Section 2: The Williston Basin

North Dakota lies on top of a large geologic feature of North America called the Williston Basin. (See Map 1.) The Basin, more than 300,000 square miles in area, includes Saskatchewan, Manitoba (both in Canada), Montana, and South Dakota. The largest portion of the Williston Basin is located in North Dakota.

Williston Basic Stratigraphy
Image 4: Williston Basin Stratigraphy. On the left side of this cut-away image of North Dakota’s geological layers is a chart indicating years each layer represents. A geologist reads this chart like a calendar. The larger periods are called Eras. The next smaller divisions of the eras are called Periods. The periods are divided into Epochs. Several formations might have been deposited in one epoch. The far left hand column tells us when these geological events took place (millions of years ago or Before Present Era.) There is also a column describing how deep you would have to dig to encounter a particular formation. The black boxes with numbers represent a core sample. The deepest core sample on the chart is more than 9,000 feet deep. These core samples are stored at the Wilson M. Laird Core Sample Library. You can see portions of these core samples at the Heritage Center Museum in Bismarck. Courtesy North Dakota Geological Survey.

Williston Basin
Map 1: The tan shaded area shows part of the Williston Basin. It extended north into Saskatchewan and Alberta and south into South Dakota. Below the surface, many formations were laid down by millions of years of geological events. Over time, pressure converted the organic materials in the formations into oil and natural gas. The Williston Basin is not visible on the surface. Courtesy, United States Geological Survey.

As Williston Basin formed over millions of years, layers of sedimentary rocks more than 16,000 feet deep accumulated near the center of the Basin.  The center is close to the present-day town of Williston. (See Image 4) The sedimentary layers along the eastern edge of the Basin in eastern North Dakota are about 300 feet deep.

How Coal Was Formed
Image 6:How Coal Was Formed. This graphic image shows how plants that died near a swamp 60 million years ago turned into coal as the weight of other layers compressed the organic material. Courtesy, ND Geological Survey, Becky Barnes, artist.
Image 7: Lignite coal has been mined in North Dakota for more than a century. Today, most coal is removed by digging through the surface layers with large machines. Coal is used to produce electricity. SHSND 10692-Box 12-18.

We know today that the Williston Basin is a major source of coal, oil, and natural gas. These resources are located in the Basin area because the geologic history of the region. While the Basin was forming, shallow seas covered the area, but receded several times. Animal and plant organisms lived and died in or near the seas and their remains were deposited on the bottom of the sea. Some of this organic material became oil. The woody debris along the shores of the sea became coal.

Vegetation grew in the swamps and river edges. (see Document 1) In time, dense layers of decaying vegetation in this damp environment formed a moist loose layer called peat. The plants decayed very slowly because peat forms in wetlands where periodic flooding prevents oxygen from aiding in the decay process. In time, the bogs were buried under many layers of rocky sediment. With time and pressure, the plants that had once lined the shores of the inland sea became lignite coal. (See Image 6.)

How Petroleum Formed
Image 8: How Petroleum Formed. Oil and natural gas began with the small plankton of the Western Interior Sea. As these microscopic plants and animals died they fell to the floor of the sea. Over the next 360 million years or so, the plankton was buried beneath new layers as the seas dried up and rivers carried new layers from the Rocky Mountains to western North Dakota. Pressure and time converted this organic matter into oil which flowed through microscopic pores in the rock layers towards the Williston Basin. Courtesy, ND Geological Survey, Becky Barnes, artist.

Lignite is a soft coal with high moisture content. It occurs near the surface in the east central zone of the Williston Basin. The Freedom Mine and the Knife River Mine near Beulah extract coal deposits laid down in the Fort Union formation of the Paleocene (65 to 55 million years BPE.) (See Image 7.)

About 500 million years ago, the earth’s crust began to fold along a line in western North Dakota. Folds of this type are called anticlines. Around that time, another process began to create oil. Plankton (microscopic algae, bacteria, protozoans) floating in the inland sea, died and fell to the sea floor. Over millions of years, these one-celled organisms were covered by layers of sediments and more dead plankton. As the sediments built up, the pressure and temperature increased within the buried layers. Finally, the organisms turned into oil and gas. (See Image 8.) Oil and gas flowed through the tiny spaces in the rock. As oil and gas moved into the anticlines, they flowed upward, closer to the earth’s surface.

Anticlines trap oil in spaces that can be tapped by drilling from the surface. The Nesson anticline is the most important of these formations in North Dakota. It is located in the Williston Basin a few miles east of Williston.

Document 1: Tour a swamp.


Why is this important?  Williston Basin geology has been responsible for a great deal of economic activity since the 19th century. While access sometimes has been difficult, the value of these mineral resources has repaid the cost of mining and drilling. The minerals found in Williston Basin formations have provided jobs and helped people heat their homes, cook their food, and pay their bills for more than 125 years.