Wyoming's Coal Geology

Geologic History of Wyoming Coal

Most coal in Wyoming formed during the Cretaceous and Paleogene periods, and are associated with coastal and fluvial depositional environments.

The oldest economic coal deposits preserved in Wyoming are in the Lakota Formation coals near the Black Hills. These coals were deposited in broad floodplains across the continental interior during the Early Cretaceous.

A large inland sea, known as the Western Interior Seaway (WIS), spread across western North America during the Late Cretaceous. The shoreline of this sea migrated back and forth across Wyoming for nearly 40 million years, leaving behind a wide range of coastal and marine deposits. Coals formed along the western shore of the WIS in environments like coastal marshes, back-barrier swamps, and interdeltaic wetlands. Many of the Cretaceous coal-bearing formations were deposited during this time, including the Rock Springs, Adaville, Frontier, Allen Ridge, and Almond.

As the WIS retreated near the end of the Cretaceous Period, broad coastal plains developed and fluvial systems came to dominate the landscape. The coal-bearing Lance Formation records this transition from marine and coastal environments to inland fluvial settings.

The thickest coal seams were deposited during the Paleocene and Eocene epochs on broad floodplains and are preserved in the Fort Union and Wasatch formations. During this time Wyoming and surrounding areas were undergoing a major episode of deformation known as the Laramide orogeny, a mountain-building event that uplifted much of the Rocky Mountains. Basins formed between the mountain ranges, and subsidence in these basins contributed to thick accumulations of sediment. Thick coal deposits accumulated in wetlands, around lake margins, and in association with meandering rivers. These coal deposits are the thickest in the Powder River Basin (PRB), where Fort Union coal beds like the Big George coal and the Wasatch Formation’s Lake DeSmet coal bed can be more than 200 feet thick.

For more information on what coal is and how it forms, see the What is Coal page.

Wyoming’s Coal Quality

There are many physical parameters that determine a coal deposit's quality. Most important are the heat value, sulfur, and moisture contents of the coal.

The heat value is reflected in the rank of the coal, which is measured in British thermal units per pound (Btu/lb). In southwestern Wyoming, the heat values average 10,000 Btu/lb (as-received raw coal sample, not heat dried or baked). PRB coals range from 7,710 Btu/lb to 9,410 Btu/lb, averaging 8,580 Btu/lb.

Sulfur content is not rank dependent, but rather controlled by the depositional environment and the water chemistry of the peat bog. Seawater contains higher concentrations of sulfur compounds than freshwater, so coals that formed in coastal settings and were exposed to marine waters typically contain higher concentrations of sulfur than coals deposited in freshwater settings. This difference is evident in the variable sulfur contents of Cretaceous and Paleogene coals in Wyoming. Cretaceous coals that formed along the shores of the Western Interior Seaway have 0.4 to 2.0 percent sulfur, while Paleogene coals, which were deposited in freshwater terrestrial environments, have 0.2 to 0.9 percent sulfur. Most of the Paleogene coal mined in the Powder River Basin today has less than one percent sulfur, which makes it the lowest sulfur coal mined in the United States.

WY coal strata
Coal-bearing stratigraphic units of Wyoming. [Credit: WSGS]

Moisture is also an important parameter in Wyoming coal. Lower-rank (and younger) subbituminous PRB coals have much higher moisture than the higher rank (and older) bituminous coals of the Green River Basin region. Moisture content of PRB coals is between 20 and 30 percent, while coals in the Green River Basin typically have moisture contents below 15 percent.

Volatile matter and fixed carbon contents vary throughout Wyoming and are also rank dependent. Calcium and sodium concentrations in Cretaceous coals are up to six times greater than those found in Tertiary Wyoming coals. Wyoming coals also tend to have very low concentrations of trace elements, such as mercury and arsenic, when compared to eastern U.S. coals.

Additional information about Wyoming coal can be found in these WSGS publications:


Kelsey Kehoe, kelsey.kehoe@wyo.gov