Insights into the Cretaceous Dakota Aquifer System of central Kansas: applying sequence stratigraphy to unravel hydrogeologic complexities

High-resolution analysis of outcrops, cores, logs, and hydrogeologic datasets from central Kansas provide new insights into the Cretaceous units comprising the Dakota Aquifer System. This succession, spanning the Kiowa Fm., Dakota Fm., and Graneros Shale, is a groundwater resource that is increasingly being relied on by communities and businesses. While previous interpretations of geologic and hydrostratigraphic units relied on lithostratigraphic concepts, this study employs sequence-stratigraphic principles to redefine important framework surfaces. Numerous implications include our fundamental understanding of the timing and basin dynamics of the Greenhorn Cyclothem and its application to understanding and predicting hydraulic connectivity and changes in aquifer properties. Timing and correlation of these units relies on new U–Pb radiometric datasets combined with carbon stable isotope analyses to produce chronostratigraphic constraints. Initial datasets reveal a lower package comprising fluvial sandstones and thick coastal plain paleosols of the Dakota Fm. A sharp erosional contact is overlain by a conformable succession of tidally influenced sandstones and argillaceous mudstones. While previous interpretations classify the tidally influenced sandstones as Dakota Fm., this interpretation recognizes their genetic relation to the transgression of the Western Interior Seaway shoreline and the conformably overlying Graneros Shale. The argillaceous mudstones contain at least six parasequences, recording the successive drowning of the margin. The transgressive succession includes an important, regionally correlative ash bed previously hypothesized to be the “X-Bentonite” in upper Graneros Shale. Data from a newly installed high-resolution (38 monitoring zones) multilevel well reveal several changes in vertical gradient indicative of contrasts in vertical hydraulic conductivity with the largest change occurring across the transgressive surface of erosion that separates tidally influenced strata from underlying paleosols and fluvial sandstones. Together, these datasets allow integration of geologic and hydrogeologic data into new subsurface modeling and interpretation of the Dakota Aquifer System, providing pivotal information to Kansas water management agencies.