Dakota Formation

Dakota Formation Stratigraphic range: Upper Cretaceous
Outcrop of Dakota Formation at crest of Dinosaur Ridge, near Golden, Colorado
Type	Geologic formation
Overlies	Morrison Formation (Rocky Mountains)
Lithology
Primary	sandstone
Other	shale
Location
Named for	Dakota City, Nebraska
Named by	Meek and Hayden, 1862[1]
Region	Rocky Mountains, Great Plains
Country	United States, Canada

The Dakota Formation is a geologic formation composed of sedimentary rocks deposited on the western side of the Late Cretaceous Western Interior Seaway.^[2] F.B. Meek and F.V. Hayden named it for exposures along the Missouri River near Dakota City, Nebraska.^[1] The strata lie unconformably atop Paleozoic and Precambrian rocks, and are the oldest Cretaceous rocks in the northern Great Plains, including Kansas, Nebraska, Iowa, Minnesota, and Wisconsin.

Small exposure of the Dakota Formation in Central Kansas.

Geological history

Schematic reconstruction of the eastern side of the Cretaceous seaway during deposition of the sediments that eventually became the Dakota Formation. Erosional highlands occur in South Dakota and Minnesota.

Deposition of the sediments that would become the Dakota Formation began during the early Late Cretaceous (Cenomanian). This deposition marked a reversal from millions of years of erosion. This reversal was due to rising of the mouth of the rivers, called a rise in base level, as the Cretaceous Seaway formed. This rise lowers the gradient of the rivers causing them to deposit sediment because their velocity can no longer sustain high volumes of sediment.^[3]

Measurements show that the rivers flowed westward and southwestward towards the encroaching sea from source areas near the present-day Great Lakes. The point of deposition slowly moved eastwards as the seaway rose. This change is seen by a gradual shift in the composition of sandstones from having a lot of Paleozoic-age rock detritus in Kansas to sandstones having all Precambrian crystalline rock debris in Iowa.^[4]

This shift means that the rivers had completely eroded away the Paleozoic rocks in the river source area by the time the Seaway rose high enough for the rivers to deposit sediments in Iowa. The very top of the Dakota Formation was deposited along the coast as indicated by some fossil marine invertebrates. Fossil plants, coal deposits and kaolinite clays show that the climate was warm and wet during deposition of the Dakota Formation.^[4] Some of the ancient preserved soils show that an extensive flood plain forest was present.

Two sides of the seaway

Historically, Lower Cretaceous strata in the Rocky Mountain region have been called the Dakota Formation based on assumed correlation with the type section of the Dakota of the Great Plains. Witzke and Ludvigson have argued that use of the name "Dakota" must reflect actual, not presumed correlation based on stratigraphy and composition of the sedimentary rock.^[4] In the west, this sequence of early Cretaceous, predominately sandstone, sedimentary rocks is now known as the Dakota Group,^[5] to dispel any suggestion of direct facies correlation.

Beginning in the Early Cretaceous, the Cretaceous Seaway spread south from what is now the Arctic Ocean and connected with a short northward extension from the Gulf of Mexico.^[6] This marine transgression of the ocean onto what was formerly land, was completed by the late Albian (~100 MA) thereby dividing North America in half. On the eastern side of the Seaway, sediments that would become the Dakota Formation were deposited as coastal and nearshore marine sands and silts. As the seaway continued to deepen and widen, this eastern shoreline moved progressively eastward throughout the Cenomanian. Meanwhile, on the western side of the seaway, sediments were carried eastwards and northeastwards by rivers from mountains located along what is the Nevada-Utah border.

These western sediments accumulated as nearshore and coastal sands and silts as well, and are counterparts to the Dakota Formation on the eastern side of the Seaway. However, these counterpart sediments originated from the other side of the sea and were carried by rivers flowing in opposite directions. These western sediments are equivalent to the Dakota Formation of the Great Plains, but are not exactly the same strata. Individual formations in the western Dakota Group have local names. In Wyoming, the term Cloverly Formation has been expanded by some authors to include sediments formerly placed within the Dakota Formation.^{[citation needed]} Along the Colorado Front Range, the lower, terrestrial beds, or facies, of the Dakota Group are sometimes called the Lytle Formation, and near-shore marine facies are called the South Platte Formation.^[7] In eastern Utah and western Colorado, Young introduced the term Naturita Formation for a series of facies in the larger "Dakota Group".^[5] However, despite Witzke and Ludvigson logic, geologist have continued to refer to the Lower Cretaceous sequence of formations on the Colorado Plateau and the Rocky Mountains as the Dakota Group.^[8]

Vertebrate paleofauna

Dinosaur fossils are very rare in the Dakota Formation and most of them come from Kansas. The best specimen is a partial skeleton of a nodosaurid ankylosaur called Silvisaurus condrayi.^[9][10] Other isolated ankylosaur material may also belong to Silvisaurus.^[11] Fossil dinosaur tracks are also known and include theropod and ankylosaur.^[11] A large ornithopod femur is known from Nebraska.

• cf. Troodon sp
• cf. Paronychodon (? troodontid indet)
• cf. Richardoestesia sp. (theropod indet)
•  ? Barosaurus lentus
• Silvisaurus condrayi – "Partial skeleton with skull, sacrum."^[12]

Hypsilophodon wielandii – "Femur."^[13]

Pterosaurs

Pterosaurs of the Dakota Formation
Taxa	Presence	Description	Images
Suborder: Pterodactyloidea[14] Tracks.	Known from both early and late Cretaceous strata in the Dakota Group.[14] Found at the John Martin Reservoir in Colorado.[14]	Specimens kept at the Dinosaur Tracks Museum, of the University of Colorado at Denver.[14]

See also

• List of dinosaur-bearing rock formations

References

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	Paleontology portal
	Dinosaurs portal

1. ^ ^{a b} Meek, F.B. and Hayden, F.V., 1862, Descriptions of new Lower Silurian, (Primordial), Jurassic, Cretaceous, and Tertiary fossils, collected in Nebraska, by the exploring expedition under the command of Capt. Wm F. Reynolds, U.S. Top. Engineers, with some remarks on the rocks from which they were obtained: Academy of Natural Sciences of Philadelphia Proceedings, v. 13, p. 415-447.
2. ^ Monroe, James S. and Wicander, Reed (1997) The Changing Earth: Exploring Geology and Evolution (2nd edition) Wadsworth Publishing Company, Belmont, California, page 610, ISBN 0-314-09577-2
3. ^ Bejnar, C. R. and Lessard, R. H. (1976) "Paleocurrents and depositional environments of The Dakota Group, San Miguel and Mora Counties, New Mexico" in Ewing, Rodney C. and Kues, Barry S. (eds.) (1976) Guidebook of Vermejo Park, Northeastern New Mexico: Twenty-seventh Field Conference, September 30, October 1 and 2, 1976 New Mexico Geological Society, Socorro, N.M., pp. 157–163, OCLC 2754478
4. ^ ^{a b c} Witzke, B.J., and Ludvigson, G.A. 1994. The Dakota Formation in Iowa and its type area. In Shurr, G.W., Ludvigson, G.A., and Hammond, R.H. (eds). Perspectives on the eastern margin of the Cretaceous Western Interior Basin. Geological Society of America, Special Paper 287:43–78.
5. ^ ^{a b} Young, Robert G. (1960) "Dakota Group of Colorado Plateau" American Association of Petroleum Geologists Bulletin 44(2): pp 156–194
6. ^ Kauffman, E.G. 1984. Paleobiogeography and evolutionary response dynamic in the Cretaceous Western Interior Seaway of North America. In Westermann, G.E.G. (ed), Jurassic-Cretaceous Biochronology and Paleogeography of North America, Geological Association of Canada Special Paper 27: 273–306.
7. ^ Waage, K. M. (1955) Dakota Group in northern Front Range Foothills, Colorado U.S. Geological Survey Professional Paper 274-B:15–49.
8. ^ For example in east-central and northeast New Mexico the Dakota Group cpnsists of the Mesa Rica Sandstone, the Pajarito Shale, and the Romeroville Sandstone, and includes the underlying Tucumcari Shale in the Tucumcari area and Glencairn Formation in Union County. "Dakota Group" U.S. Geological Survey
9. ^ Eaton, T.H. 1960. A new armored dinosaur from the Cretaceous of Kansas. University of Kansas Paleontological Contributions, Vertebrata, 8:1–24.
10. ^ Carpenter, K. and J.I. Kirkland. 1998. Review of Lower and Middle Cretaceous ankylosaurs from North America. Lucas, S.G., Kirkland, J.I. and Estep, J.W., (eds.), Lower and Middle Cretaceous Terrestrial Ecosystems. New Mexico Museum of Natural History and Science Bulletin No. 14:249–270.
11. ^ ^{a b} Liggett, G.A. 2005. A review of the dinosaurs from Kansas. Kansas Academy of Science, Transactions 108(1–2), p.1-14.
12. ^ "Table 17.1," in Weishampel, et al. (2004). Page 365.
13. ^ "Table 18.1," in Weishampel, et al. (2004). Page 396.
14. ^ ^{a b c d} Lockley, M.; Harris, J.D.; and Mitchell, L. 2008. "A global overview of pterosaur ichnology: tracksite distribution in space and time." Zitteliana. B28. p. 187-198. ISSN 1612 – 4138.