Uranium in Sharon Springs member of Pierre shale, South Dakota and northeastern Nebraska

Abstract

A study of the uranium content of the Sharon Springs member of the Pierre shale in South Dakota and northeastern Nebraska was made in 1954. More than 300 samples of this black organic marine shale of Late Cretaceous age from 30 exposures and 2 core holes show that the uranium content averages about 0.0015 percent throughout the region studied. The most uraniferous parts are about 3 inches thick and contain as much as 0.025 percent uranium. The richest sample comes from an exposure along the Missouri River in Nebraska and is thought to be secondarily enriched by weathering. Water from the Sharon Springs member generally has a pH of less than 4, and 7 samples from seeps and springs contained from 7 to 780 parts uranium per billion. The thickness of the Sharon Springs member ranges from 1 to 42 feet in exposures along the Missouri River and is as much as 106 feet on the eastern flank of the Black Hills. Gamma-ray logs and published lithologic logs from holes drilled for oil and gas indicate that rocks equivalent to the member extend northward from central South Dakota into North Dakota and that the radioactivity decreases northward. Along the Missouri River in eastern South Dakota, the noncalcareous Sharon Springs member lies on the calcareous Niobrara formation. At some places the two units appear conformable; at others a disconformity separates them. Along the southeast flank of the Black Hills and northward to North Dakota, the Sharon Springs rests conformably on rocks equivalent to the Gammon member of the Pierre shale. The upper boundary of the Sharon Springs member is poorly defined except at a few exposures along the Missouri River where a marly zone marks the base of the overlying Gregory member of the Pierre. Elsewhere, the upper boundary is placed at the change from the darker color and steeper slopes of the Sharon Springs member to the lighter color and gentler slopes of the overlying Gregory member. Semiquantitative spectrographic analyses of 82 samples of black shale from the Sharon Springs member indicate that the molybdenum, iron, and phosphate contents tend to vary directly with the uranium content, whereas the aluminum, gallium, boron, titanium, magnesium, and sodium contents vary inversely with the uranium content. The uranium in the Sharon Springs member is thought to have been emplaced during or shortly after the deposition of the shale. 577 578 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM