PALYNOSTRATIGRAPHIC FRAMEWORK FOR MID-CRETACEOUS STRATA, EASTERN MARGIN OF WESTERN INTERIOR BASIN
B.J. Witzke, G.A. Ludvigson, R.L. Ravn, R.L. Brenner, R.M. Joeckel
The Geological Society of America
1996 GSA Annual Meeting
Denver, Colorado, October 28-31, 1996
1996 Abstracts with Programs, v. 28, no. 7, p. A-185
The Dakota Formation along the Western Interior Basin's eastern margin in western Iowa and eastern Nebraska, consists of lithologies with complex stratigraphic relationships that include non-marine and marginal-marine mudstone and sandstone strata that grade laterally from fluvial facies, westward into coastal facies. Palynologic sampling has produced 140 genera and 265 species of palynomorphs from four general palynostratigraphic units that subdivide the "mid-Cretaceous" into 0.5 to 2.0 m.y. increments: 1) A basal Dakota (lower Nishnabotna Member) assemblage contains key forms (Eucommiidites minor, Foveosporites labiosus, Cicatricosisporites patapscoensis, others) of mid-Late Albian age (Kiowa Formation equivalent). Marine palynomorphs are associated in southeast Nebraska. 2) A lower to mid Dakota (Terra Cotta, basal Woodbury, members) assemblage contains the highest occurrences of typical Albian palynomorphs (e.g., Exesipollenites tumulus, Cedripites canadensis, others), equivalent in part to the Muddy Sandstone of Wyoming. 3) A mid to upper Dakota (lower Woodbury Member, early Cenomanian) assemblage is marked by the first occurrences of characteristic Cenomanian taxa, constrained below the last-appearance datum of Neoraistrickia robusta. 4) Upper Dakota (mid to upper Woodbury and Janssen Clay members, mid and late Cenomanian) assemblages are marked by a general upward loss of forms common in older Albian units and upward increase in angiosperm diversity, including morphologically complex forms. The first-appearance datums of Cicatricosisporites crassiterminatus and Motiapollis hallii are used to subdivide unit 4. Marine palynomorphs generally increase in abundance upward.