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Petrified Wood from the Triassic

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Chinle Formation of Arizona (208 to 245 million years ago)

During the Triassic period over 200 million years ago, ancestral Arizona was located in the northwestern corner of the supercontinent Pangaea. At that time, the area had a tropical climate and a vast forest grew upon a relatively flat-lying terrain. Downed trees that accumulated in river flood plains and channels were periodically buried by floods carrying fine-sediment. Over time, these logs were buried deep under thousands of feet of sediment and eventually became petrified. Uplift and erosion once again exposed the wood at the surface; however, this time the wood was replaced with quartz and agate and more resistant to weathering than the fine-sediments that encompassed them. Now, the petrified wood is exposed and broken logs and fragments cover the ground in places.

Araucarioxylon arizonicum, is an extinct species of conifer and the state fossil of Arizona. It represents the most abundant petrified wood found in the Chinle Formation. It is commonly called “Rainbow Wood” due to the large variety of colors it can exhibit. Iron and its oxide mineral forms produce the varying colors within the agatized structure of the wood. A. arizonicum grew to 200 feet (60 meters) tall.

Woodworthia arizonica is probably the second most common tree in the Petrified Forest. It is characterized by circular small scars on the outer surface of the tree rounds. In cross-section view these scars are usually shone as white veins radiating outward from the central core of the tree. W. arizonica grew to 105 feet (32 meters) tall.

Schilderia adamanica is somewhat rare in the Petrified Forest. Based upon a cross-sectional view, it is recognized by its numerous thin veins radiating outward from the central core. Its classification in relation to other members of the plant Kingdom is uncertain. S. adamanica grew to 118 feet (36 meters) tall.

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