Arikaree Formation

Arikaree Formation
Name	Arikaree Formation
Type	Geological formation
Period	Oligocene–Miocene
Primary lithology	Sandstone, siltstone, conglomerate
Named for	Arikaree River
Region	Great Plains
Country	United States

Arikaree Formation The Arikaree Formation is a late Oligocene to early Miocene stratigraphic unit exposed across the North American Great Plains, noted for its coarse clastic sediments, caliche horizons, and vertebrate fossils. It has been studied in the contexts of regional tectonics, paleoclimate, and basin evolution by researchers associated with institutions such as the United States Geological Survey, University of Kansas, and University of Nebraska. The formation provides key evidence for understanding faunal turnover, fluvial systems, and pedogenesis during the Oligocene–Miocene transition.

Description and Geology

The formation consists of heterogeneous deposits including paleosols and channel fills that record interactions between tectonism in the Laramide Orogeny aftermath, drainage reorganizations tied to the Rocky Mountains, and sediment supply from uplifted terrains documented by studies at the Badlands, Pawnee National Grassland, Chadron Formation interfaces, and exposures near the Niobrara County outcrops. Regional mapping by the Kansas Geological Survey, Nebraska Geological Survey, and South Dakota Geological Survey correlates the unit with contemporaneous packages in the White River Group and sections examined by workers from Harvard University and the Smithsonian Institution. Published stratigraphic syntheses reference lithofacies variation comparable to units described in monographs from the American Association of Petroleum Geologists and field guides produced by the Geological Society of America.

Stratigraphy and Lithology

Stratigraphically, the unit overlies older Chadron Formation and Brule Formation equivalents in places and is overlain by younger Ogallala Formation and Pliocene deposits recognized in basin-fill successions charted by the USGS and regional surveys. Lithologies include friable red to buff sandstones, cemented conglomerates, silty matrices with calcrete nodules, and lens-shaped channel deposits similar to descriptions in publications by the Interstate Oil Compact Commission and field reports from the Kansas School of Mines and Metallurgy. Petrographic studies from laboratories at the University of Colorado and detrital zircon work from teams at Stanford University and University of Arizona tie provenance to the Laramide uplift and recycled sedimentary sources documented in basin analysis workshops hosted by the Society for Sedimentary Geology.

Fossil Content and Paleontology

Paleontological assemblages include vertebrate faunas—rodents, oreodonts, rhinocerotids, camelids, and early Equidae—collected by institutions such as the American Museum of Natural History, Field Museum of Natural History, and University of California Museum of Paleontology. Invertebrate and palynological records studied at the Smithsonian National Museum of Natural History and University of Michigan Museum of Paleontology complement vertebrate biostratigraphy used in regional correlation schemes employed by researchers from Yale University and Princeton University. Notable fossil localities investigated by crews from the University of Nebraska State Museum and Denver Museum of Nature & Science preserve taphonomic signatures comparable to those in the John Day Fossil Beds and Agate Fossil Beds National Monument, and have been cited in faunal turnover analyses in journals associated with the Paleontological Society and the Journal of Vertebrate Paleontology.

Depositional Environment and Age

Interpretations of depositional settings invoke fluvial braid-plain systems, ephemeral stream channels, overbank fines, and paleosol development under semi-arid climates reconstructed using stable isotope work from teams at University of California, Santa Cruz and paleosol models promoted by researchers at the University of Illinois. Radiometric constraints and magnetostratigraphic correlations tied to regional chronologies published by the Seismological Society of America and geochronology groups at Massachusetts Institute of Technology place much of the formation in the late Oligocene to early Miocene, roughly contemporaneous with the Arikareean North American Land Mammal Age used by paleontologists at the Carnegie Museum of Natural History and referenced in compilations by the Natural History Museum, London.

Geographic Distribution and Occurrence

Exposures and subsurface equivalents occur across parts of Nebraska, Kansas, Colorado, Wyoming, South Dakota, and North Dakota, with type-area exposures along the Arikaree River corridor mapped by state geological surveys and documented in field guides produced by Rocky Mountain Geological Society and regional park services including Agate Fossil Beds National Monument and Fort Laramie National Historic Site. Petroleum and groundwater studies by the Kansas Geological Survey and Colorado Oil and Gas Conservation Commission include cross-sections illustrating lateral facies changes and thickness variations observed in well logs archived at the National Geologic Map Database.

Economic Uses and Quarries

Locally cemented members and caliche horizons have been quarried for roadstone, aggregate, and historic building stone in counties administered by state transportation departments and listed in construction inventories by the Federal Highway Administration. Quarries and borrow pits documented by the U.S. Bureau of Land Management and county planners show use as subgrade material and riprap comparable to other Miocene units exploited for aggregate by corporations regulated by the Environmental Protection Agency. Paleontological resources in certain outcrops are managed under permits issued by the National Park Service and state agencies, with specimens curated at museums such as the Museum of the Rockies and the Denver Museum of Nature & Science.

Category:Geologic formations of the United States Category:Oligocene geology Category:Miocene geology