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General Geology:
Petrified Forest National Park was a dense forest over 200 million years ago, during the Late Triassic period, and the trees from the forest were preserved as petrified wood through a process of sedimentation and volcanic ash. Over millions of years, erosion of the surrounding land has exposed the petrified wood and created a unique landscape of colorful badlands. The park showcases two prominent geological formations: the Chinle Formation and the Bidahochi Formation.
Map of Petrified Forest National Park,
via npmaps.com/petrified-forest/
Rainbow Forest
Rainbow Forest
Rainbow Forest is located in the southernmost area of Petrified Forest National Park. Rainbow Forest is where you will find the most petrified wood.
GEOLOGICAL SUMMARY
A majority of the petrified wood is derived from a tree species called araucarioxylon arizonicum, which was common in the region during the Late Triassic Period. Fallen trees were buried in sediment, preventing them from decomposing. They then underwent a gradual process of fossilization over the span of millions of years.
Ash from volcanoes in the west moved through the wind and became incorporated into river sediment. Silica from the ash was carried into the wood via the river water. As a result, quartz (most common), citrine, amethyst, and other vibrant minerals formed inside of the wood, giving the petrified wood its color and glisten.
Environmental setting
Once buried under sediment in Chinle Formations, the logs are now completely exposed as a result of wind and water erosion cutting away at the formations.
HISTORICAL SIGNIFICANCE
The petrified wood found at Rainbow Forest provides a window into the past, offering insights into the ancient forests that were once in the region.
Aside from its geological significance, Rainbow Forest has cultural and historical importance as well. The area was once inhabited by indigenous people, who used the petrified wood for tools and ceremonies.
Painted Desert
Painted Desert
Painted Desert is an area of badlands located in the northern side of Petrified Forest National Park, occupied by Chinle Formations and Bidahochi Formations.
GEOLOGICAL SUMMARY
The Chinle Formations in Painted Desert are made up of 5 main members.
Mesa Redondo Member (226 million years old): The oldest member, made up of red sandstone with some mudstone.
Blue Mesa Member (223-225 million years old): Made up of purple, grey, blue, and green mudstones with some sandstone beds.
Sonsela Member (213-215 million years old): Made of 3 parts
Rainbow Forest Beds: white cross-bedded sandstone and conglomerates (with the petrified wood)
Jim Camp Walsh Beds: purple, grey, and blue mudstones, as well as white and grey sandstone
Flattops One Bed: brown cross-bedded sandstone
Petrified Forest Member (209 million years old): red mudstone and brown sandstone
Owl Rock Member (207 million years old): pink and orange mudstone and limestone
The Bidahochi Formations are made up of sediments during the Miocene and Pliocene periods (4-8 million years ago) that were deposited into a large lake basin. The sediment layers include silt, clay, sand, and volcanic ash.
Environmental Setting
Painted Desert's signature look is due to the different colors of each layer of its members, which are separated by millions of years of time. Reddish hues formed when iron minerals oxidized because of fluctuating water levels, while greenish and bluish hues formed due to iron minerals in reducing environments with low oxygen.
Over time, the badlands of Painted Desert faced vertical water and wind erosion, as well as deposition, shaping the landscape that we see today.
HISTORICAL SIGNIFICANCE
With each member of the Painted Desert's Chinle Formations being from vastly different time periods, geologists can study each layer to learn about the environmental conditions of its respective time period. This allows us to learn about the geological history of Petrified National Park overall.
Blue Mesa
Blue Mesa
Blue Mesa is a location in the southern area of Petrified Forest National Park known for its blue, purple, and grey landscapes.
GEOLOGICAL SUMMARY
As the name suggests, Blue Mesa is primarily made up of the Blue Mesa member of the sedimentary Chinle Formations. The mesas are made up of mudstone with some sandstone beds.
Environmental setting
While badlands have slopes when eroded, mesas have flatter tops. This is because the erosion happens along horizontal rock layers, sweeping away the softer sediments.
The mesas seen in the image to the left have their blueish hue because of reductive dissolution. The iron in the formations were formed in reducing conditions with low oxygen.
HISTORICAL SIGNIFICANCE
The Blue Mesa area of Petrified Forest can tell geologists much about rocks 223-225 million years ago.
In regards to human history, Pueblo, Zuni, and Hopi Native Americans would use this area as an important trade route.
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