Origin and distribution of selenium in the upper Cretaceous Niobrara and Pierre formations, northeastern North Dakota
High concentrations of selenium, often as much as 28 ppm, have been found in the bentonitic zones near the contact of the Niobrara and Pierre Formations of Cretaceous age in northeastern North Dakota. A 64-foot section in the Pembina Hills area reveals that the highest selenium content occurs in shale and marlstone beds adjacent to the bentonite beds which, themselves, contain comparatively little selenium. Higher concentrations of selenium were found in the brown shale than in the black, with a small variation in color making a big difference in selenium content. The selenium content of the shale and marlstone drops off considerably with increasing distance from the bentonitic zone in the vertical section. This suggests that the selenium had a volcanic source and accompanied the volcanic ash that formed the bentonite beds. Soils formed from rock in the bentonitic zone above and below the Pierre-Niobrara contact can be presumed to be seleniferous and cap able of producing toxic vegetation. These soils present a potential danger of selenium poisoning to livestock and to humans who inhabit the area.
Selenium; Geology--North Dakota; Geology, Stratigraphic--Cretaceous, Geology
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