RECENT RESEARCH ACTIVITY ON THE DAKOTA FORMATION IN THE TYPE AREA, NORTHWEST IOWA AND NORTHEAST NEBRASKA
G.A. Ludvigson, B.J. Witzke, R.L. Ravn, and R.M. Joeckel
Water and Energy Resources of the Dakota
Aquifer: Workshop and Clinic
Holidome, Great Bend, Kansas
April 11-13, 1994
Program and Abstracts, p. 9-11
The type area for the Dakota Fm was designated for natural exposures of mudstones, sandstones, and lignite along bluffs of the Missouri River near Dakota City in Dakota Co, NE. In this area, the Dakota reaches thicknesses greater than 150 m, with only the upper 40 m actually exposed. Research drilling focused on the Dakota during the late 1970s and early 1980s by the Iowa Survey (Munter et al., 1983) led to resumption of White's (1870) subdivision into two major lithostratigraphic units. The lower Nishnabotna Mbr reaches thicknesses to 90 m, and consists of 80 % coarse-fine gr. quartzarenite, with lesser amounts of kaolinitic mudstone and lignite, and has a sheetlike geometry. This unit comprises the Dakota Aquifer of western IA. The overlying Woodbury Mbr reaches thicknesses to 60 m, and is dominantly a mudrock unit, with lignites and lenticular v. fine-fine gr. micaceous quartzarenites. Regionally, the Woodbury overlaps the Nishnabotna eastward to rest on eroded Paleozoic strata, and to the northwest where it rests on erosional highlands of Proterozoic quartzite on the Sioux Ridge. Witzke et al. (1983) showed that the Nishnabotna passes southwestward into laterally equivalent mudrock-dominated strata in NE. Palynostratigraphic evidence for late Albian strata in the Nishnabotna of IA and laterally equivalent mudrocks in southeast NE indicate that detailed time- and sequence-stratigraphic relationships with the Kiowa Fm in Kansas need to be clarified.
Work on Woodbury exposures in the type area (Witzke and Ludvigson, 1987; in press) indicate that the upper Woodbury interval was deposited marginal to the encroaching Greenhorn sea (middle to upper Cenomanian) in shallow marine, deltaic, and coastal lowland settings. Exposures of underlying lower to middle Woodbury nonmarine strata appear to the south at Sergeant Bluff, IA and in the Winnebago and Omaha Reservations in Thurston Co., NE. Closely-spaced cores at Sergeant Bluff show that the Woodbury contains a 64 m interval of interbedded mudstones, siltstones, sandstones, and lignites with several tens of discrete and/or welded paleosols. Typical paleosol profiles extend vertically 1-2 m, and consist of red-mottled gray blocky mudstones passing down into gray blocky mudstones with sphaerosiderites. Mesoscopic evidence for pedogenesis includes blocky structure, slickensides, rooting, and color mottling. Cross-striated birefringent clay fabrics in groundmass of blocky mudstones, and birefringent clay coatings along fracture planes provide further evidence of pedogenic alteration. Petrographic fabrics of sphaerosiderites (mm-scale spherulitic ferroan carbonate concretions) indicate early diagenetic origin. Electron probe microanalyses show that they approach end-member FeCO<sub>3</sub> compositions indicative of formation in freshwater (Mozley, 1989).
Carbon-oxygen isotopic sampling of a sphaerosiderite horizon shows 13C ranging from -39.05 to -7.90 o/oo, while 18O ranges only from -4.31 to -2.02 o/oo. In carbon-oxygen isotope space, data are arrayed in a linear trend with negligible variation in 18O, interpreted as a meteoric carbonate line (MCL). MCL for the Dakota (18O = -3 o/oo) compares to MCL of 18O = -6.75 o/oo from the Middle Turonian Carlile Fm, further supporting an argument by Ludvigson et al. (in press) that MCLs from the eastern margin of the Western Interior Basin are fundamentally different from those of the western margin. Differences are attributed to regional dominance of western margin groundwater systems by runoff that condensed at high altitudes in the Sevier orogen (MCLs with 18O <-10 o/oo), as compared to eastern margin groundwater systems fed by recharge on coastal lowlands. Moreover, decreasing MCL 18O recorded over the Dakota-Carlile interval (95-90 Ma) suggests long-term cooling following peak global warming at 100 Ma.
Palynostratigraphic investigations of lignitic and carbonaceous Dakota strata in the type area provide a basis for chronostratigraphic resolution of the Dakota sequence. Samples from the Woodbury have yielded abundant palynomorphs, primarily fern-dominated, that support lower, middle, and lower upper Cenomanian ages ascending through the unit (Ravn and Witzke, in press a, b). Foraminifera and inoceramids at the top of the Woodbury indicate a lower upper Cenomanian age (Witzke and Ludvigson, in press). Palynomorphs are notably rarer in the coarser-grained Nishnabotna, but limited collections are suggestive of, but probably not definitive for, an upper Albian age assignment (Ravn and Witzke, in press a, b). Southwestward from the type Dakota area, the lower mudrock-dominated Dakota interval in southeast Nebraska yields definitive upper Albian palynomorphs. Additional work on new samples recovered from the Sergeant Bluff cores and exposures in the Winnebago and Omaha Reservations is planned.
Ludvigson, G.A., Witzke, B.J., González, L.A., Hammond, R.H., and Plocher, O.W., in press, in Geological Society of America, Special Paper 287 (due in early 1994). Mozley, P.S., 1989, Geology, v. 17, p. 704-706.
Mozley, P.S., 1989, Relationship between depositional environment and elemental composition of early diagenetic siderite: Geology, v. 17, p. 704-705.
Munter, J.A., Ludvigson, G.A., and Bunker, B.J., 1983, Iowa Geological Survey, Water Supply Bull., No. 13, 55 p.
Ravn, R.L., and Witzke, B.J., in press a, Palaeontographica, Abteilung B. (due in early 1994).
Ravn, R.L., and Witzke, B.J., in press b, in Geological Society of America, Special Paper 287 (due in early 1994).
White, C.A., 1870, Report on Geological Survey of the State of Iowa, v. 1: Mills and Co., Des Moines, IA, 391 p.
Witzke, B.J., Ludvigson, G.A., Poppe, J.R., and Ravn, R.L., 1983, in Reynolds, M.W. and Dolly, E.D., eds., Mesozoic Paleogeography of West-Central United States: RMS-SEPM, Denver CO, p. 225-252.
Witzke, B.J., and Ludvigson, G.A., 1987, in Biggs, D.L., ed., North-Central GSA, Decade of North American Geology Centennial Field Guide, Geological Society of America, v. 3, p. 97-102.
Witzke, B.J., and Ludvigson, G.A., in press, 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 (due in early 1994).