Cryptic Tuff (Bellavista Canyon, Santa Clara Canyon)

Central Chihuahua, Mexico is host to a stratigraphic sequence of calc-alkaline, alkaline, and peralkaline eruptives of varying composition exposed primarily at two localities: Bellavista Canyon and Santa Clara Canyon. Underlying the sequence at Bellavista are Cretaceous limestones followed by a tilted volcanic sequence of intermediate volcanics, both of which are intruded by a small mafic pluton. These units are overlain by the calc-alkaline, Eocene, and welded Acantilado Tuff, followed by a mildly alkaline basaltic andesite suite capped by peralkaline rhyolitic ash-flow known as the Cryptic Tuff. Following this, are lavas of a mildly alkaline basaltic andesitic suite, overlain by the same sequence of alkaline lavas and Cryptic Tuff found at Bellavista. The latter unit is a comendite with notable minerals including riebeckite and aegirine. These rocks are thought to be the source rocks for uranium mineralisation within the Chihuahua City Uranium Province. The shift from calc-alkaline to alkaline lithologies is thought to result from a change from a compressional stress regime during the latter stages of the Laramide Orogeny. Detailed stratigraphy of Bellavista, Santa Clara, and surrounding localities can be found in McDowell and Mauger (1994).