Fossil Creek - Geology

Geology

Fossil Creek Canyon is about 1,600 feet (490 m) deep and varies in width from 2 miles (3 km) at Fossil Springs to less than 1 mile (1.6 km) in places further downstream. Rocks in the canyon vary in age from the Precambrian through the Cenozoic. North of the springs the canyon cuts through Paleozoic sedimentary rocks including the Redwall Limestone; Naco, Supai, and Schnebly Hill formations; Coconino Sandstone, and Kaibab Formation. South of the springs, the canyon's rocks are Tertiary volcanic, mostly dark-gray basalt and yellow-gray tuff.

Fossil Springs, the source of about 80 percent of the water in Fossil Creek, issues from several vents in close proximity to one another at the base of the Mogollon Rim.The rim is the southern edge of the Colorado Plateau. Water percolating through limestone layers in the rim becomes laden with dissolved carbon dioxide and calcium carbonate. When the water emerges at the springs, it begins to release carbon dioxide as a gas and calcium carbonate as travertine, a porous form of calcite often found around hot springs such as Mammoth Hot Springs in Yellowstone National Park. The release, affected by stream turbulence and other factors, does not occur all at once but continues downstream.

Nineteenth-century observers of the creek reported seeing many naturally occurring travertine dams with deep pools behind them. A study conducted in 1996 identified the eroded remnants of at least 80 such dams. After the creek's natural flow was restored in 2008, hundreds of new travertine dams began to form along a 6-mile (10 km) stretch below Fossil Springs. About 13 short tons (12 t) of travertine per day are deposited along the stream. This rate of deposition makes Fossil Creek the fourth largest travertine system in the United States; the three larger systems are Mammoth Hot Springs, Havasu Creek, and Blue Springs of the Little Colorado River.