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The purpose of this reconnaissance is to assess the potential of the Pierre Shale, of Late Cretaceous age, as a possible isolation medium for radioactive wastes. The regional stratigraphic and structural setting of the Pierre Shale in the northern Great Plains is summarized from subsurface data.Geologic attributes mapped and employed in the identification of areas warranting further evaluation are: depth to the base of the Pierre Shale, shale thickness, overburden thickness, lithology and mineralogy of the shale, and penetrations by oil and gas wells. Three areas emerge as most favorable; each may contain many potential disposal sites. These large geologic study areas are further described on the basis of general structural and seismic considerations and are compared with respect to topography and mineral and water resources.A large area in west-central South Dakota is recommended for extensive further study. A smaller area in northeastern Colorado also may warrant additional investigation. A relatively small area in north-central North Dakota is also delineated, but currently is not proposed for further studies.

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The eastern margin of the Western Interior Seaway was located on the stable North American craton. Epeirogenic tectonism in continental lithosphere was sufficient, however, to influence sedimentation and generate geologic structures in southcentral South Dakota. Paleotectonic and post-Cretaceous tectonism affected lithosphere blocks that are bounded by fault zones in the Precambrian basement and are marked at the surface by linear features and lineament zones visible on satellite images.Regional blocks outlined by lineament zones reflect the eastern side of a Proterozoic convergent margin, have expression on surface and subsurface cross sections, and subdivide the southern margin of the Williston basin into a mosaic of lithosphere blocks. Individual linear features within the regional blocks and lineament zones further subdivide south-central South Dakota into a series of broad, anticlinal blocks separated by narrow synclinal troughs and linear features.Paleotectonic movements on the regional blocks are documented by thickness and lithologic variation in Cretaceous depositional cycles found in the Dakota Sandstone through the Pierre Shale. Outcrops along the Missouri River expose thickness and facies changes in the upper Niobrara Formation and lower Pierre Shale across specific linear features that separate local blocks. In particular, the Crow Creek Member of the Pierre Shale has differences in texture and composition that correspond to differences in block position.Postdepositional tectonism included vertical displacements that produced the characteristic pattern of broad anticlinal blocks separated by narrow synclines and linear features. In addition, assemblages of folds, faults, and joints provide evidence for horizontal, strike-slip displacements. Along the Missouri River near Chamberlain, small faults parallel a linear feature trending N35°W and folds and extensional joints lie oblique to the linear feature, suggesting left-lateral displacement. Farther south along the river near Platte, a subtle monocline is found in lower units of the Pierre Shale. Small faults trending east-west generally parallel the monocline axis, which is marked by a linear feature. Joint patterns have modes oblique to the

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