THE EMERGING DEEPER UNDERSTANDING OF CENOZOIC STRATIGRAPHY IN NEBRASKA AND THE POWER OF INTERSTATE COOPERATION (Invited Presentation)

A deeper understanding of the Cenozoic stratigraphic succession of Nebraska is crucial for a comprehensive characterization of the geohistory and hydrogeology of the High Plains. Decades of outcrop studies and rotary test-hole drilling by Nebraska’s Conservation and Survey Division (CSD) provide an excellent basis for the voluminous work required to complete an impactful synthesis of regional geology. Continuing cooperative studies by CSD (in conjunction with the USGS StateMap program) and the Kansas Geological Survey/University of Kansas utilize continuous wireline and auger coring, facies analyses, chemostratigraphy, geochronology, and other modern stratigraphic techniques to upgrade and maximize the geological knowledge base.

Efforts have recently focused on coring and mapping in the Niobrara Valley in the north, where major uncertainties about stratigraphy persist even after the superb work of M. F. Skinner and others. New results include: (1) partial verification of a consistent regional stratigraphy within the Miocene Valentine Formation (Ogallala Group) in the middle to eastern valley; (2) development of a novel detrital-zircon geochronology and stable-isotope chemostratigraphy of the Valentine Formation in Boyd County; (3) wireline coring and, eventually, the detailed characterization of the Paleogene stratigraphy beneath the Ogallala Group in the middle valley and beyond. The latter endeavor has produced two >135 m-long wireline cores that capture nearly the entirety of the Rosebud Formation (Arikaree Group) and the Brule and Chadron formations (White River Group). The Rosebud Formation in Nebraska has never been adequately characterized, although its importance (1) as an record of late Oligocene (and perhaps earliest Miocene) sedimentation and paleoenvironments is manifest, and (2) it functions as a regional aquitard below the High Plains aquifer (although we have found scant water-bearing sands within it). Preliminary observations on the Rosebud Formation establish that it is dominated by pedogenically-modified siltstones, and that conspicuous, light-colored clay-rich bands—especially in the upper part of the unit—are present not only in outcrop, but also in cores. These and other features suggest a loessial origin.