MAXIMUM DEPOSITIONAL AGE CONSTRAINTS FROM U-PB DATING OF ZIRCONS IN CENOZOIC DEPOSITS OF THE HIGH PLAINS AQUIFER, SOUTHWESTERN KANSAS

A new approach targeting paleosols as a source of geochronologically useful detrital zircons suggest a significantly older and more complex depositional history than previously conceived for the Cenozoic High Plains Aquifer system in western Kansas. Volcanic ashbeds are an excellent source for datable zircon grains and common in High Plains deposits, though such beds are typically of a very limited geographic extent. Mature paleosols, as condensed terrestrial sections, are likely to incorporate air fall zircons via piping through root channels, desiccation cracks, and animal burrowing activity. Thus, mature paleosols are likely to preserve a time-averaged record of zircon crystal populations that overlap with air fall tephra events. To test this hypothesis, initial work has concentrated on a 98 m-long core (HP1A) in southwestern Kansas previously assumed to be from the Miocene Ogallala Formation and composed of thick fluvial sands interbedded with thin mature paleosols. Bulk samples of sediment were processed from paleosols at the 15–16 m interval and the 33.5–34.5 m interval. Approximately 300 zircon grains from the non-magnetic heavy mineral fraction of each interval were hand-picked, mounted, and then analyzed for U-Th-Pb isotopes by laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Maximum depositional ages generated at these intervals were 27.8 ± 1.3 Ma with 6 concordant grains (youngest grain being 26.2 ± 1.4 Ma ) and 35.5 ± 0.3 Ma with 18 concordant grains, respectively. These results are consistent with 13 previous samples from HP1A and three other cores from southwest Kansas now totaling 1495 zircons grains analyzed, though of typically less than 50 zircon per sample. The complete absence of Neogene zircons in any of these cores is striking given that Miocene age zircons are readily identified in cores of the Ogallala Formation in northwestern Kansas and Nebraska using the same techniques. These maximum depositional ages in southwestern Kansas suggest Eocene to Oligocene age deposits equivalent to the White River Group in Nebraska and previously unknown from Cenozoic strata in Kansas.