Petrified Forest National Park Geologic Formations

The badlands of the Painted Desert with Pilot Rock in the background. NPS Photo Erosion has sculpted and shaped intriguing landforms. The rocks reveal an enthralling chronicle of time that is unfolding and ever-changing. What can the rocks tell us? Think of the colorful layers as pages in a massive book. As the pages are turned, we discover that the words are a language we don't completely understand. The pictures help, but we must put together the story of this ancient book with fragmented clues. The first chapter of this geological text is the Chinle Formation. Chinle Formation The colorful badland hills, flat-topped mesas, and sculptured buttes of the Painted Desert are primarily made up of the Chinle Formation, mainly fluvial (river related) deposits. Within Petrified Forest National Park, the layers of the Chinle Formation include the Blue Mesa Member, the Sonsela Member, the Petrified Forest Member, and the Owl Rock Member. The Blue Mesa Member consists of thick deposits of grey, blue, purple, and green mudstones and minor sandstone beds, the most prominent of which is the Newspaper Rock Sandstone. This unit is best exposed in the Tepees area of the park. The Blue Mesa Member is approximately 234 million years old. The Sonsela Member is divided into three parts:

1) the lower Rainbow Forest bed consisting of white cross-bedded sandstone and cobbles and containing the logs of the Rainbow Forest,

2) the middle Jim Camp Wash Beds of blue, grey, and purple mudstones and numerous small grey and white sandstone beds, and

3) the upper Flattops One Bed, which consists of a thick cliff-forming brown, cross-bedded sandstone.

This sandstone caps Blue Mesa, Agate Mesa, and the mesa north of the Rainbow Forest. The Sonsela Member is approximately 227 million years old. The Petrified Forest Member consists of thick sequences of reddish mudstones and brown sandstone layers. This member is exposed in the Flattops and the red part of the Painted Desert. The Black Forest Bed, park of the Petrified Forest Member north of Kachina Point, has been determined to be 214 million years old. The Owl Rock Member consists of pinkish-orange mudstones mixed with hard, thin layers of limestone. This member is exposed on Chinde Mesa at the northernmost border of the park. The Owl Rock Member is 205 million years old. During the Late Triassic, this region was located on the southwestern edge of the supercontinent Pangaea and just north of the equator. Evidence from ancient soils as well as fossil plants and animals indicates that the climate was humid and sub-tropical during the Late Triassic. The sedimentary layers of the Chinle Formation consist of sandstone, mudstone, and conglomerate deposited by a large river system that had cycles of droughts and floods, similar to those affecting many modern river systems. The colorful layers in the Chinle Formation represent ancient soil horizons. The coloration is due to the presence of various minerals. While the red and green layers generally contain the same amount of iron and manganese, differences in color depend on the position of the groundwater table when the ancient soils were formed. In soils where the water table was high, a reducing environment existed due to a lack of oxygen in the sediments, giving the iron minerals in the soil a greenish or bluish hue. The reddish soils were formed where the water table fluctuated, allowing the iron minerals to oxidize (rust). Bidahochi Formation During the Miocene and Pliocene of the Tertiary period (4-8 million years ago) a large lake basin with ephemeral lakes covered much of Northeastern Arizona. Fine-grained fluvial and lacustrine (lake related) sediment such as silt, clay, and sand represent the lower part of the Bidahochi Formation.

Volcanoes, both nearby and as far as the Southwestern Nevada Volcanic field, spewed ash and lava over the land and into the basin. Many of the volcanoes were phreatomagmatic, when ground or lake-water mingled with eruptive material (magma) to cause explosive eruptions. The resulting ash formed fine-grained deposits that were deposited within the lake sediments. After a few million years of erosion, most of the Bidahochi Formation has been removed from the park area, exposing the volcanic landforms known scoria cones and maars (flat-bottom, roughly circular volcanic craters of explosive origin). The vent from one of these maars is exposed in the Painted Desert Rim across the park road to the east from Pintado Point. The Hopi Butte Volcanic Field, which can be seen from the northern overlooks of the park extending northwest, is considered one of the largest concentrations of maar landforms in the world, covering about 965 sq. miles (2,500 sq. km). The erosion-resistant lava flows, such as Pilot Rock and the Hopi Buttes, protect the softer lake-bed deposits beneath. Quaternary Sediments Quaternary Period (1.8 million years ago to present) deposits of windblown sand and alluvium (deposited by flowing water), now cover much of the older formations of the park. At higher elevations in the northern part of the park, 500,000-year-old dunes can be found. Younger dunes, around 10,000 years old, are found in drainage areas that contain sand such as Lithodendron Wash. The youngest dunes are found throughout the park, in all settings, deposited around a thousand years ago. These dune deposits are largely stabilized by vegetation, especially grasses. While not as numerous as the fossils of the Chinle Formation, fossils have been found even in the quaternary sediments, including fragments of an ancestral proboscidean (elephants and their relatives, such as mammoths). The Little Colorado River and its tributaries, including the Puerco River, have cut their own valleys into the soft Chinle and Bidahochi Formations of the Painted Desert.

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