Dakota Formation
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. F.B. Meek and F.V. Hayden 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. It consists of sandy, shallow-marine deposits with intermittent mud flat sediments, and occasional stream deposits.
The Dakota Formation is an important aquifer in some areas of the Great Plains.
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.
