow water crept back in during the Pennsylvanian beginning the Absaroka Sequence with Tyler Formation sandstone and shale, overlain by carbonates and brown clastic rocks in the Amsden Formation and Broom Creek Formation sandy carbonates. Another 10 million years of erosion is marked by an unconformity. Through the Permian, salt and red bed formations filled the Williston Basin belonging to the Opeche and Spearfish formations, along with the Minnekahta Formation limestone.^[2]

In the Triassic, at the beginning of the Mesozoic, a meteorite struck McKenzie County, rearranging older sediments. Some salt and gypsum remains from the time period, indicative of the vast deserts that covered Pangea at the time. An unconformity wipes out 45 million years of the early Jurassic before the beginning of the Zuni Sequence. North Dakota was a low forested landscape experiencing ongoing erosion. Rivers and streams moving across the eroded Jurassic landscape deposited the sandstone and siltstone Inyan Kara Formation. Thick layers of shale, such as the Pierre Formation, formed in the Western Interior Seaway during a major global marine transgression in the Cretaceous.^[3]

In the early Cenozoic, uplift and erosion of the Rocky Mountains with the continuation of the Laramide orogeny dumped sediment into the Williston Basin, creating the sandstone and shale of the Ludlow Formation, Cannonball Formation and Slope Formation together with a final marine transgression into North Dakota from 65 to 55 million years ago. The Bullion Creek Formation and the Sentinel Butte Formation covered over these units with lignite coal. They outcrop in the west and contain most of the state's coal reserves (although the single most productive unit is the Sentinel Butte Formation). Coal formation took place in a coastal swamp environment akin to large coastal marshes that exist currently in the Holocene along the US Atlantic coast. Even as Bullion Creek sediments were being deposited in the center of the state, swamps to the west were filling and covering them over with Sentinel Butte material.

From 50 to 60 million years ago in the Paleocene and Eocene the clay and sand units of the Golden Valley Formation deposited in lakes and streams and lie above Sentinel Butte units in some locations in the west. During the Eocene, mammals and grasses diversified in the area as North Dakota transitioned from a warm temperate to subtropical climate. With long running dryland conditions, North Dakota has gone through a period of extensive weathering that continues in the modern period, often producing unconsolidated sediments rather than rocks. The Tejas Sequence began to form in the Oligocene, starting off with conglomerate, siltstone, clay, volcanic ash, freshwater limestone and sands of the White River Group. Widespread volcanism to the west deposited volcanic ash in Miocene and Pliocene lake beds, which now forms the peaks of the Killdeer Mountains.

From around 35 million years ago, large quantities of gravel and sand from the Absaroka Mountains, Big Horn Mountains and Black Hills deposited in western North Dakota and up to 400 feet of limestone plated the bottom of a lake that remains as the Killdeer Mountains. Estimates suggest several thousand cubic miles of sediment were eroded from five million years ago until three million years ago. Evidence of the erosion is found in the Turtle Mountains, which were originally part of a continuous plateau before up to 600 feet of sandstone and shale eroded between the two features prior to glaciation.

Pliocene erosion created the Red River Valley, as a tributary of the Cheyenne River eroded a gently sloping escarpment 1000 feet down (the ground surface of the valley is higher, due to glacial and lake bed sediments). The Killdeer Mountains are up to 1300 feet above the Little Missouri River, leading to inferences about the previous height of the plain.^[4]

The state was glaciated six to seven times during the Pleistocene, contributing to erosion and rechanneling rivers. During the last 11,000 years of the Holocene, an initial warm period led to widespread sagebrush grasslands around 7000 years ago with large dunes formed at Wyndmere, Walhalla and Denbigh. This gave way to a wetter period with more widespread forests. Devils Lake and Stump Lake have periodically dried up or fluctuated in response to climate changes. In the 1800s, when European settlers arrived in large numbers, Stump Lake contained the stumps of trees that had grown on dryland in the 1300s.