DEVELOPING A REFINED MAGNETOSTRATIGRAPHY FOR THE WHITE RIVER GROUP AND OGALLALA FORMATION IN SOUTHWESTERN KANSAS: PRELIMINARY RESULTS FROM THE HIGH PLAINS-OGALLALA DRILLING PROJECT

Seventy samples were collected from a 98 meter core (HP1A) of the High Plains aquifer in southwestern Kansas retrieved as part of the High Plains-Ogallala Drilling Program (HDODP). The drill cores were collected using Rotosonic (saturated zone) and Hollow-stem auger with split-spoon core barrel (unsaturated zone) drill rigs. The sampled horizons were identified by relatively high bulk magnetic susceptibility value (2.0-3.0 x 10^-3 SI) relative to less magnetic intervals. Additional samples are being analyzed between these high bulk susceptibility zones to develop as complete a magnetostratigraphy as possible. Extraction methods did not allow the cores to be oriented, thus declination values are unknown. Samples, prepared into 1.5 cm³ specimens, were subjected to AF demagnetization and yield one of four responses: normal polarity (positive, steep inclination), reverse polarity (negative, steep inclination), shallow positive/negative inclinations, anduninterpretable behavior. Samples analyzed for Curie point estimates and magnetic property measurement system (MPMS) experiments indicate that the magnetic mineralogy is almost entirely of single domain to pseudosingle domain grain size with a restricted composition of near pure to low-Ti titanomagnetite. Oxide petrology reveals very fine grained opaque grains that were likely transported into the basin by eolian processes, given their limited grain size (between 0.15 and 0.25 mm). Large opaque grains are rare to non-existent, even in the coarsest grained horizons, suggesting that fluvial addition of magnetic grains was not a major input source. Based on optically stimulated luminescence age determinations, the upper 12 meters of the core is Quaternary in age. Magnetic polarity data document at least three possible excursion or high amplitude secular variation events in the Quaternary portion of the core. One event is tentatively correlated to the Laschamp excursion at ~41 ka, although declination data are lacking and thus the VGP position is unknown. New preliminary age determinations from detrital zircon grains indicate that most, if not all, of the HP1A core, assumed to be Miocene Ogallala Formation, is substantially older and tentatively correlated with part of the Paleogene White River Group (Eocene-Oligocene).