Tucson Mountain Chaos

Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona. The Arizona Geological Society field trip today revealed that there are huge gaps in our understanding of this unit and the adjacent units that make up the rugged mountains on the west side of Tucson. [right, gray limestone blocks in Chaos above Trails End Wash. An excavated pit just below and to the right reveals a block (?) of 1.7 bya Pinal Schist]


There is no consensus even on the usage of the term "Chaos." What's the origin of thousands of feet of Cretaceous tuffs? Did they fill a caldera? Did flows outside the caldera pile up against local topographic barriers? Are the rhyolite peaks blocks in the Chaos, or later intrusions? Is the Chaos the result of ancient landslides? What is the green volcanic-looking material surrounding so many units? There are questions about every unit and every contact between units. [left, Bob Powell and Bob Hildebrand examine one of the multiple lithologies exposed in the Tucson Mtn Chaos]

Field trip leader Doug Shakel fostered discussion and debate at each stop. One of the more interesting ideas came from economic geologist Jim Briscoe who noted that the West Silverbell block appears to be laterally offset from the main Silverbell area by 2-3 miles. He postulated that the Tucson Mountain caldera could be the 'missing half' of the Silverbell caldera.