Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 258, part 2
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In 1994, a long-term research program to investigate the stratigraphy of the Cretaceous Dakota Formation in Iowa and immediate environs (Brenner et al., 1981; Witzke and Ludvigson, 1982, 1987, 1994; Witzke et al., 1983; Ravn and Witzke, 1994, 1995) was geographically expanded to include exposures and research drillcores in Nebraska and Kansas, under the auspices of NSF grant number EAR-9628128. This research program included 1) systematic collection, identification, and correlation of palynostratigraphic samples, with the goal of improving biostratigraphic and chronostratigraphic resolution in the interval, and 2) systematic collection of stable isotope paleoclimate proxy data to address the mid-Cretaceous climate-change record contained within the Dakota Formation.
Initial major findings on the regional stratigraphy of the Dakota Formation were published by Brenner et al. (2000). They showed that strata of the Dakota Formation, as originally defined by Meek and Hayden (1862) from its type region in the Missouri River valley (fig. 1), include Albian units that are time-stratigraphic correlates to the marine shales of the Kiowa Formation in Kansas, and Cenomanian units that are time-stratigraphic correlates to the Graneros marine shale in Colorado (fig. 2). Brenner et al. (2000) partitioned the Dakota Formation into three major sedimentary sequences bounded by correlative unconformities (D0, D1, and D2), correlations that were based on recognition of four palynostratigraphic zones (fig. 3). This report presents a more complete discussion of the palynostratigraphy of the Dakota Formation, with updated information on the stratigraphic ranges of some key palynologic taxa.
Figure 1--Generalized regional outcrop map of the Dakota Formation and equivalent strata between southern Minnesota (MN) and northern Kansas (KS). Blue-green-colored outcrop denotes the Dakota Formation in western Iowa (IA) and eastern Nebraska (NB), the Dakota and Kiowa Formations in Kansas, and equivalent units in Minnesota. Eastern outliers (dots) include the Dakota Formation in Iowa; the Windrow Formation in Iowa, Wisconsin (WS), and Minnesota; and the Baylis Formation in western Illinois (IL). Cretaceous strata lap the margins of the Sioux Ridge, a long-lived paleotopgraphic feature composed of resistant Precambrian Sioux Quartzite. General study areas are circled and labeled. Guthrie County, Iowa, is outlined. Abbreviations: L.R.--Little River; M--Manson Impact Structure (of late Campanian age). State boundaries are dashed. From Ludvigson and Witzke (1996).
Figure 2--Regional cross section of the Dakota Formation and temporal equivalents denoting three unconformity-bounded sequences based on lithostratigraphy and the palynostratigraphy outlined in this report. Graphic logs in study areas are based on measured sections, archived drillcores, well logs, and composites of these as noted. Solid lines represent sequence boundaries (D0, D1, D2); dashed lines represent major facies boundaries. From Brenner et al. (2000). An Acrobat PDF version of this figure is available.
Published findings on the stable isotope paleohydrology and paleoclimatology of the Dakota Formation have described work on pedogenic sphaerosiderites from paleosols (Ludvigson et al., 1998c; White et al., 2001; Ufnar et al., 2002, 2004a, 2004b; White et al., 2005). Concomitant works on other terrestrial stable isotope proxies from the unit are just beginning to emerge in the peer-reviewed scientific literature (see Ufnar et al., 2004a; Phillips et al., 2007). One important facet of this emerging work pertains to the field recognition of early diagenetic cementation of high-frequency (i.e. parasequence) boundaries in estuarine facies of the Dakota Formation. We present previously unpublished results on this topic in this report.
Mid-Cretaceous sediments of the midcontinent, as in much of North America, contain extraordinarily rich and well-preserved fossil palynomorphs. Through much of the region, these populations are dominated by the spores and pollen of terrestrial plants, reflecting the mid-Cretaceous seaway regression that punctuated Albian-Cenomanian time. These microfossils record a biostratigraphic succession of great utility for correlation and age interpretation. Detailed documentation of the regional Albian-Cenomanian microfloral succession is in preparation and is beyond the scope of this publication, but a number of key forms are illustrated here, and some brief comments on their significance are in order.
Numerous comparative studies are valuable in assessing the ages of strata examined in the region. In the immediate area, the publications of Pierce (1961) from Minnesota, Ward (1986) from Kansas, and Ravn and Witzke (1994, 1995) from Iowa are especially relevant. From surrounding areas, the studies of Brenner (1963), Singh (1964, 1971, 1983), Hedlund (1966), Norris (1967), Agasie (1969), Playford (1971), Phillips and Felix (1972a, 1972b), Romans (1975), Srivastava (1977), Wingate (1980), Nichols and Jacobson (1982), and Ravn (1995) are especially useful. Overall, the assemblages and succession from nearby areas are well documented, and a record of the palynostratigraphy from this area central to the Cretaceous midcontinent seaway is of great value in interrelating these earlier studies.
The taxa illustrated in plates 1 and 2 are from numerous localities in Kansas, Nebraska, Iowa, and South Dakota, and represent characteristic material from strata of late Albian through Cenomanian age. Of particular interest for Albian-age interpretation are the fern-related spore species Plicatella jansonii (Pocock) Dörhöfer 1977, Plicatella unica (Markova) Dörhöfer 1977, Impardecispora marylandensis (Brenner) Venkatachala et al. 1969 (not illustrated), Impardecispora apiverrucata (Couper) Venkatachala et al. 1969, Impardecispora excavata Ravn 1995, the gymnospermous pollen species Nicholsipollis mimas Ravn 1995, and the angiospermous pollen species Quadricolpites reticulatus Wingate 1980, all of which appear to be restricted in the region to Albian strata. Species appearing to be restricted to Cenomanian strata include Microreticulatisporites sacalii (Deák & Combaz) Ravn 1986, Lycopodiacidites arcuatus Hedlund 1966, and Cicatricosisporites crassiterminatus Hedlund 1966. The other taxa illustrated have observed ranges spanning various portions of late Albian and Cenomanian strata in the region.
Also of note is the presence of the gymnospermous pollen grain designated here as Jiaohepollis n. sp. (formal description of this new species is in preparation). As far as we know this report documents the first known occurrence of pollen corresponding to this genus in North America. The genus is originally based on species described from strata of earlier Cretaceous age in China.
Plate 1--All micrographs are at x625 magnification. Plates 1 and 2 are available as an Acrobat PDF file containing both plates.
Plate 2--All micrographs are at x625 magnification. Plates 1 and 2 are available as an Acrobat PDF file containing both plates.
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Kansas Geological Survey
Placed online April 6, 2010