Tracing the Origins and Significance of the Miocene Ogallala Formation, Great Plains, USA

The Miocene Ogallala Formation blankets the Great Plains east of the Rocky Mountains and has tentatively been linked to a cryptic period of late Cenozoic topographic reorganization along the Front Range. Despite the widespread spatial footprint of the Ogallala Formation and potential connections with topographic reorganization, source-to-sink pathways using modern provenance techniques are lacking. We present new detrital zircon U-Pb geochronology from a spatially dispersed sample set from the Ogallala Formation and age-equivalent strata in the Rio Grande Rift to establish source-to-sink linkages. Five new detrital zircon U-Pb geochronology samples across the Ogallala Formation show seven distinct age groups, >1825, 1800-1535, 1500-1332, 1300-920, 750-300, 250-44, and 40-24 Ma, and are lacking prominent syndepositional components. These samples, along with data from several previous studies, show north to south differences that inform sediment-routing reconstructions. Additionally, two new samples from quasi-contemporaneous units in the Rio Grande Rift show similar age distributions, with differences in the abundances of some age modes. We compare these results with compiled data sets from potential source regions and age-equivalent units. Composite age spectra from Cretaceous to late Cenozoic strata in Rocky Mountain basins, along with basement exposed in the Rocky Mountains, mirror the results from the Ogallala Formation, offering a compelling source. Recent work suggests that evacuation of the Rocky Mountain basins occurred contemporaneously with, or immediately predated, deposition of the Ogallala Formation, supporting our provenance interpretations. Similarities to Miocene strata in the Gulf of Mexico establish a transport pathway where Mesozoic to Cenozoic sediment accrued in the Rocky Mountain basins, was exhumed, and then was transported to the Gulf of Mexico in the Miocene, depositing the Ogallala Formation along the way. We consider these results in light of models that link late Cenozoic topographic reorganization of the Rocky Mountains to deposition of the Ogallala Formation and consider the potential driving mechanism behind depocenter reorganization.