Bighorn Dolomite

Bighorn Dolomite
Name	Bighorn Dolomite
Type	Formation
Age	Ordovician
Period	Ordovician
Primary lithology	Dolomite
Named for	Bighorn Mountains
Region	Wyoming, Montana
Country	United States

Bighorn Dolomite The Bighorn Dolomite is an Ordovician carbonate formation prominent in the Bighorn Mountains region of the United States, recognized for its dolomitized limestone, fossil assemblages, and role in regional stratigraphy. It has been studied by geologists from institutions such as the United States Geological Survey, University of Wyoming, and Montana Bureau of Mines and Geology and appears in mapping conducted during surveys by figures linked to the Powell Expedition and early 20th‑century field parties. The unit is important for correlations with contemporaneous units like the Garden City Formation, Red River Formation, and the Ellenburger Group.

Description and lithology

The formation comprises predominantly dolostone with interbeds of fine‑grained dolomitic limestone, oolitic textures, and sparry calcite veining described in reports by the United States Geological Survey and sedimentologists from the American Association of Petroleum Geologists. Petrographic studies published in journals associated with the Geological Society of America document primary dolomite, replacive dolomite, and locally preserved micritic fabrics, with euhedral dolomite rhombs and stylolites noted in core logs curated at the Rocky Mountain Geology collections. Accessory components include chert nodules, argillaceous laminations, and silicified horizons correlated with diagenetic influences recognized by researchers at the Smithsonian Institution and the American Museum of Natural History.

Stratigraphy and age

Positioned stratigraphically above Cambrian units such as the Deadwood Formation and below Silurian and younger Ordovician strata like the Bighorn Formation and Jefferson Formation, the Bighorn Dolomite occupies a key interval in the Ordovician chronostratigraphy. Biostratigraphic control comes from conodonts and brachiopod assemblages analyzed by paleontologists affiliated with Harvard University, Yale University, and the University of Minnesota, enabling correlation to the Middle to Late Ordovician global stages recognized by the International Commission on Stratigraphy. Radiometric constraints tied to volcanic ash layers in nearby sequences and regional chemostratigraphy compared with datasets from the Geological Survey of Canada refine its age placement within the Ordovician.

Depositional environment and paleoecology

Sedimentological and paleontological evidence indicates deposition in a warm, shallow epicontinental sea that inundated parts of the Laurentia craton during the Ordovician, a setting comparable to facies described from the Niagara Escarpment and Trenton Group localities. Fossil assemblages include brachiopods, trilobites, bryozoans, and stromatolites recorded by field teams from the Paleontological Society and repositories such as the Peabody Museum of Natural History, indicating reefal to peritidal shoal environments. Trace fossils and supratidal features documented by researchers at the University of California, Berkeley and the Carnegie Institution for Science suggest episodic exposure and evaporitic influence, while sequence stratigraphic models used by the American Petroleum Institute help interpret transgressive–regressive cycles.

Geographic distribution and thickness

The unit crops out extensively in the Bighorn Mountains, Wind River Range, and adjacent basins including the Big Horn Basin and stretches into parts of Montana and South Dakota where mapped by state surveys and the United States Geological Survey. Thickness varies regionally, reported in mapping bulletins from the Wyoming Geological Association and publications of the Geological Society of America to range from tens to several hundred meters depending on paleotopography, erosion, and thinning over structural highs such as the Absaroka Range and along the Beartooth Plateau.

Economic uses and resources

The formation has been evaluated for reservoir potential in petroleum systems by companies with ties to the American Association of Petroleum Geologists and for carbonate-hosted lead‑zinc mineralization explored during campaigns involving the U.S. Bureau of Mines. Dolomite from the unit has been quarried locally for aggregate and chemical feedstock, supplying construction projects overseen by agencies like the Wyoming Department of Transportation and industrial processors associated with trade groups such as the National Stone, Sand and Gravel Association. Groundwater in fractured and karstified portions of the unit provides wells used by municipalities cataloged in reports by the United States Environmental Protection Agency and state water resources departments.

Geological history and correlation

The Bighorn Dolomite records part of the broader Ordovician carbonate platform evolution on Laurentia during a time of eustatic changes and biotic diversification tied to the Great Ordovician Biodiversification Event, and it correlates with carbonate successions in the Canadian Rockies, the Appalachian Basin carbonate ramps, and midcontinent shelves. Correlative frameworks have been developed through collaborative projects involving the International Union of Geological Sciences, the Geological Survey of Canada, and multiple university research groups, integrating biostratigraphy, chemostratigraphy, and regional structural histories influenced by events recorded later in the Laramide Orogeny and preserved across western North American basins.

Category:Geologic formations of the United States Category:Ordovician