The Coconino Sandstone is a Permian age geologic formation composed of light-colored quartz arenite of eolian origin. It erodes to form conspicuous, sheer cliffs in the upper walls of Grand Canyon, as part of the Mogollon Rim to the south and east, and in many other parts of the Colorado Plateau region. The Coconino Sandstone is well known for its fossil trackways of terrestrial invertebrates and vertebrates and large-scale cross-stratification.Middleton, L.T., D.K. Elliott, and M. Morales (2002) Coconino Sandstone, in S.S. Beus and M. Morales, eds., Grand Canyon Geology. Oxford University Press, New York.
Eastward of a north–south line from Monument Creek to Fossil Creek, the Coconino Sandstone overlies and interfingers with and grades into the Schnebly Hill Formation, which is equivalent in part to the De Chelly Sandstone in Utah. In this area, it underlies the Kaibab Limestone. Further eastward, the Coconino Sandstone likely correlates with and is contemporaneous with the Glorieta Sandstone of New Mexico. Westward of this line, the upper part of Coconino Sandstone is known as the White Rim Sandstone in Utah and the Cave Springs Member in Arizona. It interfingers and merges westward into the Toroweap Formation. The remaining lower part of the Coconino Formation is known as the Harding Point Member and underlies the Toroweap Formation and uncomfortably overlies the Hermit Formation. Between the Toroweap and Hermit formations, the Harding Point Member thins westward until it disappears.Blakey, R.C., and Knepp, R., 1989. Pennsylvanian and Permian geology of Arizona, in: Jenney, J.P., and Reynolds S.J., eds., Geologic Evolution of Arizona, Arizona Geological Society Digest, 17. pp. 313-347.
Nomenclature In 1910, Darton named and mapped the Coconino sandstone as a member of the now abandoned Aubrey group for its widespread distribution in the Coconino Plateau. He named the Coconino sandstone for the cross-bedded gray to white sandstones that form a conspicuous sheer cliff in walls of Grand Canyon and underlies entire Coconino Plateau and the extensive Colorado Plateau north of the Grand Canyon. As defined at that time, it lay between the overlying Kaibab (Aubre) limestone and the underlying Supai formation. The Kaibab limestone was later divided onto the Kaibab Limestone and Toroweap Formation, and the Supai formation was later subdivided into the Hermit Formation and Supai Group.Noble, L.F., 1922. A section of the Paleozoic formations of the Grand Canyon at the Bass Trail. U.S. Geological Survey Bulletin. 131-B, pp. B23-B73. Later, the Coconino Sandstone was recognized and mapped in the San Rafael Swell in the Emery County, Utah, region.Gilluly, J., and Reeside, J.B., Jr., 1928, Sedimentary rocks of the San Rafael Swell and some adjacent areas in eastern Utah. in Shorter contributions to general geology, 1927, U.S. Geological Survey Professional Paper, 150-D, p. D61-D110.
In 1982, Hamilton recognized the fine-grained vitreous quartzites exposed in the Salton basin as the metamorphosed equivalent of Coconino Sandstone in the Big Maria Mountains of southeast California. Because of the change in lithology, this fine-grained quartzite was named and mapped as the Coconino Quartzite. Located in the Big Maria Mountains exposures, the Coconino Quartzite lies between the Hermit Schist and Kaibab Marble.Hamilton, W. H., 1982, Structural evolution of the Big Maria Mountains, northeastern Riverside County, southeastern California. in E. G. Frost and D. L. Martin, eds., pp. 1–27, Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada. Cordilleran Publishers, San Diego, California, United States. 608 pp.
Description The Coconino Sandstone consists predominately of well-sorted, uniformly fine grained , nearly pure quartz sand grains held together by siliceous cement. It contains a few scattered potassium feldspar grains and traces of heavy minerals. Many of the sand grains are frosted or pitted and nearly all of them are rounded to subangular. Iron oxide staining and cements are commonly absent, which is reflected in its pale, white to buff color. However, locally, the Coconino Sandstone is iron-stained and, as a result, is either a brownish color, as in Marble Canyon, or bright red, as near Flagstaff, Arizona.McKee, E.D., 1934. The Coconino sandstone--its history and origin, in Papers concerning the Palaeontology of California, Arizona, and Idaho. Carnegie Institution of Washington Publication, 440, pp. 77-115.McKee, E.D. (1979) A study of global sand seas. Professional Paper 1052. U.S. Geological Survey, Reston, Virginia. 429 pp.
The Coconino Sandstone exhibits a number of primary sedimentary structures. The most conspicuous of these is ubiquitous large-scale, wedge-planar, cross-stratification. It consists of long sweeping layers that often are long and as much as long as . The cross-stratification dip mostly at 25°- 30° with few at a maximum of about 34°. Their dip is generally unimodal southward, but with a spread of readings ranging between southwest and southeast. Truncated by overlying beds, they form large, irregular wedges. In addition to the wedge-planar, cross-stratification, the Coconino Sandstone exhibits rare, large-scale, low-angle, less than 15°, cross-stratification that dip in the opposite direction. They are only found in limited numbers in a few localities within exposures of this sandstone. The basal of the Coconino Sandstone at many outcrops exhibits horizontal laminae. A distinctive characteristic of this sandstone is that the cross-stratification readily splits into thin plates.
Although ripple marks are not abundant within the Coconino Sandstone, they are distinctive and locally numerous. They consist typically of low, wide, and asymmetrical ripples with ripple indexes, the ratio of wavelength to amplitude, greater than 17 with most considerably higher. Typically, the ripple crests and troughs lie parallel to the direction of dip of the foreset slopes. The ripple crests are rounded and generally consist of coarser sand grains on or about their crests. The crests and troughs of these ripple are straight and parallel and, where exposed, exhibit little change in direction for distances of or more. They are less common in the Coconino Sandstone than in modern sand dunes because of the general lack of preservation of both the windward-side deposits of sand dunes and of ripple marks formed in dry sand without special circumstances.McKee, E.D. (1945) "Small-scale structures in the Coconino Sandstone of northern Arizona". "The Journal of Geology". 53(5):313–325.
On the bedding planes of Coconino Sandstone, the small, crater-like pits of raindrop impressions are recognized at several outcrops. They are oriented in respect to the sloping surface of laminations such that these circular pits tend to face upward, or vertically, with a raised downslope rim. Rain pits have been reproduced in the laboratory on sloping surfaces of fine dry sand to provide positive evidence of subaerial formation by brief rain showers.
Slump marks from mass movement or avalanching when lee-side slopes of dunes near the angle of repose are oversteepened are also found in the Coconino Sandstone. The slump marks range from those resulting from "successive discontinuous jerks with miniature landslides" to a series of variable and irregular lines that are roughly parallel to the lee side's slope and mark the edge of a slumped sand mass, miniature terrace-and-cliff structures, and other types of dry-sand slump marks.
The Coconino Sandstone uncommonly exhibits deformed bedding in the form of penecontemporaneously deformed cross-strata. In the few outcrops in which they are found, they can be quite abundant as near Doney Crater northeast of Flagstaff, Arizona. Typically, they consist of dipping foresets within a bed that for many feet in length have been folded locally, while the cross-strata above and below them are undisturbed.
The thickness of the Coconino Sandstone varies due to regional structural features. In the Grand Canyon area, it is only thick in the west, thickens to over in the middle and then thins to in the east.
Fossils
The only known fossils found in the Coconino Sandstone are trace fossils. Plant fossils have yet to be reported from the Coconino Sandstone. The body fossils of invertebrates and vertebrates are also lacking. However, it is a common occurrence, especially in continental desert deposits, that trace fossils provide the only available paleontologic and palaeobotanic information for paleoenvironmental reconstructions.Miller, A.E., Marchetti, L., Francischini, H., and Lucas, S.G., 2020. Chapter 8. Paleozoic invertebrate ichnology of Grand Canyon national Park. In: Santucci, V.L., Tweet, J.S., ed., pp. 277–331, Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version). Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.Spamer, E.E., 1984. Paleontology in the Grand Canyon of Arizona: 125 years of lessons and enigmas from the late Precambrian to the present. The Mosasaur. 2, pp. 45–128.
The invertebrate trace fossils are known from the Coconino Sandstone include possible producers, such as worms, millipedes, isopods, spiders, scorpions. Paleohelcura, a possible scorpion track, is the most common invertebrate trace fossil. Other arthropod tracks, meniscate horizontal burrows, and conical pits have also been documented in the Coconino Sandstone. Many of the invertebrate trace specimens been collected from it include long, complete, and beautifully preserved trackways and burrows. Typically, these trace fossils are reported from the lower half of the Coconino Sandstone. They are commonly preserved on the surfaces of foreset laminae of eolian sand dunes. The invertebrate tracks were likely made on dry sand that was then moistened and covered by sand before the surface dried out, or on dunes dampened by dew. Because of the preservation of so much extramorphological variation, the effects of the trackmaker's travels across inclined sand, foreset beds can be recognized and studied. The number of valid ichnotaxa known from the Coconino Sandstone is low and consists of only six ichnogenera: Diplichnites, Diplopodichnus, Lithographus, Octopodichnus, Palaeohelcura, and Taenidium.
One fossil specimen from the Mogollon Rim area consists of two trackways, one composed of two parallel rows of tracks, Lithographus isp (possibly insect such as blattoids). The insect trackway ends abruptly after a change its path in at a tetrapod trackway trending transverse to it at a quite high pace. This association of insect and tetrapod trackways is interpreted as predation behavior by the tetrapod relative to an insect prey.Kramer, J.M., Erickson, B.R. Lockley, M.G. Hunt, A.P., and Braddy, S., 1995. Pelycosaur predation in the Permian: Evidence from Laoporus trackways from the Coconino Sandstone with description of a new species of Permichnium. New Mexico Museum of Natural History and Science Bulletin. 6, pp. 245–249. This interpretation has been questioned, although predation cannot be excluded.Marchetti, L., Francischini, H., Lucas, S. G., Voigt, S., Hunt, A. P., and Santucci, V.L., Chapter 9. Paleozoic Vertebrate Ichnology of Grand Canyon National Park In: Santucci, V.L., Tweet, J.S., ed., pp. 333-379, Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version)'. Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.