Geologic Formations

Stratigraphy Column of Petrified Forest, detail below

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.The first chapter of this geological text is the Chinle Formation.

Chinle Formation

The Chinle Formation was deposited over 200 million years ago during the Late Triassic Period. 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 Chinle Formation is further divided to include the Blue Mesa Member, the Sonsela Member, the Petrified Forest Member, and the Owl Rock Member.

 

Geological Layers, Youngest to Oldest

 

Dune near South Wilderness Unit

NPS

Pleistocene and Holocene Sediments

Pleistocene and Holocene Epoch (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.

 

Bidahochi Formation

NPS

Bidahochi Formation

During the Late Miocene and Early Pliocene Epochs of the Neogene 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, leaving volcanic scoria cones and maars (flat-bottom, roughly circular volcanic craters of explosive origin). The vent from one of these maars is exposed on the Painted Desert Rim across the park road to the east of 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 square miles (2,500 square km). The erosion-resistant lava flows, such as Pilot Rock and the Hopi Buttes, protect the softer lake-bed deposits beneath.

 

Unconformity between Bidahochi basalt on top and red sedimentary Chinle below

NPS

Where the Bidahochi and Chinle Formations make contact is an unconformity. An unconformity represents missing rock layers which in turn represents missing time. It's like a geology textbook with missing pages. You can tell that a page is missing but you can't tell what was on them. The Chinle Formation was deposited over 200 million years ago but the Bidahochi Formation is only about 8 million years ago. The contact between the Bidahochi and Chinle Formations represents 192 million years of missing time!

 

Owl Rock Member, Chinle Formation

NPS

The Owl Rock Member consists of pinkish-orange mudstones mixed with hard, thin layers of limestone. Lenses of selenite gypsum are scattered periodically throughout the Owl Rock Member representing the minerals left behind after evaporation of inland lakes. This member is exposed on Chinde Mesa at the northernmost border of the park. The Owl Rock Member is approximately 205 million years old.

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).

 

Petrified Forest Member, Chinle Formation

NPS

The Petrified Forest Member consists of thick sequences of reddish mudstones and brown sandstone layers. This member is exposed in the Flattops and is the white and pink reworked volcaniclastic unit of the Painted Desert. It contains large amounts of petrified wood. The Black Forest Bed, part of the Petrified Forest Member north of Kachina Point, has been dated isotopically at 213 +/- 1.7 million years old.

 

Sonsela Member, Chinle Formation

NPS/Andrew V. Kearns

The Sonsela Member is divided into three parts: 1) the upper Flattops One Bed, which consists of a thick cliff-forming brown, cross-bedded sandstone, 2) the middle Jim Camp Wash Beds of blue, grey, and purple mudstones and numerous small grey and white sandstone beds, and 3) the lower Rainbow Forest bed consisting of white cross-bedded sandstone and conglomerate of rounded pebbles and cobbles which contains the logs of the Rainbow Forest. This sandstone caps Blue Mesa, Agate Mesa, and the mesa north of the Rainbow Forest. The Sonsela Member is approximately 216 million years old.

 

Blue Mesa Member, Chinle Formation.

NPS

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 220-225 million years old.

 

Mesa Redondo Member outcrop, Chinle Formation

NPS

The Mesa Redondo Member is the oldest member of the Chinle Formation exposed in the Petrified Forest, mainly in the Tepees area. The Mesa Redondo consists of dark red siltstones as well as sandy conglomerates. The top of the member is very colorful hard layer with yellow, red, and purple mottles. The Mesa Redondo was deposited between 225-227 Million years ago and represents floodplain deposits that were adjacent to the sandstone channels of the Shinarump Member.

 

NPS