Terry reported in 1998 that lithologic correlations between type areas of the White River Group in Nebraska and South Dakota have resulted in a revised lithostratigraphy for the lower part of the White River Group. The following pedostratigraphic and lithostratigraphic units, from oldest to youngest, are newly recognized in northwestern Nebraska and can be correlated with units in the Big Badlands of South Dakota: the Yellow Mounds Paleosol Equivalent, Interior and Weta Paleosol Equivalents, Chamberlain Pass Formation, and Peanut Peak Member of the Chadron Formation.
The term "Interior Paleosol Complex" used for the brightly colored zone at the base of the White River Group in northwestern Nebraska, is abandoned in favor of a two-part division. The lower part is related to the Yellow Mounds Paleosol Series of South Dakota and represents the pedogenically modified Cretaceous Pierre Shale. The upper part is composed of the unconformably overlying, pedogenically modified over-bank mudstone facies of the Chamberlain Pass Formation (which contains the Interior and Weta Paleosol Series in South Dakota). Greenish-white channel sandstones at the base of the Chadron Formation in Nebraska (previously correlated to the Ahearn Member of the Chadron Formation in South Dakota) herein are correlated to the channel sandstone facies of the Chamberlain Pass Formation in South Dakota. The Chamberlain Pass Formation is unconformably overlain by the Chadron Formation in South Dakota and Nebraska.
The Chadron Formation in South Dakota is divided into the Ahearn, Crazy Johnson, and Peanut Peak Members in South Dakota. Of these three, only the bluish-green, hummocky weathering mudstones of the Peanut Peak Member are present in northwestern Nebraska. These mudstones were previously classified as the lower part of the "Chadron B" in Nebraska and correlated with the Crazy Johnson Member in South Dakota. The term Chadron B is abandoned and the correlation to the Crazy Johnson Member rejected.
The remainder of the Chadron Formation in northwestern Nebraska (the Chadron B and "Chadron C"), along with the basal beds of the Brule Formation ("Orella A"), are included within the newly defined Big Cottonwood Creek Member. The Chadron C was initially used to define strata between the "upper" and "lower purplish-white layers" and was correlated to the Peanut Peak Member in South Dakota. The term Chadron C is abandoned and the correlation
with the Peanut Peak Member of South Dakota is rejected. The contact between the Peanut Peak Member and Big Cottonwood Creek Member of the Chadron Formation in northwestern Nebraska is intertonguing, except where strata of the Big Cottonwood Creek Member fill depressions and minor valleys in the Peanut Peak Member.
The Peanut Peak Member of northwestern Nebraska differs from the Big Cottonwood Creek Member in that the Peanut Peak Member is composed of smectite-rich mudstone and claystone, weathers into hummocky hills and slopes, is less variegated in color, and has less silt. The Big Cottonwood Creek Member is siltier, cliff forming, and contains the various purplish-white layers of Schultz and Stout (1955). These proposed revisions and correlations are applicable to other exposures of the White River Group across the Great Plains.
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