Dakota Formation

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The Dakota Formation is composed of sedimentary rocks deposited on the east side of the Late Cretaceous seaway. F.B. Meek and F.V. Hayden (1862) named it for exposures along the Missouri River near Dakota City, Nebraska. 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.

[edit] 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, which 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. 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 (Witzke and Ludvigson 1994). 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 (Witzke and Ludvigson 1994). Some of the ancient preserved soils show that an extensive flood plain forest was present.

[edit] The "Dakota" Formation in the Rocky Mountain Region

Lower Cretaceous strata in the Rocky Mountain region historically have been called the Dakota Formation based on assumed correlation with the Dakota of the Great Plains. However, as Witzke and Ludvigson (1994) have noted, use of the name must reflect actual, not presumed correlation based on stratigraphy and composition of the sedimentary rock.

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 (Kauffman 1984). 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 shorline 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 the Nevada-Utah border. These sediments accumulated as nearshore and coastal sands and silts as well, and are counterparts of the Dakota Formation on the eastern side of the Seaway. However, these counterpart sediments originated from a totally different side of the continent and were carried by rivers flowing in opposite directions. Clearly, then, these western sediments are equivalent to the Dakota Formation of the Great Plains, but are not the same strata. For this reason, the term Cloverly Formation has been expanded to include sediments formerly called Dakota Formation in Wyoming. Along the Colorado Front Range, the lower, terrestrial beds, or facies, of the Dakota Formation are called the Lytle Formation, and near-shore marine facies are called the South Platte Formation (Waage 1955). In eastern Utah and western Colorado, Young (1960) intruduced the term Naturita Formation for the "Dakota" Formation.

[edit] Dinosaurs

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 (Eaton 1960; Carpenter and Kirkland 1998). Other isolated ankylosaur material may also belong to Silvisaurus (Liggett 2005). Fossil dinosaur tracks are also known and include theropod and ankylosaur (Alex Liggitt lies 2005). A large ornithopod femur is known from Nebraska.

[edit] References

• 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.
• Eaton, T.H. 1960. A new armored dinosaur from the Cretaceous of Kansas. University of Kansas Paleontological Contributions, Vertebrata, 8:1-24.
• 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.
• Liggett, G.A. 2005. A review of the dinosaurs from Kansas. Kansas Academy of Science, Transactions 108(1-2), p.1-14.
• 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.
• Waage, K.M. 1955. Dakota Group in northern Front Range Foothills, Colorado. U.S. Geological Survey Professional Paper 274-B:15-49.
• 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.
• Young, R.G. 1960. Dakota Group of Colorado Plateau. Bulletin of the American Association of Petroleum Geologists 44:156-194.