PALEO-TOPOGRAPHY IN THE CRETACEOUS/TERTIARY ANGULAR UNCONFORMITY AND ITS INFLUENCE ON URANIUM MINERALIZATION IN SHIRLEY BASIN, WYOMING

The Shirley Basin in northern Carbon County, Wyoming is small asymmetric synclinal structure located approximately 40 miles south of Casper, Wyoming. The basin formed during the Late Cretaceous to Early Tertiary (78 Ma – 49 Ma) and contains economically significant uranium deposits. The underlying Cretaceous units create an angular unconformity (K/T boundary) with the overlying Tertiary units. This K/T unconformity is significant as it represents a paleo-topographic erosional surface and functions as the lower confining unit/aquitard required in In-Situ Recover (ISR) uranium solution mining. The boulder to cobble sized sediments and other sedimentary elements found in the Wind River formation indicate a period of erosion and deposition characterized by alluvial fans and braided channel systems adjacent to topographically higher features.

This study uses resistivity curves from geophysical electric logs (e-logs) to identify and create subsurface structure maps of the K/T boundary. Using a large geographical distribution of exploratory wells and their associated subsurface K/T elevation, a paleo- topographic map identifies features such as drainages, ridges, hills and river channels. Exploration geologist can use these features to target the best depositional environments that favor uranium mineralization. This study uses Esri’s ArcGIS and Golden Software’s Surfer to compare interpolation methods such as kriging and natural neighbor to analyze the subsurface elevations and generate paleo-topographic maps.

Historical uranium mineralization data from the exploratory wells will be compared to these paleo-topographic maps for any association with river system features and uranium mineralization, specifically the depositional bars of river meanders that contain organic material which enhances uranium mineralization. Geologists can use these relationships to predict other areas of favorable mineralization and reduce the number of exploratory drill holes and associated exploration costs.