PRELIMINARY MAGNETOSTRATIGRAPHIC ANALYSES OF THE NEOGENE OGALLALA FORMATION IN SOUTHWESTERN KANSAS AND NORTHEASTERN NEW MEXICO FOR THE HIGH PLAINS-OGALLALA DRILLING PROGRAM

The High Plains-Ogallala Drilling Program (HPODP) has been developed to advance scientific understanding of sedimentary facies, stratigraphic framework, and chronostratigraphy of the Ogallala Formation and overlying units of the central High Plains aquifer. The High Plains aquifer has come under increasing stress with growth of urban and agricultural areas and developing the best groundwater management policies will depend on the most accurate characterization of the aquifer, the aquifer materials and their stratigraphic and geochronologic framework. In northeastern New Mexico and southwestern Kansas, the Neogene Ogallala Formation represents sediment accumulation as a clastic apron shed to the east from the Rocky Mountains. Although mammalian faunal assemblages and volcanic ash bed tephrochronology provide a basic geochronologic framework for the Ogallala Formation, better precision in terms of correlation is needed in order to understand formative processes and depositional histories for the primary water-bearing units in the High Plains aquifer and confining strata. In order to develop this framework, six drill cores are currently being extracted from southwestern Kansas that capture most of the Ogallala Formation and overlying units. Additional sampling of surficial outcrops of the Ogallala Formation in Union County, northeastern New Mexico, is being performed in order to develop a better understanding of the relationships among complex facies as the Ogallala Formation pinches out to the west. The first drill core was recovered from Kansas in summer 2011 and samples collected for bulk magnetic susceptibility, magnetostratigraphic and rock magnetism analyses. Samples from the surface exposures in northeastern New Mexico are also currently being analyzed. A better understanding of the Ogallala Formation is critical in terms of developing better groundwater flow models and management policies for communities relying on aquifers in heterolithic sequences around the globe.