CHRONOSTRATIGRAPHY OF THE OGALLALA FORMATION IN WESTERN KANSAS: A NEW APPROACH USING PALEOSOLS

Continental sedimentary layers are notoriously difficult to date because the poor preservation potential of stratigraphic marker beds and the paucity of biostratigraphic markers complicates the task of chronostratigraphic data collection. Tephra are well understood as marker beds because they define instantaneous depositional events, but their limited extent in terrestrial settings hinders their potential for regional correlations. We test the hypothesis that paleosols contain populations of volcanogenic zircon grains accrued from tephra deposition during periods of subaerial exposure to reliably date the paleosol. Airfall zircons from volcanic events coeval with paleosol deposition could preserve the depositional age although the ash layers are not preserved. If significant populations of age appropriate zircons are identified, paleosols could be used as chronostratigraphic marker beds. The Ogallala Fm. in western Kansas offers a unique opportunity to test this hypothesis because there are ten distinct ash beds and as many as thirty-three paleosols described in the formation. A section of stacked paleosols and an ash lens from Devil’s Backbone and Battle Canyon outcrops, near Lake Scott, KS were sampled for this type of geochronologic analysis. The 11.4 +/- 0.5 Ma (Bruneau-Jarbidge, 12.7-10.5 Ma) ash lens in the sampled section serves as a datum against which the U-Pb zircon data from the under- and overlying paleosols are compared. Analyses were completed using n=300 measurements per sample, resulting in only a 5% chance of missing a small population (1% of total grains). Significant populations of zircons were found in the paleosols 2m below the ash lens and have similar dates to the Bruneau-Jarbidge eruptions. Zircon grains found in a paleosol 12m above the Bruneau-Jarbidge ash yield dates of ca. 6 Ma. Further research in this study may provide high-resolution temporal relationships between other paleosols layers in the section. Interpretation of these results will be used to constrain timing of deposition, depositional rates, and paleoclimate information. This detailed stratigraphic information, which is currently lacking, is crucial in order to understand stratigraphic controls on groundwater flow in the High Plains Aquifer.