Geology of Petrified Forest National Park

Did you know that October 15 was National Fossil Day? Did you know that petrified wood is the official Arizona state fossil? (Did you know that Arizona had a state fossil?) And there is plenty of petrified wood in Petrified Forest National Park about 25 miles east of Holbrook.petrified-forest

Last year, the Arizona Geological Survey released a new geologic map (65Mb) and 18-page report (9Mb) on Petrified Forest National Park. Both can be downloaded for free here.

General Geology

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 the 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, however, 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. More recently, dinosaur fossils were also discovered. They were small dinosaurs, just three to 20 feet long (see here on page 2).

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.

Citation for the report and map:

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.

For more Arizona geology see:

A Brief Geologic History of Arizona Chapter 1 Precambrian

A Brief Geologic History of Arizona Chapter 2 Cambrian and Ordovician time

A Brief Geologic History of Arizona Chapter 3 Silurian to Permian

Arizona Geological History Chapter 4: Triassic Period

A brief geologic history of Arizona Chapter 5: Jurassic Time

A brief geologic history of Arizona Chapter 6: Cretaceous Time

A brief geologic history of Arizona Chapter 7, the Cenozoic Era