Notes on the Geology of Southeast Arizona

The Coronado Mountains and part of the Montezuma Caldera, looking east from Montezuma Pass (National Park Service Photo)

There’s no easy way to summarize the geology of Southeastern Arizona except to describe it as a complex mess. WC doesn’t want and isn’t qualified to write the multi-volume treatise that would be required to begin to detail it. So here is the briefest possible summary of about 1.4 billion – that’s billion with a “b” – years that are shown in the rocks. Note to Real Geologists: Yes, this is greatly simplified. There’s no other way to approach it.

Between about 1.4 billion and 550 million years ago, a series of exotic terranes, probably volcanic arcs, docked onto the North American craton, becoming a permanent part of the North American Plate. The added terranes included the area that is now Arizona. Most of the geology of those dockings is lost; there is an unconformity, a layer or layers of rock, that is simply missing, presumably eroded away over most of a billion years.

Sometime after those events, rising sea levels covered southeast Arizona with a shallow sea. The sea persisted for a long time, most of a quarter billion years, near enough to all of the Permian Period. The marine life, as it died, settled to the bottom of the sea, where the shells and bodies slowly pile up in a layers of calcium carbonate, a thick enough layer to compress the inorganic remains into limestone.

Beginning about 250 million years ago, the Farallon Plate began to subduct beneath that North American Plate. Like the Cascade Volcanoes today, the subducted ocean floor melted, rose up, and erupted as volcanos. At first the volcanism was fairly modest, but beginning in the Jurassic Period, the volcanism switched to massive caldera-type eruptions.¹ One well-studied such eruption is the Montezuma Caldera.

That was a ten mile wide caldera in what is today the Coronado Mountains; part of its is preserved in the Coronado National Monument. The eruption of the Montezuma Caldera was so extreme that the magma ejected created a vast empty underground chamber. That chamber collapsed, mixing the layers of native rock and new volcanic materials in a stew of rock. The steep walls of the caldera later collapsed and eroded, mixing materials still more. Nearby volcanism ejected still more ash, which mostly filled the Montezuma Caldera.

Montezuma Peak in the Coronado Mountains, a granitic intrusion with limestone blocks still stuck to its sides (National Park Service Photo)

A series of gigantic blobs of granite – failed eruptions, as it were – were pushed up through all that earlier rock so quickly that the native rock, chiefly limestone, remember, was floated up as well, in chunks and layers of varying sizes. In the meantime, the ongoing plate tectonics were squeezing southwestern North America, pressing it, forcing portions to uplift. New mountains rose up, increasing the rate of erosion. These aren’t the mountains you see today, these eroded away. That erosion created vast piles of mountain rubble which was pressed into conglomerate and breccia.

Finally, about 30 million years ago, basin and range faulting began, creating the “sky island” mountain ranges that characterize much the Intermountain West today. The mechanism may have been a little different in Southeast Arizona, but the result was the same: big blocks of the countryside tilted up, creating a shallow slope on one side and a steep slope on the other, the “trap door lid” metaphor. That mixture of earlier limestone, volcanic debris, granite, conglomerate and breccia were tilted up. Modern day Montezuma Pass features breccia with chunks of embedded limestone, in a matrix of tuft, hoisted 3,000 feet above the surrounding country. And, of course, those new mountains have themselves eroded, sending even more mixed debris down slope to the broad valleys that separate the fault block mountains.

Messy doesn’t being to describe it.

The same processes occurred across most of Southeast Arizona. Geologists are still working to understand it all. But today all that geology has created a terrific variety of ecozones, which means an excellent variety of birds and other fauna. The summits of the Chiracahua Mountains, for example, are forested with Ponderosa Pines, the trees of Central Idaho. Cave Creek Road takes you through five or six different ecozones, from baking hot desert at the bottom to cool moist forests at the top. Each has its own flora and fauna, and, specifically, its own birds.

Canyon walls along Cave Creek; granite intrusions rising through limestone, Cave Creek in the foreground

The ranges are high enough that they pull moisture out of the air, generating permanent streams like Cave Creek. Those streams have carved deeply into the rock of the mountains, creating steep canyon walls floored in trees and shrubs.

Don’t misunderstand; WC wouldn’t want to live there. But it sure is interesting geology and terrific birding.

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¹ One theory is that the rate of subduction increased as a result of the North American Plate being more rapidly forced west as the Atlantic Ocean opened, and the Farallon Plate being forced east as a result of a developing mid-ocean ridge west of North America. More rapid subduction, the theory goes, means the subduct plate if forced into a more horizontal plane, allowing greater and more shallow reservoirs of magma – melted ocean plate – to accumulate. YMMV.