Volcanogenic zircon geochronology from paleosols in Cenozoic terrestrial strata

Volcanic ash beds are unrivaled as geochronological marker horizons because radiometric dates from volcanic minerals yield high-precision absolute ages of deposition. Such chronostratigraphic information is difficult to obtain however because tephras are rare. Our study tests the hypothesis that paleosols are significant reservoirs for volcanogenic zircons representing the depositional age of continental strata. Paleosols, as time-rich stratigraphic horizons, are likely to incorporate airfall zircons from volcanic eruptions that overlapped with extended periods of landscape stability and pedogenesis, even in areas where volcanicity is low or volcanic sources are far away. Zircon-bearing ashes from late Miocene calderas on the Snake River Plain were deposited along with the Ogallala Formation on the Great Plains. The combination of nearly continuous volcanic activity and the high stratigraphic frequency of paleosols in the Ogallala Formation make it an ideal testbed of the application of ancient soils as repositories for chronostratigraphically significant zircons. We sampled ten paleosols in the Ogallala Formation in the central Great Plains for U Pb zircon dating. Six paleosols produced maximum depositional ages (MDAs) ranging from 11.4 ± 0.4 Ma to 6.3 ± 0.3 Ma and correct order of superposition. Paleosols are the only facies in the study areas that yield Miocene MDAs likely to coincide closely with depositional ages. This methodology has the clear potential to greatly decrease uncertainties in paleontological and biostratigraphic studies. In addition, our results suggest that paleosol geochronology can be used on regional or basin-wide scales to make higher order correlations. Future work using paleosols as geochronometers could unravel the complex depositional history of the Cenozoic Great Plains succession and resolve previously problematic internal heterogeneities. • Volcanic ash beds and associated datable minerals are ideal geochronologic marker beds, but are rarely preserved. • Paleosols likely incorporate zircons from contemporaneous volcanic airfall that would otherwise erode from paleolandscapes. • Volcanic ash and zircons erupted from calderas on the Snake River Plain were deposited with the Miocene Ogallala Formation. • Six paleosols produced maximum depositional ages (MDAs) ranging from 11.4 ± 0.4 Ma to 6.3 ± 0.3 Ma and in superposition. • Paleosol geochronometry can be used on a regional or basin-wide scale to make higher order correlations.