The Geology of Coronado National Memorial, Arizona

Coronado National Memorial lies on the U.S.–Mexico border in Cochise County, southeastern Arizona. Congress first authorized it as an international memorial on August 18, 1941, and it was redesignated on July 9, 1952. Covering about 1,954 hectares (4,830 acres), it commemorates Francisco Vásquez de Coronado's expedition of 1540–1542 — the first major European exploration of the American Southwest — and the ties linking the United States with Mexico and Spain. The expedition brought deep and lasting changes to the region's language, technology, religion, livestock, farming and food. From the memorial there are wide views of the border and of the San Pedro River Valley, the route the expedition is believed to have followed in its search for the "Seven Cities of Gold."

The National Park Service's Geodiversity Atlas summarizes the park's rocks, minerals, sediments, fossils, landforms, soils and the processes that shape them, drawing on the Geologic Resources Inventory.

300 million years in the rocks

The memorial rests on a complex mix of igneous and sedimentary rocks spanning the last 300 million years. Oceans once covered what is now a semi-arid land; after they withdrew, southeastern Arizona became a scene of catastrophic volcanism.

The defining event came in the Jurassic Period (about 201–143 million years ago), when violent eruptions in the southern Huachuca Mountains formed several calderas, among them the Montezuma Caldera — a great hole in the ground left by the eruption. The eruption also set off landslides and earthquakes and left very large blocks of broken, shattered strata. Most rock exposed at the surface in the memorial is Jurassic and tied to the volcanism that made the caldera.

• The north is rugged country of Jurassic tuffs and related granitic intrusions, the Huachuca Granite.
• The southwest holds a jumble of volcanic tuff, lava flows, limestone, sandstone and shale (mudstone), capped by the Jurassic Glance Conglomerate.
• The middle holds the oldest bedrock mapped in the memorial: Permian layers of the Naco Group.
• The southeast has the youngest deposits, Pleistocene–Holocene terrace gravels, alluvium, colluvium and debris-flow deposits.

Starting about 90 million years ago, west–east compression from colliding tectonic plates raised long north–south mountain ranges stretching from Arizona to Canada. About 15 to 10 million years ago the crust began pulling apart, and widespread faulting created the basin-and-range landscape of southeastern Arizona. The memorial is part of the Basin and Range Physiographic Province and shares its history with a far larger region.

Today four major biological provinces meet in the memorial — a richness of rock and life that Coronado's expedition passed without notice in 1540.

Fossils

Paleozoic limestones in the Huachuca Mountains, as elsewhere in Arizona, hold fossils of many marine invertebrates. Here, though, heat from igneous intrusions, volcanism, faulting, and the recrystallization of limestone to dolomite destroyed many of them. All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.

Caves

During the Quaternary (2.6 million years ago to the present), groundwater seeping through cracks in the limestone dissolved calcite and hollowed out caves. The memorial has nine known solution caves. The largest and best known, Coronado Cave, lies 1.2 km (0.75 mi) north of the visitor center; graffiti in the caves date as far back as 1892. Caves in national parks are protected by the Federal Cave Resources Protection Act of 1988.

Old mines

The memorial's inventory of abandoned mineral lands counts 62 mine shafts, adits and test pits. The State of Texas Mine, inside the memorial, and the Morgan Mine, just north of it in South Fork Canyon, are two of the largest abandoned mines in the area. Because they endanger visitors, most mines in the memorial carry warning signs and are closed to the public. Abandoned mines can be valuable cultural sites and habitat, but many threaten people and wildlife and harm water quality, landscapes and other resources. The Park Service's advice: stay out and stay alive.

Maps and reports

An overview of the digital geologic data for Coronado National Memorial. Image from the National Park Service's page.

The Geologic Resources Inventory makes digital geologic maps and reports for more than 270 natural-resource parks. For Coronado it offers a scoping summary, digital geologic map files for GIS, a report on the park's setting, features, management issues and geologic history, and a poster. A soil resources inventory has also been completed. All are on the NPS DataStore.

Montezuma Peak. Image from the National Park Service's page.

Sources

• National Park Service: "NPS Geodiversity Atlas—Coronado National Memorial, Arizona," citing Graham (2011) and National Park Service publications (2007, 2016). Its image of the inventory report's cover is not reproduced.