Abstract

As part of an extensive research program, a borehole was drilled to extend sedimentary facies models of the Cretaceous into the subsurface on the eastern side of the western interior Cretaceous basin in Kansas. Lithodensity-neutron and spectral gamma-ray logging runs were completed to facilitate interpretation of rocks penetrated by the borehole. Th/K, Th/U, and composition profile logs based on apparent grain density and photoelectric absorption index were prepared and used to show vertical changes in geochemical facies and clay mineralogy. These logs were compared with the gamma-ray log and drill cuttings to interpret depositional environments. The Th/K log sharply defines the Cretaceous/Permian boundary and, together with the Th/U log, emphasizes the contrast between marine Upper Cretaceous rocks of the Greenhorn depositional cycle and nonmarine to transitional rocks of the Lower Cretaceous. The long-term cyclic pattern of the Th/U log is an excellent indicator of a broad transgression/regression during the Greenhorn cycle on an open marine shelf, whereas extreme fluctuations of Th/U in the Lower Cretaceous rocks suggest a high degree of short-term environmental variability. Interpretation of the RHOMAA-UMAA compositional profile in the Dakota Formation indicates several pulses of marine transgression and regression prior to the initiation of the Greenhorn cycle in central Kansas.

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