Geology of Petrified Forest National Park, Arizona

Geodiversity is the full range of a park's geologic resources — rocks, minerals, sediments, fossils, landforms and the processes that shape them — together with its soils. The National Park Service's Geodiversity Atlas, produced by the Geologic Resources Inventory, collects that information for education, geoconservation and the joint management of each park's living and non-living resources.

The park

• Petrified Forest National Park lies about 160 km (100 miles) east of Flagstaff, in Apache and Navajo counties, Arizona.
• Established as a national monument on December 8, 1906, it became a national park on December 9, 1962.
• It covers about 89,603 hectares (221,416 acres).
• It protects globally significant fossils: one of the largest and most colorful concentrations of petrified wood in the world — including logs of ancient conifers more than 58 m (190 feet) long — and one of North America's most complete records of Late Triassic vertebrate evolution.

The rocks

Two formations dominate:

Between them is an unconformity — a gap in the rock record of about 190 million years.

The Chinle's members have been renamed several times since the early 1990s. The accepted sequence at the park, oldest to youngest, is the Mesa Redondo, Blue Mesa, Sonsela, Petrified Forest and Owl Rock members.

A stratigraphic column of the park's rock units. Image from the National Park Service's page.

A natural laboratory for paleontology

The two formations have yielded more than 700 fossil sites. The park's petrified wood and other Late Triassic fossils document diverse land ecosystems during "the dawn of dinosaurs," between about 223 and 208 million years ago — a time when land animals, especially tetrapods, were changing fast. Excellent, accessible exposures make the park one of the best places in the world to study that transition, and a valuable site for sedimentology, stratigraphy, geomorphology and structural geology too.

All fossils in national parks are protected under the Paleontological Resources Preservation Act of 2009.

The park belongs to the Colorado Plateaus province and shares its geologic history with a region far beyond it.

Maps and reports

Part of the Geologic Resources Inventory map of Petrified Forest. Image from the National Park Service's page.

The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks — including a geologic report for Petrified Forest — available through the NPS DataStore.

Sources

Based on "NPS Geodiversity Atlas—Petrified Forest National Park, Arizona," National Park Service; a work of the United States government in the public domain.