Petrified Forest National Park - Geology

Geology

Petrified Forest National Park is known for its fossils, especially of fallen trees that lived in the Late Triassic period of the Mesozoic era, about 225 million years ago. During this period, the region that is now the park was near the equator on the southwestern edge of the supercontinent Pangaea, and its climate was humid and sub-tropical. What later became northeastern Arizona was a low plain flanked by mountains to the south and southeast and a sea to the west. Streams flowing across the plain from the highlands deposited inorganic sediment and organic matter, including trees as well as other plants and animals that had entered or fallen into the water. Although most organic matter decays rapidly or is eaten by other organisms, some is buried so quickly that it remains intact and may become fossilized. Within the park, the sediments containing the fossil logs for which the park is named are part of the Chinle Formation.

The colorful Chinle, which appears on the surface in many parts of the southwestern United States and from which the Painted Desert gets its name, is up to 800 feet (240 m) thick in the park. It consists of a variety of sedimentary rocks including beds of soft, fine-grained mudstone, siltstone, and claystone—much of which is bentonite—as well as harder sandstone and conglomerate, and limestone. Exposed to wind and water, the Chinle usually erodes differentially into badlands made up of cliffs, gullies, mesas, buttes, and rounded hills. Its bentonite clay, which swells when wet and shrinks while drying, causes surface movement and cracking that discourages plant growth. Lack of plant cover makes the Chinle especially susceptible to weathering.

About 60 million years ago, tectonic movements of Earth’s crust began to uplift the Colorado Plateau, of which the Painted Desert is part. Eventually parts of the plateau rose to 10,000 feet (3,000 m) above sea level. This warping of Earth’s surface led to the gradual and continuing destruction of the plateau by erosion. An unconformity (break in the rock record) of about 200 million years occurs within the park, where erosion has removed all the rock layers above the Chinle except geologically recent ones. The Bidahochi Formation, laid down only 8 to 4 million years ago, rests directly atop the Chinle, and rocks laid down in the Jurassic, Cretaceous, and much of the Tertiary are absent.

During the period of the Bidahochi deposition, a large lake basin covered much of northeastern Arizona. The older (lower) layers of the formation consist of fluvial and lacustrine (lake-related) deposits of silt, sand, and clay. The younger (upper) Bidahochi contains ash and lava from volcanoes that erupted nearby and as far away as southwestern Nevada. Although much of the Bidahochi has since eroded, a small part of it outcrops in the northern part of the park—on Pilot Rock in the park’s wilderness section and along the rim of the Painted Desert between Pintado and Tawa points. Exposed by erosion of the Bidahochi are volcanic landforms called maars (flat-bottomed, roughly circular volcanic craters of explosive origin). A maar vent can be seen from the Pintado Point lookout.

During the Quaternary Period (1.8 million years ago up to today), deposits of windblown sand and alluvium covered much of the Chinle and Bidahochi. Older dunes range in age from 500,000 years at higher elevations in the northern part of the park to about 10,000 years in sandy drainage areas such as Lithodendron Wash. Stabilized by grasses and other vegetation, young dunes of about 1,000 years old are found throughout the park.