Stratigraphic Relationships of Some Great Plains Formations.

<p>Both the Poleslide Member, Brule Formation in South Dakota and the Whitney Member, Brule Formation in Nebraska are overlain conformably by lithologic bodies of approximately the same thickness: the Sharps Formation and the Brown Siltstone Beds <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047759#pone.0047759-Harksen1" target="_blank">[3]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047759#pone.0047759-Tedford1" target="_blank">[7]</a>. In the vicinity of the Chadron Arch, a thick tephra is identified at the base of both: the Rockyford Ash Zone (RAZ) in South Dakota <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047759#pone.0047759-Nicknish1" target="_blank">[9]</a> and the Nonpareil Ash Zone (NPAZ) in Nebraska <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047759#pone.0047759-Tedford1" target="_blank">[7]</a>. The lower portions of the Brown Siltstone Beds and Sharps Formation are more stratified than their upper portions. The upper portions of each are massively bedded siltstone with intermittent, vertically oriented, spindle/potato-shaped nodules. The lithologies of the Brown Siltstone Beds and the Sharps Formation are similar. These data are consistent with the subsurface evidence of lateral continuity across the Chadron Arch <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047759#pone.0047759-Swinehart1" target="_blank">[12]</a>. The Nonpareil Ash Zone (NPAZ) of the Brown Siltstone Beds can be traced in the subsurface across the Chadron Arch. This ash complex northeast of the Chadron Arch is known as the Rockyford Ash Zone (RAZ) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047759#pone.0047759-Nicknish1" target="_blank">[9]</a>. Swinehart, <i>et al.</i> (1985) provides direct subsurface evidence of the correlation of the NPAZ with the RAZ and the subsequent correlation of the Sharps Formation with the Brown Siltstone Beds.