Radioactivity and uranium content of some Cretaceous shales, central Great Plains

Abstract

The Sharon Springs member of the Pierre shale of Late Cretaceous age, a hard black organic-rich shale, similar in many characteristics to the Chattanooga shale, is radioactive throughout central and western South Dakota, most of Nebraska, northern Kansas, and northeastern Colorado. In the Missouri River valley, thin beds of the shale contain as much as 0.01 per cent uranium. Beds correlated with the Sharon Springs are as much as 20 feet thick or more and have a radioactivity of about 0.01 per cent equivalent uranium in southwestern Nebraska according to interpretation of gamma-ray well logs. The radioactivity and uranium content are highest in the Missouri River valley in South Dakota and in southwestern Nebraska where the shale rests disconformably on the underlying Niob ara formation, also of Cretaceous age. Near the Black Hills, and in the area on the north, the shale of the Sharon Springs member rests on a wedge of the Gammon ferruginous member of the Pierre, which is represented by a disconformity on the east and south, and the radioactivity of the shale is low although greater than that of overlying strata. The shale also contains a suite of trace elements in which arsenic, boron, chromium, copper, molybdenum, nickel, selenium, and vanadium are conspicuous. Molybdenum and tin are less abundant in the Sharon Springs than in similar shales of Paleozoic age, and silver and selenium are more abundant. In the Great Plains region, the upper 30-50 feet of Cretaceous shales overlain unconformably by the White River group of Oligocene age have been altered to bright-colored material. This altered zone is chiefly the result of pre-Oligocene weathering although post-Oligocene ground-water conditions also have affected the zone. The greatest radioactivity occurs in masses of unaltered shale measuring about 1 × 4 feet in cross section included in the lower part of the altered zone. Where the zone is made up of shale and marl of the Niobrara formation, parts of the included unaltered shale contain as much as 0.1 per cent equivalent uranium and 0.03 per cent uranium. The disequilibrium between equivalent uranium (radioactivity) and the uranium content of the shales is believed to be a su face feature caused by relatively recent leaching of uranium from the present outcrops. The co-extensive distribution of the altered Cretaceous zone and of the overlying White River group suggests that most of the uranium in the small masses of unaltered marl in the altered zone has been derived from the White River group.