Painted Desert Geology

To set the stage, you’re facing northwest and looking at Pilot Rock, elevation 6234 feet, across the Painted Desert from Chinde Point in the northern portion of Petrified Forest NP, Arizona. Note the late Cenozoic lava flow at the lower right. (November, 2007 photo © Donna Mason-Spier)

Painted Desert Geology – Part 1, Northern Petrified Forest NP, AZ — © Dave Spier

From east of the Grand Canyon, the Painted Desert crosses the Colorado Plateau in a long curving arc southeastward to encompass the Petrified Forest east of Holbrook, Arizona, and then continues into New Mexico and northeastern Arizona. The bright colors, primarily shades of red, are due to iron oxides in the late Triassic Chinle formation deposited as clay, silt and sand in lakes and wetlands that covered a broad plain over 200 million years ago (mya). Most of these sediments are now soft mudstones and related rocks with a few harder layers of sandstone. Color contrast in the northern section of the Petrified Forest is provided by a light-colored band of volcanic ash known as the Black Forest bed.

The bright-colored layer on top of the low hills is volcanic ash in the Black Forest Bed, named for the unusually-dark tree trunks it contains. In the foreground, we are standing on basalt lava of the Bidahochi Formation exposed at the rim of a mesa overlooking the Painted Desert north of Kachina Point in Petrified Forest NP, AZ.
(23mm/full-frame photo © Dave Spier)

Bentonite clay layers are formed from the volcanic ash of the Black Forest bed and provides contrast in the Chinle Formation seen from Kachina Point using a 300mm telephoto. (© Dave Spier)

On top of the Chinle layers there are younger deposits of clay, silt and sand plus lava flows belonging to the Bidahochi Formation, much of which has been removed by erosion. These Tertiary layers can be seen from several of the overlooks north of I-40. Flat-topped mesas to the north are capped by this formation.

The buff-colored band at top is assigned to the Bidahochi Formation and was deposited in a lake around 50 million years ago [per Dr. Sydney Ash] during the Tertiary Period. The red layers below it are the colorful and much older Chinle Formation. (November photo © Donna Mason-Spier)

Photography Notes: On cloudy days, the diffuse lighting makes topographic features look flat and featureless. If possible wait for a sunny day or time and choose locations where the sun will be to your side (left or right). This side-lighting creates shadows for contrast and brings out the texture and details. The effect is maximized soon after sunrise and before sunset, which are the golden times of daylight and give the warmest color tones.

Painted Desert from the overlook between Tiponi and Tawa Points, taken at noon on a cloudy day. (© Donna Mason-Spier)

We were on our way to the Grand Canyon, so time constraints reduced our ability to revisit this section under better lighting conditions. There is no campground [or car camping] in the park, [although you can backpack which requires a permit], and the extra travel further reduced time for photography.

Painted Desert from Tawa Point using a 300mm telephoto on Canon XT; even with the sun out, mid-day lighting tends to be flat without strong sidelighting. (November 10, 2007 photo © Dave Spier)

 Corrections, comments and questions are always welcome at northeastnaturalist@yahoo.com or connect through Facebook (Dave Spier, photographic naturalist and northeast naturalist).

Raven flying by Tiponi Point around noon – © Dave Spier

References:

Petrified Forest – a Story in Stone, by Dr. Sidney Ash (© 2005), by Petrified Forest Museum Association

Roadside Geology of Arizona, by Halka Chronic (© 1983/reprinted 2002), published by Mountain Press

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One thought on “Painted Desert Geology

  1. One new photo was added [volcanic ash using 300mm from Kachina Point], and a misleading reference to Miocene vulcanism was removed. At this point, I do not know the age of the lava capping the northern overlooks in the park, although it’s likely late Miocene or early Pliocene. The Bidahochi is around 4-8 million years old. A reference to 50 million year old Bidahochi layers [in one of the photo captions] by Dr. Ash may be a typographical error in his book.

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