Abstract

Uranium deposits occur in the upper part of the Fox Hills Sandstone and the lowermost part of the Lance Formation, both of Late Cretaceous age, in the eastern Powder River Basin, northeastern Wyoming. The Fox Hills is a sequence of marginal marine and estuarine deposits that were laid down during an eastward regression of the Late Cretaceous Pierre Sea. A lower unit of the Fox Hills, which does not contain significant quantities of uranium, consists of offshore marine and transitonal marine shale, siltstone, and fine-grained sandstone beds that grade upward into littoral or nearshore marine sandstones deposited in shoreface and foreshore environments. The upper unit of the Fox Hills Sandstone locally is uranium-bearing and is composed largely of thin-bedded claystone, siltstone, and sandstone beds, many of which are rich in organic matter. The upper Fox Hills is considered to be largely estuarine in origin and is separated from the lower Fox Hills by a scour surface in some places. The upper unit grades into the overlying Lance Formation which is composed of fresh-water delta-plain deposits.

The lower unit of the Fox Hills thins both regionally and locally. There is a regional northward depositional thinning from about 200 m near Lance Creek in central Niobrara County, Wyo., to 40-50 m in west-central Crook County, Wyo. Regional thickness variations are attributed to slightly greater subsidence rates to the south in Niobrara County. Local erosional thinning of this unit also occurred before deposition of the upper unit as seen in an isopach map of the lower unit of the Fox Hills in west-central Crook and northeastern Campbell Counties, Wyo. The pattern of the isopachs shows an erosional thinning of the lower unit superimposed on the regional northward depositional thinning of the formation. The erosional pattern is interpreted to represent a generally easterly-draining dendritic estuarine system about 20 km wide, 25-30 km long, and as much as 40 m deep.

On the basis of cores and logs from exploratory holes, uranium mineralization is known to be present east of, and associated with, the easterly-draining dendritic estuarine system. It is believed that plant matter was carried into estuaries and associated environments by high-energy fluvial systems. Upon reaching the low-energy estuarine environments, the organic matter dropped out of suspension. The organic matter was the focal point for much of the later uranium mineralization although cores taken recently in this area show that some of the uranium is not directly related to organic matter.