Paleopedology of the Chadron Formation of Northwestern Nebraska: implications for paleoclimatic change in the North American midcontinent across the Eocene–Oligocene boundary

Abstract

The Big Cottonwood Creek Member is a newly recognized lithostratigraphic unit of the Chadron Formation of northwestern Nebraska. I measured 23 superimposed paleosol profiles within 29 m of strata exposed 0.4 km (0.25 mile) east of the type section of the Big Cottonwood Creek Member at Toadstool Park in northwestern Nebraska and recognized four main types of paleosols. The paleosols are all well drained, moderately alkaline to alkaline, oxidized, and range from weakly developed azonal Ustifluvents, to moderately and well-developed Paleustalfs and Argiustolls with argillic and petrocalcic horizons. Based on changes in soil structure, horizons, root traces, and geochemical trends, these paleosols record a gradual change from humid, forested conditions of the late Eocene to more seasonal, semi arid, open range conditions leading up to the global climatic deterioration event of the early Oligocene at 33.2 Ma. This interpretation is supported by associated changes in sedimentology, fluvial architecture, floras, and faunas over this period of time. The section at Toadstool Park is significant in that it establishes a link between the Big Badlands of South Dakota and correlative deposits of Wyoming. The contact of the Chadron and Brule Formations in the Big Badlands of South Dakota is marked by an unconformity that is in part correlative to the earliest Oligocene sediments that elsewhere record the global climatic deterioration event at 33.2 Ma. The paleosols of the Big Cottonwood Creek Member of northwestern Nebraska fill in a significant part of this gap and record paleoclimatic conditions of the late Eocene leading up to the global climatic deterioration event of the early Oligocene. Using the latest lithostratigraphic, biostratigraphic, magnetostratigraphic, and tephrostratigraphic data for the White River Group, this change to more arid conditions in the midcontinent appears to have progressed from west to east over time.