Brule Formation

Brule Formation
Name	Brule Formation
Type	Formation
Period	Oligocene
Lithology	Claystone, siltstone, sand, volcanic tuff
Region	Great Plains, Rocky Mountains
Country	United States
Namedfor	Brule County, South Dakota
Namedby	N. H. Darton
Year ts	1898

Brule Formation The Brule Formation is an Oligocene geologic unit in the central United States Great Plains and Rocky Mountains region, notable for continental sedimentary successions. It crops out in areas of South Dakota, Nebraska, Wyoming, Montana, and Colorado and interfaces with regional units studied by workers from institutions such as the United States Geological Survey, University of Nebraska–Lincoln, South Dakota School of Mines and Technology, University of Wyoming, and University of Colorado Boulder. The formation figures in research on fossil mammals, volcanic ash correlation, and basin evolution adjacent to the Black Hills and Laramide orogeny-affected provinces.

Description

The Brule Formation is described in early surveys by geologists including N. H. Darton and later synthesizers such as H. T. U. Smith and researchers affiliated with the United States Geological Survey and U.S. National Park Service. Outcrops form badlands landscapes within protected areas like Badlands National Park and on public lands managed by the Bureau of Land Management. The unit contains paleosols and fossil localities that attracted paleontologists from institutions such as the American Museum of Natural History, Field Museum of Natural History, University of California Museum of Paleontology, and the Smithsonian Institution.

Stratigraphy and Lithology

Stratigraphically the Brule Formation lies above the Chadron Formation and below the Arikaree Formation in classic Great Plains successions established by N. H. Darton and later refined by stratigraphers at the United States Geological Survey and universities including University of Nebraska–Lincoln. Lithologically it comprises claystone, siltstone, fine sand, and silicic volcanic tuff layers correlated to ash beds used by teams from Iowa State University, University of Washington, and Massachusetts Institute of Technology for tephrochronology. Paleosol horizons and calcrete nodules are noted in stratigraphic descriptions produced by researchers at Colorado State University and Montana State University, while sedimentologists from University of Kansas and Yale University characterized fluvial and pedogenic facies.

Age and Correlation

The Brule Formation is dated to the Oligocene epoch and correlated to North American Land Mammal Ages such as the Orellan, Whitneyan, and parts of the Arikareean by paleontologists at the American Museum of Natural History and the Smithsonian Institution. Radiometric ages from volcanic tuffs within the unit have been obtained by geochronologists at Berkeley Geochronology Center, University of California, Berkeley, and Lawrence Berkeley National Laboratory and tied to global stratigraphic markers used by the International Commission on Stratigraphy. Correlations extend to coeval sequences in the White River Group of Nebraska and Wyoming, building on comparative studies by teams from Princeton University and Harvard University.

Fossil Content

Fossil assemblages recovered from Brule exposures include diverse Oligocene mammals such as oreodonts, early camels (members studied by paleontologists at the Natural History Museum of Los Angeles County), nimravids examined by researchers from the Field Museum of Natural History, and various rodents and insectivores cataloged at the University of Michigan Museum of Paleontology. Fossil birds, reptiles, and plant remains have been described by specialists affiliated with the Smithsonian Institution, American Museum of Natural History, and University of California Museum of Paleontology. Vertebrate taphonomy and paleoecology studies have been carried out by teams from University of Chicago, Yale University, and University of Utah, while microvertebrate and pollen analyses have been performed at laboratories in Iowa State University and Pennsylvania State University.

Depositional Environment

Interpretations of the depositional environment invoke fluvial, floodplain, and loessial settings influenced by volcanic ash fall and climatic regimes explored by paleoclimatologists at National Center for Atmospheric Research and Columbia University. Sediment provenance and paleodrainage reconstructions linking to the Laramide orogeny and hinterland erosion have been advanced by researchers at University of Arizona and University of Colorado Boulder. Studies by geochemists at Stanford University and University of California, Santa Cruz have used tephra geochemistry and stable isotopes to infer arid to semi-arid paleoenvironments and seasonal variability.

Economic and Engineering Significance

Although not a major hydrocarbon reservoir, the Brule Formation influences groundwater flow and engineering geology in regions managed by the U.S. Geological Survey and Bureau of Reclamation, affecting municipal and agricultural water supply studies conducted by University of Nebraska–Lincoln and South Dakota State University. Clay-rich horizons impact foundation engineering and road construction overseen by state departments such as the South Dakota Department of Transportation and Nebraska Department of Transportation, while paleontological resources are protected under statutes administered by the National Park Service and state historic preservation offices. Tephra layers serve as chronostratigraphic markers used in regional mineral exploration programs coordinated with the United States Geological Survey and university geology departments.

Category:Oligocene geology of North America Category:Geologic formations of the United States