Petrified Forest

Geology of Petrified Forest National Park

Petrified1The Arizona Geological Survey just released a new geologic map (65Mb) and 18-page report (9Mb) on Petrified Forest National Park in Arizona (located 25 miles east of Holbrook). Both can be downloaded for free here. Printed copies can be purchased from the Arizona Experience Store, 416 W. Congress, Tucson, AZ 85701; e-mail –, ph. 520.770.3500.

The Upper-Triassic age (ca. 225 million years old) Chinle formation on the Colorado Plateau hosts the petrified wood and represents deposits of river systems originating in what is now western Texas and fed by tributary streams from highlands to the south and north of National Park. The Chinle Formation preserves a suite of lowland terrestrial environments that includes river channels, floodplains, swamps, and small lakes operating in a strongly seasonal subtropical climate. The rocks show that with time, the climate became arid.

Within the National Park, the Chinle formation contains fossils of petrified wood, boney fish, sharks, locally abundant bivalves, gastropods, freshwater crustaceans and trace fossils of a wide variety of insects and other small arthropods.

The well-illustrated report shows the geologic history of the area through a series of cross-sections. That history includes the existence of an extensive lake which persisted for about 12 million years. The report also contains many photographs.

One thing the report does not do is explain how petrified wood forms but the Spring 1989 issue of Arizona Geology has a good description:

After the trees were transported downstream and became trapped in shallow waters, fluvial deposits of silt, mud, and volcanic ash from volcanoes to the south or west buried the logs and cut off the supply of oxygen; decay was thus retarded. Ground water percolating through the sediments dissolved silica from the volcanic ash. As the silica filtered through the logs, it precipitated from solution as microscopic quartz crystals in the woody tissues where air, water, and sap were originally present in the living tree. In some logs, cell structure remained intact, albeit entombed. Where the logs were hollow, woody tissue did not limit crystal growth; large crystals of rose quartz, smoky quartz, amethyst, and other gemstones or large masses of amorphous (noncrystalline) chalcedony and chert lined the cavity walls. Originally, researchers believed that minerals replaced the wood fibers. In recent experiments, however, after acid was used to dissolve the minerals, the original woody tissue was visible under a microscope.

As the silica petrified the wood, other elements in the water, such as iron, copper, manganese, and carbon, added tints of red, yellow, orange, brown, blue, green, purple, and black to the fossilized tissues. In some logs, tunnels and galleries are visible, the remains of ancient excavations dug by Triassic insects. The high degree of preservation of the logs and other fossils in the Chinle Formation is due to favorable conditions, such as warm temperatures, high moisture, and little or no oxygen, during and after deposition of the sediments.

This geologic map was a long time in coming. See notes by Bill Parker one of the authors here.


Martz, J.W., Parker, W.G., Skinner, L. and Raucci, J.J., Umhoefer,P. and Blakey, R.C., 2012, Geologic Map of Petrified Forest National Park, Arizona. Arizona Geological Survey Contributed Map CR-12-A, 1 map sheet, 1:50,000 map scale, 18 p.


Arizona Geological History Chapter 4: Triassic Period

With this chapter we begin the Mesozoic (middle life) Era which extended from 251 million years ago to 65 million years ago. The Mesozoic is divided into three Periods: the Triassic (251- to 202 million years ago), the Jurassic (202- to 145 mya), and the Cretaceous (145- to 65 mya).

The preceding Paleozoic Era (542- to 251 mya) ended with a mass extinction and with most of the landmass forming a massive continent called Pangea. Arizona was just barely north of the equator, and once again, emerging from the sea which still existed in California and Nevada.

Paleomap 237

By Triassic time, dinosaurs, pterosaurs (flying reptiles), lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock. In Arizona, there were Phytosaurs, crocodile-like animals (2- to 12 meters long) which inhabited streams and ponds.

Triassic sedimentary rocks, well-exposed on the Colorado Plateau, are represented by the Moenkopi Formation and the Chinle Formation. The Moenkopi consists of continental redbeds (sandstones, shales, and conglomerates) in the northeastern part of the plateau, and minor mixed carbonates of fluvial (river), tidal flats, and shallow marine origin in the west. After a period of erosion, continental sandstones, mudstones, and lake-formed carbonates of the Chinle Formation were deposited. Most Triassic sediments represent deposition well-inland from the sea. The climate was semi-arid in the interior and wet and swampy in the lowlands. Temperatures were 15 -to 20 F warmer than today.


Southern Arizona was a major volcanic province. Many of the mountain ranges contain Triassic volcanic rocks. In the Santa Rita Mountains, for instance, almost 10,000 feet of volcanics were deposited. The Recreation Redbeds in the Tucson Mountains represent an inter-volcanic period of erosion in upper Triassic time.

Volcanism and the high-energy continental deposits made poor hosts for fossils of terrestrial animals and plants. However, the Chinle Formation contains the silicified trunks of large trees preserved and exposed in the Petrified Forest of Arizona, and colorful Chinle rocks are exposed in the Painted Desert.

In mid-Triassic time, the mega-continent of Pangea began splitting into two parts: Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. This split caused massive volcanism along a rift that would become the Atlantic Ocean.

The Triassic Period ended with another mass extinction of about 76% of marine species and some terrestrial species. Again, the reason is not known, but speculative theories attribute it to comet impacts and volcanism. According to The Resilient Earth: ” At least two impact craters have been found from around the time of this extinction. One is in Western Australia, where scientists have discovered the faint remains of a 75 mile (120 km) wide crater. The other is a 212 million year old crater in Quebec, Canada, forming part of the Manicouagan Reservoir. The Manicouagan impact structure is one of the largest impact craters still visible on the Earth’s surface, with an original rim diameter of approximately 62 miles. Others have suggested that a sudden, gigantic overturning of ocean water created anoxic conditions causing the massive die-off of marine species.”


Blakey, R.C., 1989, Triassic and Jurassic Geology of the Southern Colorado Plateau, in Geologic Evolution of Arizona, J.P. Jenney and S. J. Reynolds, eds. Arizona Geological Society Digest 17.

Hayes, P.T. and Drewes, Harald, 1978, Mesozoic Depositional History of Southeastern Arizona, in Land of Cochise, New Mexico Geological Society Guidbook 29.

Moore, R. C., 1958, Introduction to Historical Geology, McGraw-Hill.