DETERMINING DEPOSITIONAL AGE OF DAKOTA FM. PALEOSOLS IN CENTRAL KANSAS THROUGH U-Pb DATING OF DETRITAL ZIRCONS

The Dakota Fm. holds significant value to a wide range of topics from reconstructions of Cretaceous paleoclimates and fossil records to modern day aquifer architecture. The Dakota Fm. is an important economic resource by comprising part of the Dakota aquifer system and by its clay deposits. Additionally, the paleosols within the Dakota Fm. record paleoclimatic signals of greenhouse conditions and preserve a wealth of important plant fossils. Due to the economic and scientific implications of the Dakota Fm. several studies have attempted to correlate its units throughout the Midcontinent. Bentonites from the overlying Graneros Shale and Greenhorn Fm. provide geochronologic constraints on the Cretaceous shallow marine units in the Midcontinent. However, the nature of these deposits with varied thicknesses and discontinuous lithologies formed by the transgressive and regressive cycles of the eastern margin of the Western Interior Seaway makes correlation challenging.

This study aims to determine depositional ages for Dakota Fm. paleosols from three different localities in central Kansas by interpreting the youngest zircon populations. Previous studies indicated that zircon ages from paleosols are more likely to yield true depositional ages than those based zircon from sandstones, provided that a volcanic arc is active upwind. We analyzed samples from Dakota Fm. paleosols, and from bentonites found in the Graneros Shale and Greenhorn Limestone by U-Pb laser ablation ICP-MS. We analyzed a total of 14 samples: 9 detrital samples from Dakota Fm. paleosols at 3 localities as well as 5 bentonite samples from outcrops near Russel, Kansas. One bentonite sample from the Graneros Shale is suspected to be the X-Bentonite from previous work in Nebraska, the other four are from thinner bentonites within the Greenhorn Limestone. Maximum deposition ages were derived by Bayesian modeling of the youngest population, following the approach of Keller et al. (2018). Results from these samples and localities will be used to correlate Cretaceous core and outcrop samples curated at the Kansas Geological Survey, and future stable isotope stratigraphy obtained from these sections.

Keller, C.B., Schoene, B. and Samperton, K.M., 2018. A stochastic sampling approach to zircon eruption age interpretation. Geochemical Perspectives Letters (Online), 8(LLNL-JRNL-738859).