DECODING THE DAKOTA IN KANSAS: AN EASTERN MARGIN PERSPECTIVE ON THE EVOLUTION OF THE WESTERN INTERIOR SEAWAY

High-resolution analysis of outcrops, cores, and logs from central Kansas provide new insights into the evolution of the eastern margin of the mid-Cretaceous (Albian-Cenomanian) Western Interior Seaway (WIS). Units of interest include the Cheyenne Sandstone, Kiowa Fm., Dakota Fm., Graneros Shale, and Greenhorn Limestone, and collectively make up the Dakota Aquifer System, an important regional groundwater resource. Previous research focused on the lithostratigraphy and regional distribution of these units. The current study pairs modern chronostratigraphic techniques including U-Pb radiometric age dating and carbon stable isotopic analyses paired with sequence-stratigraphic characterization to refine and reinterpret previously outlined frameworks.

Multiple orders of framework surfaces and major carbon isotope excursions are identified and correlated using absolute age control and relative markers. The lower package comprises a thick succession of coastal plain paleosols and fluvial deposits and records carbon isotope shifts consistent with OAE1d. An abrupt erosional transition is overlain by a conformable succession of amalgamated deltaic sandstones and argillaceous mudstones which record the transgression of the WIS and a second carbon isotopic excursion, the Mid-Cenomanian Event. Six parasequences in an overall retrogradational succession record successive drowning of the margin. The “X-Bentonite”, a high confidence correlative ash bed, is used to date the maximum flooding surface at 95.53 ± 0.36 Ma. The highstand succession records a transition from detrital to carbonate-dominated marine sedimentation. This transition is abrupt in outcrop in western parts of the study area and more gradual to the east. All exposures record a positive carbon isotope shift consistent with OAE2.

The proposed stratigraphic framework has implications for the connectivity of Dakota Aquifer units. For example, previous interpretations assigned amalgamated deltaic sandstones to the Dakota Fm., whereas the current work recognizes the unit as part of the overlying transgressive systems tract. Interpretations of lateral connectivity and changes in hydrologic conductivity can therefore be refined in subsurface models.