Wyoming 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 (1951 USGS coal fields map).
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
Coal in Wyoming formed during the Cretaceous and early Tertiary periods- 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 coal deposits 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.
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, WY, where they can be over 200 ft thick.