Geology of Fort Bowie National Historic Site, Arizona

Part of the NPS Geodiversity Atlas, which describes each park's significant geologic features and processes. Geodiversity is the full variety of a park's natural geologic resources — rocks, minerals, sediments, fossils, landforms and physical processes — and its soils. The atlas comes from the NPS Geologic Resources Inventory.

The site's Geologic Resources Inventory report. Image from the NPS's page.

The Apache Pass fault zone

The Apache Pass fault zone is the most significant geologic feature around Fort Bowie: steeply tilted, contorted rock in a zone 1–2 km (0.6–1.2 mi) wide, running northwest–southeast for nearly 60 km (38 mi) through Apache Pass. East of the Fort Bowie trailhead parking, Apache Pass Road crosses the zone's southwestern border fault into a thick section of sedimentary and volcanic rock. Fault slivers of Permian limestone and other sedimentary rock show along the Overlook Ridge Trail, where a marker about 0.6 km (0.4 mi) from the visitor center explains the fault.

The zone shows all three main kinds of fault:

• Strike-slip. The fault began about 1.4 billion years ago as a strike-slip fault, in which rock on one side moves sideways past the other. Shattered sedimentary rock inside the zone has moved more than 12 km (7.5 mi) southeast relative to the igneous rock beside it.
• Reverse. Precambrian rock on the southwest side has been pushed up relative to Paleozoic and Mesozoic strata on the northeast. From Overlook Ridge you can see the contact between the light-gray Horquilla Limestone (about 310 million years old) and Proterozoic granodiorite (about 1.4 billion).
• Normal. When stretching of the crust built the basin-and-range landscape, reverse faults formed by compression were reactivated as normal faults.

The fault has slipped again and again, horizontally and vertically, tilting and breaking the once-flat layers into steep, elongated slices that dip southwest. Its high-angle fractures carry groundwater that emerges as springs and seeps — among them Apache Spring.

Folds and thrusts

Between Apache Pass Road and the fort, Paleozoic and Mesozoic layers have been tilted and overturned into a tight syncline — a U-shaped fold with the youngest rock at its core — with both limbs dipping northeast. It is part of a folded package of strata above a thrust fault dipping southwest.

Normally younger rock lies on older, but thrust faults jumble the order. Compression from the southwest drove the Fort Bowie thrust fault, carrying older Horquilla Limestone over younger Colina Limestone. The Horquilla forms Overlook Ridge, and the thrust can be seen along its trail. This slice was torn from the main body of Horquilla Limestone about 65–63 million years ago, in the late Cretaceous or early Paleocene, and the movement also folded it. It is one of several thrust slices in the zone: the Colina Limestone beneath it was itself thrust from the southwest over younger siltstone and shale of the Cintura Formation.

Changed rocks

The area's sedimentary rocks have been metamorphosed to varying degrees. South of the site, carbonate rock has recrystallized into commercial-grade marble. Overlook Ridge stands because metamorphism made the Horquilla Limestone harder and more resistant to erosion than the rock around it.

In the Cretaceous Cintura Formation, metamorphism grew large crystals of andalusite and staurolite and flakes of chloritoid, tourmaline and graphite. A 1-m (3.3-ft) pod of graphitic shale is exposed in a fault zone in the gully just southeast of the visitor center.

Rounded domes of Proterozoic granodiorite, nearly 1.4 billion years old, rise in the west of the site. The magma cooled slowly, growing the feldspar and quartz crystals of its coarse texture; fresh rock is gray, but it weathers to the brown of the surrounding slopes.

Apache Spring

Because the rock here is mostly impermeable, groundwater is stored only in fractures made by folding and faulting — and, as is typical of the Basin and Range, springs lie along fault zones. Rain and snowmelt from higher ground soak through the loose sediment of Siphon Canyon into fractured rock beneath, and rise where the canyon crosses shattered bedrock along the Apache Pass fault.

The spring has been modified for human use: a masonry box collects its water, and some is piped to a stock tank on private land under established water rights. It flows on average 19 L (5 gal) a minute, varying with the season, the year and the rain.

Geologic map of the site, from the Geologic Resources Inventory. NPS.

Fossils and mines

Marine invertebrate fossils occur in the region's Paleozoic limestones. Within the site, the Horquilla Limestone has yielded large fusulinids, which helped date it; south of the site it has produced fusulinids, corals, bryozoans and brachiopods. The Colina Limestone has given echinoid spines and gastropods. All NPS fossils are protected under the Paleontological Resources Preservation Act of 2009.

The Park Service has found 32 abandoned mine features at six sites in or near Fort Bowie. Most are gated or fenced, which protects people and bat habitat; their waste-rock piles are left alone as historic features. Abandoned mines can be dangerous: stay out and stay alive.

Maps and reports

Fort Bowie is part of the Basin and Range province. Its Geologic Resources Inventory maps and reports are in the NPS DataStore, and a Soil Resources Inventory project for the site has been completed.

The fort's ruins, looking toward Bowie Peak. Image from the NPS's page.

Sources

Based on "NPS Geodiversity Atlas—Fort Bowie National Historic Site, Arizona," National Park Service; a work of the United States government in the public domain. It cites Drewes (1980, 1981, 1984), Bezy (2001) and Tweet and others (2008).