RE-EVALUATING THE RADIOMETRIC AGE AND STRATIGRAPHIC CORRELATION OF CRETACEOUS BENTONITES WITHIN THE GRANEROS SHALE AND GREENHORN LIMESTONE IN CENTRAL KANSAS

Understanding the stratigraphy of the Upper Cretaceous in Kansas is of scientific and economic importance. The Graneros Shale is an aquitard overlying the Dakota aquifer, a significant freshwater resource. Scientifically, the Graneros Shale and Greenhorn Limestone provide insight into significant shifts in the global carbon cycle including the Mid-Cenomanian Event (MCE) and Ocean Anoxic Event 2 (OAE2) (Andrzejewski et al., 2022). Additionally, the Graneros Shale and Greenhorn Limestone record significant changes during the formation of the Western Interior Seaway (WIS). However, the transgressive/regressive cycles of the WIS result in complex and discontinuous stratigraphy, posing challenges for correlating units. Historically, a thick bentonite identified in the Graneros Shale referred to as the “X-Bentonite” was used to correlate outcrops throughout Central Kansas and into Colorado (Hattin, 1965, Hamilton, 1994). Several less significant bentonites occur in the lower Greenhorn Limestone. However, limited geochemical or radiometric age data exist to confirm the age or correlation of these bentonites. An initial U-Pb radiometric age obtained by ICP-MS analyses from zircons separated from the “X-Bentonite” indicates it may be significantly younger (92.4 Ma), challenging earlier age interpretations and correlation of these units (Maresh et al., 2024).

This study aims to establish high-precision radiometric ages of bentonites from 3 localities in Russell County Kansas previously used to correlate Cretaceous stratigraphic units using the “X-Bentonite” marker. The significant bentonite within the Graneros Shale was sampled along with several overlying bentonites in the Greenhorn Limestone. Approximately 300 zircons were extracted from each sample and analyzed using U-Pb laser ablation ICP-MS. The initial dataset from Maresh et al. (2024) showed several age populations. Therefore, all samples were reassessed by chemical abrasion (CA)-TIMS, with the youngest and most concordant grains from each sample to estimate eruption ages. The results from this study will be used to assess if the age previously assumed for the thick “X-bentonite” within the Graneros Shale is correct and if these bentonites are in fact time-correlative and useful as marker beds within the complex stratigraphic sequence.