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Wyoming's Coal Geology

The Wyoming State Geological Survey (WSGS) has a long tradition of coal research. The coal fields and coal-bearing strata – successions of various ages – of Wyoming have been mapped and correlated in detail. The WSGS has published more than 40 coal publications over the past 50 years and serves as a repository for coal informational resources, which includes a collection of Wyoming historic coal maps.

Geologic History of Wyoming Coal

Resource and reserve

Coal in Wyoming formed during the Cretaceous and early Tertiary period- from 130 to 50 million years ago. The oldest economic coal deposits preserved in Wyoming are the Lakota Formation coals near the Black Hills. This organic-rich coal and black shale formation was deposited on land adjacent to an ancient inland waterway called the Western Interior Seaway, a large inland sea that spilt the continent of North America into two landmasses in Lower Cretaceous time, 100 to 130 million years ago. Coal-bearing coastal plains and shoreface barrier beach sands dominated the depositional setting of Wyoming. These coals were deposited and preserved around the present-day town of Cokeville in western Wyoming (Lincoln County), within the Bear River Formation. Although the location, quality, and quantity of the coals are geologically known, they are considered “sub-economic” because they are too thin and discontinuous to mine.

During the Upper Cretaceous (the period in Earth’s history from 100 to 65 million years ago), thin coals were deposited within the transgressing Frontier Formation in the southwestern part of the state. The oldest widespread coals deposited were from the very wide ranging Mesaverde Group, extending from northern Arizona to as far north as northwestern Wyoming. The Adaville Formation is a member of this group and contains coal up to 118 feet thick. The Westmorland Kemmerer Mine in Lincoln County extracts coal from this formation today. The Almond and Meeteetse formation coals are regional basin members of the Mesaverde Group in the Green River Basin and the Bighorn Basin, respectively. After this time, the seaway shrank and retreated northeastward to the open ocean in northeastern Montana and Canada. During this period, thick coals of the Lance Formation were deposited along the shoreline where the ocean marched across Wyoming and other neighboring states, constantly changing the position and configuration of the shoreline. The Lance coals and the adjacent Fox Hills Sandstone rocks represent the last retreat of the Cretaceous seaway from Wyoming. View the stratigraphic columns of Wyoming's principal coal-bearing and related stratigraphic units.

The greatest thickness of coal in Wyoming was deposited on broad flood plains in the Paleocene Epoch of the Tertiary Period, 65 to 55 million years ago. These coals are located within the 2,000- to 3,000-foot thick Fort Union Formation and are preserved in all Wyoming basins except the Goshen Hole area of southeastern Wyoming. These coal units range from 30 to 200 feet thick in the Powder River Basin (PRB). The Big George coal is over 200 ft thick in places. Coals were from semitropical plants such as metasequoia moss, etc. They were deposited during the Laramide Orogeny, a time of tectonic compression and are only preserved in the associated basins between the regional uplifts. The youngest coals in Wyoming are in the Eocene Wasatch Formation. These coals are also rich in uranium ore in the Green River Basin, and were deposited near the shoreline of ancient Lake Gosiute near Wamsutter. Wasatch coals also form anomalously thick coals in the PRB, especially in the Lake DeSmet area near Buffalo, Wyoming, where they can be over 200 ft thick.

How was Wyoming coal formed?


Thick Wyoming coal beds are indicative of ancient black water, slow moving hydrophyllic swamp-like environments. Cretaceous coal mires were adjacent to a long beach with a shoreface sand barrier that prevented salt water from invading the swamps. The exception of short-term storm surges and transitional mangrove swamps of brackish origin where salt water mixed with fresh degraded coalification. Coal deposits of brackish origin reflected times of major storm surges, resulting in higher ash and sodium (Na) content in some Cretaceous coal deposits. As the seaway retreated to the northeast corner of the state, the wetland environments migrated in a northeast direction behind the advancing beachfront. The coal swamp consisted of organic material from trees, palms, algae, moss, and leaf matter that composted together under freshwater (with a pH <4.5) to form peat. The climate was hot, the relative sea level was high, and the paleo-latitude was semi-tropical.

The coal swamps accumulated organic material until they were buried by sand and mud carried from inland rivers or from the ocean. The organic material became submerged under water where it was exposed to aerobic decay (decay by underwater organisms, bacteria) creating humic matter, a material formed by microbial degradation of dead plant matter, such as lignin. The underwater section of the material from freshwater systems then underwent a gelification process of anaerobic decay that changed the peat into a hydrophilic gel-creating a partially decayed peat substance called “gytta.” The burial water was subsequently removed from this material by heat, which is the second part of the coalification process. Temperatures are very important to the formation of coal types, measured by the ‘rank’ of coal. Thermal alteration or bituminization of the coal is the end result (Jones, 2010).

Coal forming

The more carbon found in coal maceral, the higher the rank of coal. Initially, the organic material is thermally altered to lignite through the coalification process. Lignite or brown coal is a low rank coal, about 60 percent carbon. Next is subbituminous coal (black) at 70 percent, bituminous coal at 85 percent, and anthracite coal is 95 percent carbon. Most of Wyoming’s Tertiary coal is subbituminous, and most of the Cretaceous coal is subbituminous to bituminous. Generally, older and deeper coals are higher in rank than the younger coals in Wyoming basins. Consequently, the deeper the Powder River Basin coal mines go the higher the rank of the coal that is extracted. Exposure to high heat sources deep in the Earth’s crust can upgrade the coal rank in many Rocky Mountain coal regions. Some coals in New Mexico and Colorado, for instance, were thermally upgraded to metallurgical-grade coking coal by intrusive and extrusive Tertiary volcanics.

Wyoming’s coal quality

There are many physical parameters that make up a coal deposit's quality. Most important are the heat value, sulfur, and ash 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). Powder River Basin (PRB) coals range from 7,710 Btu/lb to 9,410 Btu/lb, averaging 8,580 Btu/lb. The ash and sulfur contents are not rank dependent but are measured by percent. They are related to the depositional environments of the coal mire. As the coal swamp is covered by overburden sediment, it mixes with the top layers of the organic material, increasing the inorganic percentage or ash in the coal.

Sulfur content in most Wyoming coals is organically bound. Seawater is several orders of magnitude higher in SO4 than river water. As the sea retreated or prograded away from Wyoming during Cretaceous and Tertiary time, sulfur concentrations decreased. Cretaceous Wyoming coals have 0.4 to 2.0 percent sulfur, and Tertiary coals have 0.2 to 0.9 percent sulfur. Most of the coal mined in the PRB today is 0.2 to 0.3 percent sulfur, which is why it is the very lowest sulfur and naturally the “cleanest coal” in the United States. Some Wyoming coal is ‘super compliant’ meaning its sulfur quality is <0.25 percent.

Moisture is also an important parameter in Wyoming coal. Lower rank (younger) subbituminous PRB coals have much higher moisture than the higher rank (older) bituminous coals of the Green River Basin region. Moisture content of PRB coals is typically between 20 and 30 percent, while in the Green River Basin Cretaceous coals the moisture content is <15 percent. Volatile matter and fixed carbon contents also 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 have very low concentrations of trace elements such as mercury and arsenic, when compared to Eastern U.S. coals on a national level.

Reference: Jones, N.R., 2010, Genesis of thick coal deposits and their unique angular relationships: Powder River Basin, Wyoming: Wyoming State Geological Survey Report of Investigations No. 60, 85 p.

Chris Carroll (307) 766-2286 Ext. 243