| Navajo Sandstone | |
|---|---|
| Type | Geological formation |
Navajo Sandstone is a geologic formation in the Glen Canyon Group that is spread across the U.S. states of northern Arizona, northwest Colorado, and Utah; as part of the Colorado Plateau province of the United States.[1] This rock formation is particularly prominent in southern Utah, where it forms the main attractions of a number of national parks and monuments including Zion National Park, Capitol Reef National Park, Glen Canyon National Recreation Area, Grand Staircase-Escalante National Monument, and Canyonlands National Park. Navajo Sandstone frequently occurs above the Kayenta Formation and Wingate Sandstone, the other formations of the Glen Canyon Group. Together, these three formations can result in immense vertical cliffs of up to 2,200 feet (670 m). Atop the cliffs, Navajo Sandstone often appears as massive rounded domes and bluffs that are generally white in color.
Contents |
Navajo Sandstone frequently occurs as spectacular cliffs, cuestas, domes, and bluffs rising from the desert floor. It can be distinguished from adjacent Jurassic sandstones by its white to light pink color, meter-scale cross-bedding, and distinctive rounded weathering.
The wide range of colors exhibited by the Navajo Sandstone reflect a long history of alteration by groundwater and other subsurface fluids over the last 190 million years. The different colors, except for white, are caused by the presence of varying mixtures and amounts of hematite, goethite, and limonite filling the pore space within the quartz sand comprising the Navajo Sandstone. The iron in these strata originally arrived via the erosion of iron-bearing silicate minerals. Initially, this iron accumulated as iron-oxide coatings, which formed slowly after the sand had been deposited. Later, after having been deeply buried, reducing fluids composed of water and hydrocarbons flowed through the thick red sand which once comprised the Navajo Sandstone. The dissolution of the iron coatings by the reducing fluids bleached large volumes of the Navajo Sandstone a brilliant white. Reducing fluids transported the iron in solution until they mixed with oxidizing groundwater. Where the oxidizing and reducing fluids mixed, the iron precipitated within the Navajo Sandstone. Depending on local variations within the permeability, porosity, fracturing, and other inherent rock properties of the sandstone, varying mixtures of hematite, goethite, and limonite precipitated within spaces between quartz grains. Variations in the type and proportions of precipitated iron oxides resulted in the different crimson, vermillion, orange, salmon, peach, pink, gold, and yellow colors of the Navajo Sandstone. The precipitation of iron oxides also formed laminea, corrugated layers, columns, and pipes of ironstone within the Navajo Sandstone. Being harder and more resistant to erosion than the surrounding sandstone, the ironstone weathered out as ledges, walls, fins, "flags", towers, and other minor features, which stick out and above the local landscape in unusual shapes.[2][3]
The age of the Navajo Sandstone is somewhat controversial. It may originate from the Late Triassic but is at least as old as the Early Jurassic stages Pliensbachian and Toarcian.[1] There is no type locality of the name. It was simply named for the 'Navajo Country' of the southwestern United States.[4] The two major subunits of the Navajo are the Lamb Point Tongue (Cedar City area) and the Shurtz Sandstone Tongue (Kanab area).[5]
The Navajo Sandstone was originally named as the uppermost formation of the La Plata Group by Gregory and Stone in 1917. Baker reassigned it as the upper formation of Glen Canyon Group in 1936. Its age was modified by Lewis and others in 1961. The name was originally not used in northwest Colorado and northeast Utah, where the name 'Glen Canyon Sandstone' was preferred.[6] Its age was modified again by Padian in 1989.
The sandstone was deposited in an arid erg on the Western portion of the Supercontinent Pangea. This region was affected by annual monsoons that came about each winter when cooler winds and wind reversal occurred.
Navajo Sandstone outcrops are found in these geologic locations:
The formation is also found in these parklands (incomplete list):
Indeterminate theropod remains geographically located in Arizona, USA.[7] Theropod tracks are geographically located in Arizona, Colorado, and Utah, USA.[7] Ornithischian tracks located in Arizona, USA.[7]
Color key
|
Notes Uncertain or tentative data are in small text; |
| Ornithodires of the Navajo Sandstone | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Stratigraphic position | Material | Notes | Images |
|
Ammosaurus cf. major[7] |
|
|||||
|
S. halli[7] |
|
"Partial postcranial skeleton." |
||||
|
S. ruessi[9] |
|
|||||
The Navajo Sandstone is also well known among rockhounds for its hundreds of thousands of iron oxide concretions. They are believed to represent an extension of Hopi Native American traditions regarding ancestor worship ("moqui" translates to "the dead" in the Hopi language). Informally, they are called "Moqui marbles" after the local proposed Moqui native American tribe. Thousands of these concretions weather out of outcrops of the Navajo Sandstone within south-central and southeastern Utah within an area extending from Zion National Park eastward to Arches and Canyonland national parks. They are quite abundant within Grand Staircase-Escalante National Monument.[2][3]
The iron oxide concretions found in the Navajo Sandstone exhibit a wide variety of sizes and shapes. Their shape ranges from spheres to discs; buttons; spiked balls; cylindrical hollow pipe-like forms; and other odd shapes. Although many of these concretions are fused together like soap bubbles, many more also occur as isolated concretions, which range in diameter from the size of peas to baseballs. The surface of these spherical concretions can range from being very rough to quite smooth. Some of the concretions are grooved spheres with ridges around their circumference.[2][3]
The abundant concretions found in the Navajo Sandstone consist of sandstone cemented together by hematite (Fe2O3), and goethite (FeOOH). The iron forming these concretions came from the breakdown of iron-bearing silicate minerals by weathering to form iron oxide coatings on other grains. During later diagenesis of the Navajo Sandstone while deeply buried, reducing fluids, likely hydrocarbons, dissolved these coatings. When the reducing fluids containing dissolved iron mixed with oxidizing groundwater, they and the dissolved iron were oxidized. This caused the iron to precipitate out as hematite and goethite to form the innumerable concretions found in the Navajo Sandstone. These concretions are regarded as terrestrial analogues of the hematite spherules, called alternately Martian "blueberries" or more technically Martian spherules, which the Opportunity rover found at Meridiani Planum on Mars.[2][3]