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Magnetostratigraphic sampling was conducted on key fossiliferous exposures of the upper Eocene-lower Oligocene (Chadronian-Whitneyan) White River Group in North Dakota, the Pine Ridge area of Nebraska, the North Platte River-Wildcat Ridge area of Nebraska and Wyoming, and the Cedar Creek area of northeastern Colorado. Together with results from other White River outcrops reported elsewhere, a general magnetostratigraphic pattern can be seen. Most sections are of predominantly reversed polarity, but there are two short zones of normal polarity in the middle and late Chadronian, another in the early Orellan, and another in the late Whitneyan. Based on 40Ar/39Ar dates and correlation to the Berggren et al. (1995) time scale, the middle Chadronian normal magnetozone correlates with Chron C16n (35.4-35.6 Ma), the late Chadronian normal magnetozone with Chron C15n (35.7-35.9 Ma), the early Orellan normal magnetozone with Chron C13n (33.0-33.5 Ma), and the late Whitneyan with Chron C12n (30.5-30.9 Ma). The Chadronian spans the interval from about 37 Ma to about 33.8 Ma; the Orellan from 33.8-32 Ma; the Whitneyan from 32 to about 30 Ma.
The richly fossiliferous and spectacularly scenic badlands of the White River Group in the High Plains (Fig. 1) have been a magnet to fossil collectors and paleontologists ever since the first fossils were collected and described in 1846. In the century and a half since then, tens of thousands of fossils of extraordinary quality have been recovered from White River rocks, making them the most productive mammal-bearing sequence in North America.
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