TOWARD AN ALBIAN-CENOMANIAN SPHAEROSIDERITE CHEMOSTRATIGRAPHY IN THE DAKOTA FORMATION OF THE TYPE AREA, NW IOWA AND NE NEBRASKA
G.A. Ludvigson, B.J. Witzke, L.A. Gonzalez, R.L. Brenner, R.M. Joeckel, R.H. Hammond, R.L. Ravn
The Geological Society of America
29th Annual North-Central Section and South-Central Section Meeting
University of Nebraska-Lincoln, Lincoln, Nebraska, April 27-28, 1995
1995 Abstracts with Programs, v. 27, no. 3, p. 70
Sphaerosiderites (SPH) are mm-scale radial-spherulitic ferroan carbonate concretions that form as meteoric phreatic authigenic minerals in modern alluvial bog and lacustrine sediments. They are abundantly preserved in lignite-bearing fluvial mudstones of the mid-Cretaceous Dakota Fm in Iowa and Nebraska, where we are evaluating their potential for preserving a long-term paleoclimatic record of changing freshwater carbonate 18O/16O compositions. Research efforts are being focused on a 64-m cored sequence at Sergeant Bluff, Iowa, where a palynostratigraphic zonation is being constructed through studies of interstratified carbonaceous units. Multiple independent lines of evidence suggest that each SPH sample horizon preserves a primary geochemical record of early carbonate precipitation in shallow groundwaters that saturated the host sediments: 1) uniform nonluminescence of SPH suggests lack of late diagenetic alteration in burial environments; 2) concentric epifluorescence zonations indicate growth histories that are unique to each SPH sample horizon; 3) SPH are free of detrital inclusions, suggesting early displacive crystal growth in soft uncompacted muds before significant burial; 4) SPH predate formation of pedogenic argillans and ferric oxide mottles confirming early origin in Dakota Fm paleosols; and 5) highly variable 13C/12C values and invariant 18O/16O values measured from specific SPH sample horizons are analogous to meteoric calcite lines recognized in limestones diagenetically stabilized in early meteoric phreatic environments. Consequently there is no evidence for secondary alteration of 18O/16O values. Late Albian SPH has a mean 18O/16O of -1.47 PDB, while early Cenomanian SPH has a mean 18O/16O of -2.49 PDB. This one per mil decrease in SPH 18O/16O is interpreted to reflect 18O/16O depletion of groundwater recharge in response to a long-term global cooling from a thermal maximum in the Aptian-Albian. Stable isotopic data from SPH of the Dakota Fm confirm earlier results showing that mid-Cretaceous meteoric phreatic carbonates from the cratonic margin of the Western Interior Basin have 18O/16O values that are consistent with groundwater recharge by rainfall in coastal lowland settings.