Coconino Sandstone

Coconino Sandstone
Name	Coconino Sandstone
Type	Geological formation
Age	Permian
Period	Permian
Primary lithology	Sandstone
Other lithology	Siltstone, conglomerate
Named for	Coconino County
Region	Arizona, Utah
Country	United States
Unit of	Supai Group (older usage) / Chinle Formation (regional correlations)
Subunits	Upper and lower informal members
Underlies	Hermit Formation, Toroweap Formation
Overlies	Hermosa Group (regional variation), Kaibab Limestone

Contents

Coconino Sandstone The Coconino Sandstone is a distinctive Permian-age sedimentary sandstone unit prominent in the Colorado Plateau of the southwestern United States, known for its pale tan to white cross-bedded eolian deposits and widespread cliff-forming exposures. It has been central to interpretations of Permian paleoclimate, regional stratigraphy, and recreational geology, with extensive study involving institutions such as Smithsonian Institution, United States Geological Survey, Harvard University, Stanford University, and University of Arizona. The unit interfaces with landmarks and managed areas including Grand Canyon National Park, Coconino County, Kaibab National Forest, Glen Canyon National Recreation Area, and Zion National Park.

Description and Lithology

The formation consists predominantly of well-sorted, fine- to medium-grained quartzose sandstone with prominent large-scale planar and trough cross-bedding, best developed along the Grand Canyon escarpments and exposures near Sedona, Flagstaff, and Grand Staircase–Escalante National Monument. Petrographic studies from researchers at Yale University, University of California, Berkeley, Columbia University, University of Colorado Boulder, and Arizona State University document high quartz content, minor feldspar, and accessory heavy minerals such as zircon and magnetite, with cement types including calcite and silica. Diagenetic features, described in publications from Geological Society of America, American Geophysical Union, and Society for Sedimentary Geology, include authigenic quartz overgrowths, clay illitization, and rare gypsum nodules; paleosols and thin siltstone interbeds indicate episodic subaerial alteration noted in fieldwork by teams from University of New Mexico, Brigham Young University, and University of Utah.

Regional stratigraphic correlations tie the unit to mid- to late-Permian chronostratigraphy, constrained by biostratigraphic and radiometric calibrations used by specialists at University of Oklahoma, Purdue University, and Pennsylvania State University. In the Grand Canyon stratigraphic column it typically overlies the Hermit Formation or Kaibab Limestone depending on locality and is overlain by the Toroweap Formation or locally by the Coconino Sandstone equivalent units recognized in western exposures. Correlations with the Beekmantown Group and other North American Permian sequences have been debated in symposia held by International Union of Geological Sciences and reported in journals edited by Elsevier and Springer Nature contributors. Radiometric ages from detrital zircon studies by teams at University of Arizona and University of Wisconsin–Madison place maximum depositional ages within the Cisuralian to Guadalupian stages.

Interpretations emphasizing wind-blown dune deposition (eolian) were advanced by researchers affiliated with Arizona State University, University of California, Los Angeles, University of Texas at Austin, and field campaigns sponsored by National Park Service and Bureau of Land Management. The formation's large-scale cross-strata, foreset angles, and grainfall laminae support analogies to modern deserts such as the Sahara Desert, Rub' al Khali, and Namib Desert, while palaeoclimatic reconstructions reference Permian greenhouse intervals discussed at conferences by Intergovernmental Panel on Climate Change-associated researchers and in syntheses by Royal Society. Trace fossils, invertebrate burrows, and rare plant impressions reported by paleontologists at American Museum of Natural History, Natural History Museum, London, Field Museum, and Museum of Northern Arizona yield limited paleoecological data, indicating transient colonization by arthropods and cryptogamic crusts comparable to modern hyperarid settings. Competing hypotheses, including fluvial or coastal sabkha influences advocated in works from University of Kansas and Oklahoma State University, emphasize local water-table fluctuations and interdune deposition.

The unit crops out across the Colorado Plateau in Arizona, Utah, New Mexico, and Nevada with iconic exposures at Grand Canyon National Park, Coconino County, Walnut Canyon National Monument, Sedona, Vermilion Cliffs National Monument, Bryce Canyon National Park peripheries, and along the Colorado River corridor. Noteworthy study sites include the South Rim and North Rim of the Grand Canyon, the Hurricane Cliffs near St. George, Utah, and roadcuts along Interstate 17 and U.S. Route 89A, frequented by investigators from Arizona Geological Survey, Utah Geological Survey, and international delegations from University of Glasgow and University of Oxford.

As a competent, cliff-forming sandstone, the formation influences regional slope stability, reservoir potential, and aggregate resources noted in assessments by U.S. Army Corps of Engineers, Federal Highway Administration, and state departments such as Arizona Department of Transportation and Utah Department of Transportation. Its porosity and permeability characteristics are evaluated for groundwater flow near Coconino National Forest and potential unconventional reservoirs in studies by Chevron Corporation, ExxonMobil, and academic-industry collaborations at Colorado School of Mines. Quarrying for dimension stone and crushed stone has local economic impact documented by county records in Coconino County and city planning offices in Flagstaff, while engineering challenges for infrastructure crossing exposures have been addressed in reports from American Society of Civil Engineers and Transportation Research Board.

Named for Coconino County during early geological surveys by parties associated with U.S. Geological Survey and paleontologists such as those linked to George Perkins Merrill-era investigations, the formation's nomenclature and interpretation evolved through contributions from geologists at Yale University, Princeton University, University of Chicago, and California Institute of Technology. Seminal mapping and descriptive work appeared in bulletins issued by USGS and state surveys, with major revisions prompted by 20th-century fieldwork from teams at Stanford University and University of Arizona and later synthesis in volumes from the Geological Society of America. Ongoing research integrates stratigraphic, sedimentologic, and isotopic methods developed in laboratories at Massachusetts Institute of Technology, University of Minnesota, University of Michigan, and international collaborators from University of Toronto and ETH Zurich.