Mexico's largest uranium deposit occurs in the Pena Blanca Range of central Chihuahua. At Pena Blanka, Tertiary silicic pyroclastics overlie middle Cretaceous (Albian and Cenomanian) limestones. The limestones comprise a large rudistid reef, with extensive fore-reef and lagoon facies, at the edge of the Chihuahua trough. Younger, basin limestones overlap the lower edges of the fore-reef slope. The reef itself is of Albian age and shares faunal and lithologic characteristics with both the El Abra Formation of Mexico and the Edwards Formation of Texas. A total thickness of 230 m of reef limestone is exposed in the central and southern part of the range. Important rudistids include caprinids (especially Mexicaprina), radiolitids (especially Eoradiolites and Sauvegesia), and equienids (Toucasia). Other faunal elements are gastropods, corals, bivalves, algae, calcisponges, and forams (including Nummoloculina sp., useful in correlations). Lagoon, back-reef, requienid rudistid mounds, near back-reef carbonate sand, caprinid and radiolitid rudistid reef core, fore-reef carbonate sand, and fore-reef debris-slope facies are all evident in outcrop. In the reef core, rudistids predominate over all other organisms. None of the reef facies possesses significant porosity. The abundant carbonate mud and diagenetic calcite cement have occluded all available pore space.

The basin limestones include the rhythmically layered Cuesta del Cura, upper Tamaulipas, and Aurora Formations. Approximately 120 m of Tamaulipas Formation interfinger with and lap onto the fore-reef slope facies of the El Abra Formation. The Tamaulipas in this region is a foraminiferal mudstone, locally replaced by rudistid wackestone. It is a deeper water limestone characterized by calcisponges, benthonic, and planktonic Foraminifera, and rudistid debris shed from the reef. The Cuesta del Cura Formation consists of 50 m of interbedded argillaceous limestones and calcareous shales. It covers both the El Abra and Tamaulipas Formations. The calcareous units are mudstones and wackestones containing globigerinid, rotalinid, and rotaliporid Foraminifera and scattered gastropods and bivalv s.

End_of_Article - Last_Page 770------------