Abstract: Abstract: Geometry and Paleoenvironments of Uppermost Cretaceous Strata in Western Nebraska and Adjacent States

Harold M. Degraw

The upper Pierre Shale in the study area consists of the so-called "transition zone" of the Pierre and the upper part of the older typical Pierre Shale. Detailed correlations of electric logs of holes drilled into these strata show that they are composed of several time-stratigraphic units separated by disconformities. Isopach maps of selected intervals in three of these units--two in the "transition zone" and one in the older Pierre--indicate progressive onlap of younger strata onto older.

The maps for the basal parts of the two units in the "transition zone" show that both have a north-pointing lobal geometry. Their depositional axes are offset, however. In the southern part of the Nebraska Panhandle, the axis of the lower unit lies in Kimball County, whereas the axis of the overlying unit is farther east in Cheyenne County.

The older Pierre unit is isolated from the other two units, its axis lying farther east in Deuel and Keith Counties. It appears to have a northeastward depositional trend.

These time-stratigraphic units appear to consist of transgressive-regressive sequences. Their basal geometries and electric-log characteristics suggest that sedimentation occurred in a series of offsetting structural troughs that resulted in paleogeographic embayments of a gulfian sea. If this interpretation is valid, a major revision of the prevailing view of Late Cretaceous geography and tectonic evolution is required.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado