Bullion Creek Formation

Bullion Creek Formation The Bullion Creek Formation is a Paleocene stratigraphic unit of the North American Interior Plains known for lignitic coal, continental clastic sediments, and a diverse vertebrate and plant fossil assemblage. It crops out across parts of North Dakota, South Dakota, and Montana and has been investigated by institutions including the United States Geological Survey, North Dakota Geological Survey, and university paleontology departments. The unit plays a role in regional studies linking the Laramide Orogeny, the Hell Creek Formation–Fort Union Formation succession, and Paleocene biotic recovery after the Cretaceous–Paleogene extinction event.

Geology and Lithology

The Bullion Creek Formation consists predominantly of fluvial and paludal clastics: channel sandstones, overbank siltstones, mudstones, and interbedded lignite seams. Field descriptions and measured sections from the Williston Basin, Fort Union Basin, and exposures near the Missouri River show meter-scale cross-bedded sandstone bodies, laterally continuous carbonaceous shales, and root-bearing paleosols. Petrographic studies by researchers at the University of North Dakota, South Dakota School of Mines and Technology, and the Smithsonian Institution indicate quartzose arkosic sand derived from local Precambrian and Paleozoic source terranes affected by the Laramide Orogeny provenance. Palynological samples analyzed at the University of California Museum of Paleontology, University of Kansas Natural History Museum, and Yale Peabody Museum of Natural History indicate abundant angiosperm pollen, supporting interpretation of swampy floodplain lithofacies comparable to contemporaneous strata in the Fort Union Formation and Tullock Formation.

Stratigraphy and Age

Stratigraphically, the Bullion Creek Formation overlies Late Cretaceous units in places and is overlain by younger Paleogene strata; it is commonly correlated with parts of the Fort Union Group. Regional chronostratigraphic work using mammalian biozones, palynology, and isotope stratigraphy from labs at Columbia University, University of Michigan, and the American Museum of Natural History places the bulk of deposition in the late Paleocene, near the Paleocene–Eocene Thermal Maximum boundary in some localities. Correlations have been proposed with formations examined by geologists from the United States Bureau of Mines, Iowa Geological Survey, and Montana Bureau of Mines and Geology. Biostratigraphic tie-ins use faunal comparisons to assemblages described in monographs from the Carnegie Institution of Washington and the Royal Ontario Museum.

Paleontology and Fossil Content

Fossiliferous horizons in the Bullion Creek yield vertebrates, plants, mollusks, and trace fossils documented by paleontologists from the Field Museum of Natural History, Denver Museum of Nature & Science, University of Nebraska State Museum, and Paleontological Research Institution. Mammalian remains include multituberculates, early primatomorphs, condylarths, and archaic ungulates comparable to taxa described from the Willwood Formation, Puerco Formation, and Frenchman Formation. Reptilian fossils such as turtles and crocodilian fragments tie to genera reported in collections at the Natural History Museum, London and the Smithsonian Institution National Museum of Natural History. Plant macrofossils—leaves, wood, and reproductive organs—mirror floras curated by the Royal Botanic Gardens, Kew and documented in floristic surveys by botanists affiliated with the Missouri Botanical Garden. Palynological assemblages include gymnosperm pollen and diverse angiosperm taxa that permit comparison with assemblages held at the New York Botanical Garden and the University of Florida Herbarium.

Depositional Environment and Paleoclimate

Sedimentological analyses, facies models, and paleopedological studies by teams from the University of Colorado, Iowa State University, and North Dakota State University interpret the Bullion Creek as deposited in low-gradient fluvial systems, abandoned channels, oxbow lakes, and peat-forming swamps. Stable isotope work and clay mineralogy from laboratories at Oregon State University, Pennsylvania State University, and Washington State University indicate warm, humid conditions with seasonal variability consistent with late Paleocene greenhouse climates reconstructed for the Boreal Realm and mid-continent North America. Comparisons with climate proxies used in studies by the National Oceanic and Atmospheric Administration, Lamont–Doherty Earth Observatory, and the Paleontological Society help place local paleoecology within global Paleogene warming trends and post-extinction ecosystem restructuring.

Economic Significance and Resources

The Bullion Creek Formation contains economically important lignite seams and has been evaluated for coal resources by the Energy Information Administration, the Bureau of Land Management, and the North Dakota Industrial Commission. Historical and modern strip-mining and resource assessment projects involve corporations and agencies such as Westmoreland Coal Company and state energy offices; environmental and reclamation work involves the Environmental Protection Agency and state reclamation programs. Additionally, sandstone units function as aquifers exploited by rural communities overseen by county governments and public utilities; hydrogeologic studies have been conducted by consultants associated with the United States Army Corps of Engineers, HDR, Inc., and regional water management districts. Research into carbon sequestration and unconventional resource potential has engaged academic groups from the Massachusetts Institute of Technology, Stanford University, and University of Texas at Austin.

Category:Geologic formations of North Dakota