THE DAKOTA FORMATION IN IOWA AND THE TYPE AREA
B.J. Witzke and G.A. Ludvigson
Geological Society of America
Special Paper 287, 1994, p. 43-78
The historic type area of the Dakota Formation lies along the Missouri and Big Sioux River Valleys in northeast Nebraska and northwest Iowa, in the eastern margin area of the Western Interior Province. The type Dakota sequence is characterized to provide a basis of comparison with supposed "Dakota" sections elsewhere in the Wetsern Interior. The general sequence includes (1) a lower sandstone-dominated Nishnabotna Member with coarse-grained and conglomeritic facies, and (2) an upper mudstone-dominated Woodbury Member with sandstone channel bodies and lignites. The depositional sequence is interpreted to record, in ascending order: (1) aggradation of coarse-grained braided and proximal meanderbelt fluvial systems; (2) an episode of lowered base levels, channel incision, and widespread pedogenic alteration; (3) aggradation of channels and flood-basin sediments in meanderbelt fluvial systems; and (4) upward transition into deltaic and marginal-marine facies. The Woodbury Member is of Cenomanian age, and is interpreted to have aggraded in response to sea-level rise in the adjacent seaway during transgressive phases of the Greenhorn Cyclothem, with progressive eastward displacement of nonmarine by marine facies. The age of the lower Dakota sequence is known with less certainty, but is probably upper Albian in part. Aggradation of lower Nishnabotna fluvial systems may be related to sea-level rise during transgression of the Kiowa-Skull Creek Cyclothem, although the possibility that the entire Dakota sequence is post-Kiowa cannot be completely discounted. Based on the type sequence, usage of the term Dakota Formation should be restricted to include only eastern-derived detrital sequences and should exclude marginal-marine or marine facies of the Kiowa-Skull Creek Cyclothem. Usage of the term "Dakota" for western-derived sedimentary sequences in the Rocky Mountain area is discouraged.