ASSESSING DEPOSITION AGE OF DAKOTA FM. PALEOSOLS IN SE NEBRASKA THROUGH DETRITAL ZIRCON U-Pb DATING

The Dakota Fm. is important for a wide range of topics from Cretaceous paleoclimate and fossil record reconstruction to understanding aquifer architecture in the Midcontinent. Economically the Dakota Fm. serves as a water resource comprising part of the Dakota Aquifer system and also contains valuable clay deposits that are used in construction materials. Paleosols within the Dakota Formation record paleoclimatic signals during greenhouse conditions and preserve an abundance of important plant fossils. Given both the economic and scientific importance of the Dakota Fm several studies have attempted to correlate its units throughout the Midcontinent; however, the nature of these deposits makes correlation of units challenging with varying thickness, and discontinuous lithologies due to the transgressive/regressive cycles that persisted along the eastern margin of the Western Interior Seaway.

This study aims to establish deposition ages of Dakota Fm. paleosols at the Endicott Clay Pit near Endicott, NE using youngest zircon populations. Approximately 120 zircon grains extracted from each paleosol sample are dated by U-Pb laser ablation ICP-MS, and a maximum deposition age is derived by Bayesian modelling of the youngest population, following the approach of Keller et al. (2018). Prior studies have shown that zircon from paleosols is more likely to yield true depositional ages than from sandstones, given an active volcanic arc is upwind. Results obtained from paleosols at the nearby Camp Jefferson and Rose Creek Clay Pit localities yielded successful zircon extraction and generate maximum deposition ages ranging from 100-95 Ma suggesting conditions for preserving detrital zircon populations in similar age Dakota Fm. paleosols is promising. Slightly older results are expected from the much thicker section at the Endicott Clay Pit, about 1 mile SE of Camp Jefferson and 3 miles NE of the Rose Creek clay pit, given its relative stratigraphic position. Results produced from the samples collected at the Endicott Clay Pit will be used to correlate to previously sampled outcrops and cores curated at the Kansas Geological Survey.