SEDIMENT ROUTING OF THE OLIGO-MIOCENE OGALLALA FORMATION AND SUBSEQUENT EVOLUTION ON THE NORTH AMERICAN HIGH PLAINS BASED ON DETRITAL ZIRCON PROVENANCE

The Oligo-Miocene Ogallala Formation represents the sedimentary record of fluvial systems in central North America following the period of uplift of Laramide ranges. During this time, deposition of fluvial and eolian sands, conglomerates, and volcanic tuffs occurred across the US Great Plains from Nebraska, Kansas, Oklahoma and Texas, significantly influencing the High Plains topography, and leaving behind the water-bearing strata now known as the High Plains Aquifer.

In this paper, we present new detrital zircon (DZ) U-Pb geochronological data to help understand the evolution of sediment routing since Oligocene, and to provide context for understanding subsurface water flow on the High Plains. We collected samples of medium-to-fine-grained channel-belt sandstones in 13 outcrops of the Ogallala Formation from Nebraska to Texas as well as modern bar sands from major rivers across the High Plains. In total 31 samples were collected and analyzed, with ~600 DZ grains per sample by LA-ICP-MS at the Arizona LaserChron Center. In addition, we have calculated the detrital zircon age distributions for each sample and applied statistical analyses to understand significance to provenance and sediment routing.

Our results indicate that the provenance for the Ogallala Formation is dominated by the Laramide Rockies and recycled Cretaceous foreland basin fill. However, we find regional heterogeneity as well, and recognize distinct northern, central, and southern subregions for deposition based on contrasts in DZ U-Pb age distributions. Zircon grains with Mesoproterozoic ages provide specific insight: for example, Grenville grains (ca. 1250-950 Ma) are primarily constrained to the central subregion in Kansas and Oklahoma, whereas zircon grains originating from the Midcontinent province (ca. 1550-1350 Ma) are scarce in the southern subregion. Analyses of modern river sands reveal a continuity of detrital zircon U-Pb signatures from ancient to modern deposits within the same region, although modern samples tend to have more Cenozoic zircon grains. However, noteworthy changes in provenances of the South Platte River suggest potential reorganization of sediment routing and differential erosion in close proximity to the Front Range in Colorado.