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The Dakota Formation (also Dakota Sandstone and Cockrum Sandstone, more formally the Dakota Group) is a geologic formation composed of sedimentary rocks deposited on the eastern side of the Late Cretaceous Western Interior Seaway.〔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〕 F.B. Meek and F.V. Hayden named it for exposures along the Missouri River near Dakota City, Nebraska.〔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.〕 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. It consists of sandy, shallow-marine deposits with intermittent mud flat sediments, and occasional stream deposits.〔("Geology of the Quarry: Dakota Sandstone" ) Dinosaur National Monument, National Park Service〕〔McLaughlin, Thad G. (1942) ("Water-bearing Formations, continued: Cretaceous System: Dakota Group" ) ''Geology and Ground-Water Resources of Morton County, Kansas''〕 The Dakota Formation is an important aquifer in some areas of the Great Plains.〔〔Wang, Herb (2003) ("Saga of the Dakota Sandstone" )〕 ==Geological history== 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.〔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, 〕 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, 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.〕 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.〔 Some of the ancient preserved soils show that an extensive flood plain forest was present. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dakota Formation」の詳細全文を読む スポンサード リンク
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