PETRIFIED FOREST, ARIZONA 

 

 

    

 

 

 

 

Root bole on base of a long trunk

 

 

 

Accumulation of log segments weathering from strata

   

 

 

The Petrified Forest National Park of northeastern Arizona features one of the world's largest and most colourful concentrations of petrified wood.  The common fossils trees include Araucarioxylon, Woodworthia, and Schielderia, along with other plants including cycads and ferns.  Bark engravings by insects and fossil bee nests been found in logs at the Petrified Forest.  These have helped demonstrate that hymenopteran behavior has changed little over 220 million years and that bees were pollinators of plants in Triassic terrestrial ecosystems.  Much of the wood is of the species Araucarioxylon arizonicum. 

 

The northern part of the park encompasses multihued badlands known as the Painted Desert.  The Petrified Forest area was designated a National Monument in 1906.  The Painted Desert was added later, and in 1962 the whole area was declared a national park.  It covers 218,533 acres (885 km²). 

 

All the petrified wood occurs in deposits of the nonmarine Upper Triassic Chinle Group.  Two horizons, the Sonsela Member and the Black Forest Bed, contain the vast majority of the petrified wood in the park.  The Sonsela Member of the Petrified Forest Formation hosts almost all of the brightly colored, large trunks and logs for which the park is known. The Black Forest Bed in the Painted Desert Member of the Petrified Forest Formation contains another large deposit of petrified logs.  The Sonsela Member deposits are the oldest widespread deposits in the park and include the spectacular logs associated with Giant Logs, Rainbow Forest, Long Logs, Crystal Forest, Jasper Forest, Agate Bridge, and Blue Mesa in the southern portion of the park. The most extensive deposits in the Painted Desert Member are those in the Black Forest Bed in the northern portion of the park.   Available biochronological evidence, including tetrapods, megafossil plants, pollen, and calcareous microfossils, indicates that both the Sonsela Member and the Black Forest Bed are of early- to mid-Norian (220-215 Ma) age.

 

The major concentrations of petrified wood have been termed "forests" (e.g. Rainbow Forest, Crystal Forest, Black Forest, etc.) although the vast majority of the fossil tree trunks are preserved in a prone position and have been transported at least some distance from their original growth areas. However, in-place stumps of trees do occur in several areas, and many of the logs may not have moved far before burial.

 

Some of the soft mudrock and shale that encases the fossils is rich in weathered volcanic ash.  In places (e.g. Wolverine area with logs to 6 ft diameter and 90 ft length) the entombing stream sediments are regarded as volcanic-ash-derived.  In the Black Forest Bed, 60-65 m above the top of the Sonsela, most of the wood is concentrated in the upper reworked tuffaceous portion of the unit.  

 

The vast majority of the fossil wood in the park occurs in trough crossbedded sandstones and conglomerates of the Sonsela Member.  Almost all of the famous localities in the southern portion of the park, including the spectacular "forests" of trunks preserved in Giant Logs, Rainbow Forest, Long Logs, Crystal Forest, Jasper Forest, Agate Bridge, and Blue Mesa, are in the Sonsela Member, as are several "forests" on adjacent private and state lands.   The Sonsela Member represents a low sinuosity fluvial system consisting of northerly to northeasterly draining braided channels. 

 

 

 

REFERENCES

Heckert, A.B. and Lucas, S.G., 1996.  Stratigraphic description of the Tr-4 unconformity, west-central New Mexico and eastern Arizona. New Mexico Geology, 18, pp. 61-70.

Heckert, A.B. and Lucas, S. G.  Stratigraphic distribution and age of petrified wood in Petrified Forest National Park, Arizona.

Lucas, S.G., 1993.  The Chinle Group: revised stratigraphy and biochronology of Upper Triassic nonmarine strata in the western United States. Museum of Northern Arizona Bulletin, 59, pp. 27-50.

 

Sigleo, A.C., 1978.  Organic geochemistry of silicified wood, Petrified Forest National Park, Arizona.  Geochimica et Cosmochimica Acta, Vol. 42, p. 1978.