The Shinumo Quartzite also known as the Shinumo Sandstone, is a

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic ...

rock formation A rock formation is an isolated, scenic, or spectacular surface rock (geology), rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock Geological formation, formation ...

, which outcrops in the eastern

Grand Canyon The Grand Canyon is a steep-sided canyon carved by the Colorado River in Arizona, United States. The Grand Canyon is long, up to wide and attains a depth of over a mile (). The canyon and adjacent rim are contained within Grand Canyon Nati ...

Coconino County, Arizona Coconino County is a County (United States), county in the North Central Arizona, North-Central part of the U.S. state of Arizona. Its population was 145,101 at the 2020 United States census, 2020 census. The county seat is Flagstaff, Arizon ...

, ( Northern Arizona). It is the 3rd member of the 5-unit Unkar Group. The Shinumo Quartzite consists of a series of massive, cliff-forming

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s and

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

s. Its cliffs contrast sharply with the stair-stepped topography of typically brightly-colored (orange, red, yellow, etc) strata of the underlying slope-forming Hakatai Shale. Overlying the Shinumo, dark green to black, fissile, slope-forming

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s of the Dox Formation create a well-defined notch. It and other formations of the Unkar Group occur as isolated fault-bound remnants along the main stem of the Colorado River and its tributaries in Grand Canyon.

Typically, the Shinumo Quartzite and associated strata of the Unkar Group dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim).Hendricks, JD, and GM Stevenson (2003) ''Grand Canyon Supergroup: Unkar Group.'' In SS Beus and M Morales, eds., pp. 39–52, Grand Canyon Geology, 2nd ed. Oxford University Press, New York. The Shinumo Quartzite is a middle member of the Unkar Group. The Unkar Group is about thick and composed of, in ascending order, the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt.

The Unkar Group consists of a sequence of sedimentary rocks that accumulated in a variety of environments ranging from

fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

to shallow- marine. Overall, the

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

within it are conformable with the exception of a significant

disconformity An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

between the Hakatai Shale and Shinumo Quartzite. The Unkar Group is overlain in ascending order by the Nankoweap Formation, about thick; the Chuar Group, about thick; and the Sixtymile Formation, about thick. The Unkar Group, as the base section of the Grand Canyon Supergroup, overlies deeply eroded

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...

es,

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...

s, and

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

s that comprise Vishnu Basement Rocks.Elton, DP, and EH McKee (1982) ''Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona.'' Geological Society of America Bulletin. 93(8): 681–699.Karlstrom, KE, BR Ilg, Bradley, D Hawkins, ML Williams, G Dumond, KK. Mahan, and SA Bowring, Samuel (2012) ''Vishnu Basement Rocks of the Upper Granite Gorge: Continent formation 1.84 to 1.66 billion years ago.'' In JM Timmons and KE Karlstrom, eds., pp. 7–24, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado.

Description

In sharp contrast to argillaceous strata above and below it, the Shinumo Quartzite consists predominately of beds of red, brown or purple sedimentary

s that are firmly, to extremely well cemented by

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

, and characterized by poorly developed bedding. It also contains beds of massive white, red or purple

and conglomeratic sandstone. One conglomeratic sandstone layer that lies about above the base of the Shinumo Quartzite near the South Kaibab trail contains

jasper Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to ...

pebble A pebble is a clastic rocks, clast of rock (geology), rock with a grain size, particle size of based on the Particle size (grain size), Udden-Wentworth scale of sedimentology. Pebbles are generally considered larger than Granule (geology), gra ...

s.Daneker, TM (1974) ''Sedimentology of the Precambrian Shinumo Sandstome, Grand Canyon, Arizona.'' Unpublished Master’s thesis, Northern Arizona University : Flagstaff, Arizona. 390 pp.Elston, DP (1989) ''Middle and late Proterozoic Grand Canyon Supergroup, Arizona.'' In DP Elston, GH Billingsley, and RA Young, RA., eds., pp. 94–105, Geology of the Grand Canyon, Northern Arizona (with Colorado River Guides). American Geophysical Union Fieldtrip Guidebook T115/315 for International Geologic Congress, 28th. American Geophysical Union, Washington DC. 239 pp. 2021 Revised NPS Geologic Stratigraphic Column of the Grand Canyon

2021 Revised NPS Geologic Stratigraphic Column of the Grand Canyon

Typically, the Shinumo Quartzite has been subdivided into four poorly defined, unnamed members. First, the basal lower member consisting of purplish arkosic conglomeratic sandstone. Unlike the rest of the Shinumo Quartzite, it contains quartzite and

gravel Gravel () is a loose aggregation of rock fragments. Gravel occurs naturally on Earth as a result of sedimentation, sedimentary and erosion, erosive geological processes; it is also produced in large quantities commercially as crushed stone. Gr ...

s up to in diameter. As in the case of the Hotauta Conglomerate, the quartzite gravel of the lower member lacks any known equivalents in the Grand Canyon region. Second, the lower-middle member is composed of purple cross-bedded quartzite, sometimes is subdivided into two units, and overlies the lower member. The purple quartzites of the lower-middle member exhibit crisp, well-defined reduction spots. Third, the upper-middle member is composed of "rusty red" quartzite and overlies the lower-middle member. The "rusty-red" quartzite of the upper-middle member grades downward into the lower-middle member through an interval of interbedded purple and red-brown quartzite. Their colors and cementation appear to be the result of ground or connate water alteration after their deposition. Finally, the upper member of the Shinumo Quartzite is composed of reddish brown and locally purple sandstone and an overlying well-cemented gray quartzite. The Shinumo Quartzite is characterized as a quartzite, but the gray quartzite within the upper member of the Shinumo Quartzite contains

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

-rich intervals. In addition, the gray quartzite was subjected to cementation by silica and bleaching that removed its original reddish brown and purple

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

pigments, after its deposition.

Basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

ic sills and dikes intrude all formations of the Unkar Group below the Cardenas Lava. The Hakatai Shale, Shinumo Quartzite, and Dox Formation are intruded by these dikes. They can be traced, discontinuously, to within a few meters of the base of the Cardenas Lava.

Sedimentary structures

The Shinumo Quartzite exhibits a variety of sedimentary structures. The sandstones of the lower and lower-middle members exhibit centimeter- to meter-scale planar tabular

cross-stratification In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

and trough cross-beds. These cross beds record north-directed paleocurrents along with subordinate bidirectional paleocurrents. The upper-middle member exhibits abundant cross beds, clay galls, and

mudcrack Mudcracks (also known as mud cracks, desiccation cracks or cracked mud) are sedimentary structures formed as muddy sediment dries and contracts.Jackson, J.A., 1997, ''Glossary of Geology'' (4th ed.), American Geological Institute, Alexandria, VA ...

s. The sandstones of the upper member exhibit trough cross-beds, suggesting a more northerly transport direction. The sandstone and quartzite beds of the upper member exhibit an abundance of complexly contorted, gnarly bedded, fluid expulsion structures and dramatic convolute bedding. The beds that exhibit these soft-sediment deformation structures are meters- to tens-of-meters thick. They are cited as evidence for frequent

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

activity and fluid migration during the deposition of the Shinumo Quartzite. These deformed beds can be seen at river level in the vicinity of Nevilles Rapid, ( List of Colorado River rapids), at river mile 75.

Thickness

The Shinumo Quartzite ranges in thickness from in the Bright Angel Canyon area to at Papago Creek and in the Shinumo Creek area. In the Bright Angel Canyon area, the upper member is about thick, the upper middle member is about thick, and the lower middle member is about thick.

Contacts

The contact of the Shinumo Quartzite with the Hakatai Shale is the only documented

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

that has been found within the Unkar Group. This unconformity truncates across beds and channel deposits of the Hakatai Shale. The relief at this unconformity is small, being about . The Shinumo Quartzite overlying this unconformity consists of arkosic conglomeratic sandstone that contains quartzite and granite pebbles.Timmons, JM, J Bloch,K Fletcher, KE Karlstrom, M Heizler, and LJ Crossey (2012) ''The Grand Canyon Unkar Group: Mesoproterozoic basin formation in the continental interior during supercontinent assembly.'' In JM Timmons and KE Karlstrom, eds., pp. 24–47, ''Grand Canyon Geology: Two Billion Years of Earth's History''. Special Paper no. 489, Geological Society of America, Boulder, Colorado. Within the Unkar Group, the upper contact of the Shinumo Quartzite with the Dox Formation appears to be gradational and is marked by a change in topographic expression and color. The basal of the Dox Formation directly overlying Shinumo Quartzite consists of predominantly dark green to black, fissile, slope-forming shale that contains thin sandstone beds. This shale makes a distinct notch between the resistant cliff-forming quartzites of the Shinumo Quartzite underlying them and resistant cliff-forming arkosic sandstones of the Dox Formation overlying them. The change in topographic expression, color, and the

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

change, from quartz arenite, to mudstone and fine-grained arkose – is gradational.Timmons, JM, KE Karlstrom, MT Heizler, SA Bowring, GE Gehrels, and LJ Crossey, (2005) ''Tectonic inferences from the ca. 1254–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis.'' Geological Society of America Bulletin. 117(11–12):1573–1595. Between the three-member

Tonto Group The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a '' Group'', that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the T ...

(above) and the Shinumo Quartzite, and the rest of the folded and faulted Unkar Group, is a prominent

angular unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

, which is part of the Great Unconformity. Though this surface is typically a plane, differential erosion (also areal erosion) of the tilted strata of the Unkar Group left resistant beds of the Cardenas Basalt and Shinumo Quartzite as ancient hills, called

monadnock An inselberg or monadnock ( ) is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa, a similar formation of granite is known as a koppie, an ...

s, that are up to tall. Thin drapes of Tapeats Sandstone of the

now cover most of these ancient monadnocks. However, a few of these monadnocks protrude further up into the Bright Angel Shale ( Isis Temple prominence example). These monadnocks served locally as sources of coarse-grained sediments that accumulated during the

marine transgression A marine transgression is a geologic event where sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling with water ...

to form the Tonto Group.Sharp, RP (1940) ''Ep-Archean and Ep-Algonkian erosion surfaces, Grand Canyon, Arizona.'' Geological Society of America Bulletin. 51(8):1235–1270.

Fossils

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s have been reported from the Shinumo Quartzite.

Depositional environments

The sedimentary strata comprising the Shinumo Quartzite are inferred to have accumulated in a variety of environments. The lower member is regarded to have been deposited by ancient river systems and later, in coastal

tidal flats Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...

. The lower-middle, and upper-middle members are argued to have accumulated in nearshore marine environments and coastal tidal and supratidal flats. The upper member is inferred to have accumulated in the delta plain, delta front, and mouth-bar of an ancient

delta Delta commonly refers to: * Delta (letter) (Δ or δ), the fourth letter of the Greek alphabet * D (NATO phonetic alphabet: "Delta"), the fourth letter in the Latin alphabet * River delta, at a river mouth * Delta Air Lines, a major US carrier ...

. The deformed strata of the upper member are interpreted as evidence of reoccurring earthshocks along regional fault and fold systems that were active during its deposition. The contact between the Shinumo Quartzite and Dox Formation indicates a shift from deposition in coastal deltas to fluvial deposition by a large river system.

Age

The age of the Shinumo Quartzite is approximately known. Based upon recent dates of

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...

layers, detrital

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

grains, and detrital

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

flakes, geologists agree that the Unkar Group was deposited between about 1254 and 1100 Ma. The Hakatai Shale, Shinumo Quartzite, and Dox Formation samples yield clusters of

as young as 1170 Ma. Thus, these strata must be younger than 1170 Ma. The Dox Formation, which overlies the Shinumo Quartzite is estimated to have accumulated between 1150 and 1140 and 1100 Ma. Thus, the Shinumo Quartzite must date to between 1170 and 1150 to 1140 Ma. This would suggest that the disconformity at the base of the Shinumo Quartzite is a minor one.

Description

Sedimentary structures

Thickness

Contacts

Fossils

Depositional environments

Age

See also

References

Further reading