Petrified Forest N.P., AZ | GeologyVirtualTrips

PETRIFIED FOREST N.P.

FIELD TRIP STOP – TRIASSIC FOREST PRESERVED AS FOSSIL ROCK.

LOCATION: Petrified Forest Nation Park is found near Holbrook, AZ. It can be accessed from its Northern entrance off of Interstate 40 or its southern entrance off of Highway 180.

GEOLOGIC FEATURES: Triassic trees buried with organic material replaced by siliceous groundwater to form solid rock.

DESCRIPTION: Silicified trees of the Petrified Forest are found in the Upper Triassic Chinle Formation (approximately 220 mya). Trees are preserved mainly in fluvial sedimentary deposits (river-related and floodplain deposits). Wood was buried and protected from decay (by oxygen and insects). Groundwater, enriched in silica by the presence of volcanic ash (sometimes referred to as Bentonite), replaced the original plant material. Uplift of the Colorado Plateau, about 60 mya, caused increased erosion of the overlying Jurassic, Cretaceous, and some Tertiary sediments, exposing the fossil wood of the Triassic Chinle Formation due to differential erosion of the hard (siliceous) fossilized trees. Overlying the Chinle, then, is a huge unconformity with the rocks of the Bidahochi Formation above the erosional surface being 4-8 myo.

Paleogeographically, the Chinle Fm. was deposited near the equator, near the southern part of Pangaea. Most of the trees are Coniferous. The tropical environment also supported plants such as ferns, cycads, and ginkgoes as well as Phytosaurs (giant reptiles), large amphibians, and early dinosaurs.

Most of the trees preserve their external form but have lost their internal structure. Preservation is such that by looking at the bark one cannot tell the difference between real and fossil trees. Some trees have their internal cellular structure preserved.

Arizona petrified wood is known for its colors. Mineral solutions having iron, manganese, copper, cobalt, chromium, uranium and nickel give the colors such as red, orange, yellow, green, blue, violet, purple, brown, black, white and gray.

STUDENT QUESTIONS:

(1) How much time is missing at the unconformity above the Chinle Formation?

(2) Is it legal to remove a small piece of petrified wood from the park? Explain.

(3) Describe the process of Permineralization and its role in the preservation of Triassic plant material.

(4) CHALLENGE: Would the trees of Petrified Forrest have been preserved had there been no volcanic ash deposition above them? Explain

(5) CHALLENGE: How might you accurately estimate the weight of a cubic foot of a completely silicified piece of Petrified Wood?

SELECTED REFERENCES:

-Arizona Petrified Wood: A Story in Stone. Wild West Rock and Jewelry Company, Show Low, AZ. https://www.facebook.com/wildwestrock (Info displayed at the 2020 Tucson Gem, Mineral and Fossil Show)

-Harris, A. and E. Tuttle. 1983 (3rd ed.). Geology of the National Parks. Kendall/Hunt Publishing Company. Dubuque, IO. 554 pp.

-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp.

PHOTOS:

Figure 1 - Petrified log within the National Park. The preservation of the outer "bark" is often brown. Along with the detailed preservation of the outer surface, the fossilized trees are often difficult to distinguish from the real thing.

Figure 2 - The resemblance of petrified trees to recently fallen trees is often so similar that even upon close inspection the difference is not discernable. Photo taken within the National Park.

Figure 3 - Photo taken in the National Park. Though it might be tempting to some to take a small "souvenir," it is a serious crime to remove even the smallest rock from the park (true of all National Parks). Park Rangers are particularly vigilant about this and will stop vehicles to inspect for specimens illegally procured.

Figure 4 - This photo, and those below, are of petrified wood from the same rock formation as that in the National Park. The exception is that these specimens were collected in private land outside the park boundary with permission of the landowner. They were on display at the 2020 Tucson Gem, Mineral, and Fossil showcase by the Wild West Rock and Jewelry Co. Note the multitude of colors seen within the polished upper surface caused by changes in groundwater composition during burial.

Figure 5 - While the tops of many logs can be polished to show a multitude of colors, the outer "bark" often is brownish in color and retains the shape of the original tree.

Figure 6 - The internal structure is visible on many of the trees having been replaced by multicolored minerals found in the groundwater after the trees were originally buried.

Figure 7 - Fossilized Cycads can also be found.

Figure 8 - Most of the trees are Conifers. Note the slab in the foreground that has been cut and polished to form a tabletop, a common favorite among collectors.

Figure 9 - Considering that petrified wood weighs about 165 pounds per cubic foot, it was not easy to move this specimen.

Figure 10 - Petrified log exhibiting the typical brownish outer surface and multicolored internal texture. Notice the sign to the left posted by the Wild West Rock and Jewelry Co. explaining the chemical elements associated with the various colors found in the petrified wood.

Figure 11 - A cut and polished section of a petrified tree from the Chinle Fm. of Northern Arizona. Purchased from Wild West Rock and Jewelry Co.

Figure 12 - A view of the above cut and polished slab of petrified wood illustrating the outer bark.