Laramie Formation

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The Laramie Formation is a geologic formation of Cretaceous age, named by Clarence King in 1876 for exposures in northeastern Colorado, in the United States.

The formation is exposed around the edges of the Denver Basin and ranges from 400-500 feet on the western side of the basin and 200-300 feet thick on the eastern side. The Laramie conformably overlies the Fox Hills Sandstone and unconformably underlies the Arapahoe Conglomerate. The formation can be divided into a lower unnamed member containing bedded sandstone, clay, and coal and an upper unnamed member composed predominately of 90 to 190 m of drab-colored mudstone, some sandstone, and thin coal beds (Eldridge 1888; Shroba and Carrara 1996). Nodular ironstone concretions occur in the mudstones that contain plant remains. The coal and clay were once economically important (Shroba and Carrara 1996). The Laramie Formation was deposited on a coastal plain containing coastal swamp. Some of the material in the sandstones originated from silicic volcanoes far to the west (Wilson 2002).

Typical exposure of the Laramie Formation in northeastern Colorado. Dinosaur bones have been found in the area.

Fossil pollen (palynomorphs) include bryophyte and pteridophyte spores, gymnosperm pollen, and abundant angiosperm pollen, including Aquilapollenites striatus, Ilexpollenites compactus, Interpollis cf. I. supplingensis, and Tricolpites interangulus (Nichols and Fleming 2002). The palynomorphs indicate an early Maastrichtian age (68-69 mya), which is supported by magnetostratigraphy (Hicks et al. 2003). Magnetostratigraphy also shows that the Laramie Formation becomes progressively younger eastward as deposition followed the regression of the Western Interior Seaway.

Fossil leaves are abundant, especially in the shales and sandstones associated with coal (Johnson et al. 2003). Dicot angiosperms dominate, with lesser amounts of ferns, palms, and herbaceous lycopods. Interestingly, conifers are rare. Common plants include “Ficus” planicostata, “Myrica” torreyi, Sabalites sp., Platanites marginata, and Marmarthia pearsonii.

Fossil vertebrates from the Laramie Formation were among the first dinosaurs to be discovered in the American West (Carpenter and Young 2003). In 1873, Edward D. Cope accompanied Ferdinand V. Hayden, who was leader of the U.S. Geological and Geographical Survey of the Territories. The route of the expedition included eastern Colorado where Cope collected specimens in what is now the Laramie Formation along Bijou Creek on the east side of the Denver Basin (Cope, 1874). Cope named three species of dinosaurs without description: Cinodon arctatus (later changed to Cionodon arctatus), Polyonax mortuarius and Agathaumas milo (later renamed Hadrosaurus occidentalis). These specimens are currently in the American Museum of Natural History. These specimens are very scrappy and the names no longer considered valid. Subsequent discoveries of dinosaurs occur through the formation, and include a nearly complete skull of Triceratops. Non-dinosaur vertebrates also occur (Carpenter 1979).

Skull of Triceratops from the Laramie Formation. This skull may be the oldest known for the genus. Currently on display at the courthouse in Greeley, Colorado

List of Fossil Vertebrates (data from Carpenter 1979; Carpenter and Young 2002; Hutchinson and Holroyd 2003)

[edit] Uranium

Uranium deposits occur in sandstones of the Laramie Formation in Weld County, Colorado. (See Uranium mining in Colorado)

[edit] References

• Carpenter, K. and Young, B. 2002. Late Cretaceous dinosaurs from the Denver Basin, Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. I., Rocky Mountain Geology 37:237-254.
• Eldridge, G.H., 1888, On some stratigraphical and structural features of the country about Denver, Colorado: Colorado. Scientific Society Proceedings, v. 3, pt. 1, p. 86 118.
• Hicks, J.F., Johnson, K.R., Obradovich, J. D., Miggins, D.P., and Tauxe, L. 2003. Magnetostratigraphyof Upper Cretaceous (Maastrichtian) to lower Eocene strata of the Denver Basin,Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. II., Rocky Mountain Geology 38: 1-27.
• Johnson, K. R., Reynolds, M.L., Werth, K.W., and Thomasson, J.R. 2003. Overview of theLate Cretaceous, early Paleocene, and early Eocene megafloras of the Denver Basin, Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. II., Rocky Mountain Geology 38: 101-120.
• King, C. 1876. Report of the Geological Exploration of the Fortieth Parallel. U.S.Geographical and Geological Survey.
• Nichols, D.J., and Fleming, R. F. 2002. Palynology and palynostratigraphy of Maastrichtian, Paleocene, and Eocene strata in the Denver Basin, Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. I., Rocky Mountain Geology 37: 135-163.
• Shroba, R.R., and Carrara, P.E., 1996, Surficial geologic map of the Rocky Flats environmental technology site and vicinity, Jefferson and Boulder Counties, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map, I 2526.
• Wilson, M. 2002. Petrographic provenance analysis of Kiowa Core sandstone samples, Denver Basin, Colorado. In K.R. Johnson, R.G. Raynolds and M.L. Reynolds (eds), Paleontology and Stratigraphy of Laramide Strata in the Denver Basin, Pt. I., Rocky Mountain Geology 37: 173-187.