The Calera-del Nido block extends north from Chihuahua City for 140 km and from Highway 45 west to the next major valley. The block has a steep east-facing scarp and more gentle west-facing slopes. In Majalca Canyon, a rhyolite flow dome (oldest), rhyolite tuff, boulder conglomerates, and a felsic lava flow (Almireces volcanics, next oldest), and andesite-basaltic andesite flows (4+ km thick, Penas Azules volcanics) underlie a basal 45-m.y. old rhyolite tuff of the Rancheria volcanics. Above this, the Rancheria includes the Picos Gemelos andesite

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flow, a bright-red welded tuff, and the Rancheria ash-flow rhyolite which forms the prominent cliff on Cerro Rancheria, 50 km north of Chihuahua. In Bellavista Canyon these rocks are overlain by basalt flows, a rhyolite welded tuff (Acantilado tuff) and a peralkaline ash-flow rhyolite (Cryptic tuff). At Punta de Agua, rhyolite tuffs, breccias, and flows overlie these rocks on an erosion surface of considerable relief, like that of today. West of Cumbres de Majalca, flat-lying Acantilado-type (30 m.y.) ash-flow rhyolites form a dissected, westward-sloping plateau. West of Bellavista, basalt flows are overlain by the Acantilado which has a major vent in Sierra Rusia. The Sierra Campana (80 km north of Chihuahua City) is composed of Cryptic-like ash-flow rhyolites with the peralkaline Ca pana tuff on top. Thick ash-flow and vertically foliated rhyolites mark a vent for the Cryptic in lower Santa Clara Canyon. Flat-lying basalt and basaltic andesite. Acantilado tuff, and Cryptic tuff extend westward to Ojos Azules. Below the Acantilado, basalts are dominant in lower Santa Clara Canyon, but rhyolite tuffs and breccias become prominent westward. Small rhyolite vent complexes are exposed near Rancho Manta Negra and Las Varas. The large volumes of Tertiary rhyolites (including peralkaline types) indicate that this region has potential for those types of uranium deposits that are closely related to felsic volcanic environments.

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