The Vishnu Basement Rocks is the name recommended for all Early

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

crystalline rock A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosco ...

s (

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

and

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

) exposed in the

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 ...

region. They form the crystalline

basement rocks In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...

that underlie the Bass Limestone of the

Unkar Group The Unkar Group is a sequence of strata of Proterozoic age that are subdivided into five geologic formations and exposed within the Grand Canyon, Arizona, Southwestern United States. The Unkar Group is the basal formation of the Grand Canyon S ...

of the

Grand Canyon Supergroup The Grand Canyon Supergroup is a Mesoproterozoic to a Neoproterozoic sequence of sedimentary strata, partially exposed in the eastern Grand Canyon of Arizona. This group comprises the Unkar Group, Nankoweap Formation, Chuar Group and the Six ...

and the

Tapeats Sandstone Except where underlain by the Sixtymile Formation, the Tapeats Sandstone is the Cambrian geologic formation that is the basal geologic unit of the Tonto Group. Typically, it is also the basal geologic formation of the Phanerozoic strata exposed ...

of the

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 ...

. These basement rocks have also been called the Vishnu Complex and Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the ''Granite Gorge Metamorphic Suite''; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic

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 ...

, granitic

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 ...

aplite Aplite () is an intrusive igneous rock that has a granitic composition. Aplites are fine-grained to aphanitic Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not vi ...

, and

granodiorite Granodiorite ( ) is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

that have intruded these metamorphic rocks, and also intrusive Early Paleoproterozoic

ultramafic rocks Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous rock, igneous and metamorphic rock, meta-igneous rocks with a very low silica content (less than 45%), generally >18% magnesium oxid ...

.Anonymous (nd
''Vishnu Basement Rocks.''
U. S. Geological Survey, Reston, Virginia.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. The term ''Zoroaster Plutonic Complex '' is used for all Paleoproterozoic granitic and grandioritic

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

ic rocks in the Grand Canyon. Specific names have been assigned to individual

s and dike swarms because the plutons and swarms differ greatly in their age, origin, and tectonic significance. The oldest of these plutonic complexes, the Elves Chasm Gneiss, likely represents a small fragment of ''basement'' upon which the meta

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

rocks that comprise the Granite Gorge Metamorphic Suite accumulated. The remainder of the Early Paleoproterozoic granites, granitic pegmatites, aplites, and granodiorites are parts of either younger plutons or dike swarms, that have intruded the Granite Gorge Metamorphic Suite, either contemporaneously with or after their metamorphosis.Karlstrom, KE, BR Ilg, ML Williams, DP Hawkins, SA Bowring, and SJ Seaman (2003) ''Paleoproterozoic rocks of the Granite Gorges.'' In SS Beus and M Morales, eds., pp. 9–38, Grand Canyon Geology, 2nd ed. Oxford University Press, New York.Ilg, BR, KE Karlstrom, and ML Williams (1996) ''Tectonic evolution of Paleoproterozoic rocks in the Grand Canyon – Insights into middle-crustal processes.'' Geological Society of America Bulletin. 108(9):1149–66. It was named after a natural rock structure in the Colorado River valley which was named " Temple of Vishnu" from its appearance. From p. 148: "The finest butte of the chasm is situated near the upper end of the Kaibab division; but it is not visible from Point Sublime. It is more than 5,000 feet high, and has a surprising resemblance to an Oriental pagoda. We named it Vishnu's temple."

Granite Gorge Metamorphic Suite

The Granite Gorge Metamorphic Suite consists of lithologic units, the Brahma, Rama, and Vishnu

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 have been mapped within the Upper, Middle, and Lower Granite Gorges of the Grand Canyon. The Vishnu Schist consists of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

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 ...

schist,

pelitic A pelite () or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, i.e. mud or a mudstone, t ...

schist, and meta-

arenite Arenite (from the Latin ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00245 in) and 2 mm (0.08 in) and containing less than 15% matrix. The related adjective is ''arenaceous''. The e ...

s. They exhibit relict

sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different parti ...

and textures that demonstrate that they are metamorphosed

submarine A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or infor ...

sedimentary rocks 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. Sedim ...

. The Brahma Schist consists of

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

hornblende Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more al ...

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

schist, biotite-plagioclase schist, ortho

amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

-bearing schist and

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 ...

, and metamorphosed sulfide deposits. As inferred from relict structures and textures, the Brahma Schist is composed of

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

-composition metavolcanic rocks. The Rama Schist consists of massive, fine-grained quartzofeldspathic schist and gneiss that are probable felsic metavolcanic rocks. On the basis of the presence of relict pillow structures, interlayering of metavolcanic strata, and the large volumes of metavolcanic rocks, the Brahma and Rama schists are interpreted to consist of metamorphosed, volcanic island-arc and associated submarine volcanic rocks. These metavolcanic rocks are locally overlain by the metamorphosed submarine sedimentary rocks of the Vishnu Schist that are interpreted to have accumulated in

oceanic trench Oceanic trenches are prominent, long, narrow topography, topographic depression (geology), depressions of the seabed, ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers ...

es. These meta

were originally composed of particles of quartz,

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

, and volcanic rock fragments that have become metamorphosed into various schists. The Vishnu Schist exhibits relict

graded bedding In geology, a graded bed is a bed characterized by a systematic change in grain or clast size from bottom to top of the bed. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progre ...

and structures indicative of

turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...

deposits that accumulated in oceanic trenches and other relatively deep-marine settings. The Brahma Schist has been dated to about 1.75 billion years ago. The felsic metavolcanic rocks that comprise the Rama Schist have yielded an age of 1.742 billion years ago.

Early Paleoproterozoic basement

2021 Revised NPS Geologic Stratigraphic Column of the Grand Canyon

The oldest rocks that are part of the Vishnu Basement Rocks is the Elves Chasm pluton. It consists of metamorphosed mafic (hornblende-biotite

tonalite Tonalite is an igneous rock, igneous, plutonic (Intrusive rock, intrusive) rock (geology), rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali fe ...

) and intermediate-composition plutonic rocks (quartz

diorite Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is Intermediate composition, inter ...

). Within it, there are tabular amphibolite bodies that might be dikes, that have been dated at about 1.84 billion years ago. It is regarded to be an older granodioritic pluton that was exposed by erosion prior to being buried by the original volcanic and submarine sedimentary rocks of the Granite Gorge Metamorphic Suite. The Elves Chasm pluton is likely part of the basement rocks on which the original volcanic rocks and sediments of the Granite Gorge Metamorphic Suite were deposited.Hawkins, DP, SA Bowring, BR Ilg, KE Karlstrom, and ML Williams (1996) ''U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona.'' Geological Society of America Bulletin. 108(9):1167–81. The highly tectonized contact between Elves Chasm pluton and the Granite Gorge Metamorphic Suite is exposed near Waltenberg Canyon, in 115-Mile Canyon, near Blacktail Canyon, and in the Middle Granite Gorge. This contact is characterized by a high-grade orthoamphibole-bearing gneiss. This gneiss is interpreted to be a highly metamorphosed and sheared

paleosol In Earth science, geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science. In geo ...

and associated

regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...

that originally consisted of several meters of

weathered ''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...

rock debris

eroded Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. Erosion is disti ...

from older plutonic rocks.

Younger intrusive igneous rocks

On the basis of rock type, type of intrusion, chemistry, and age of rocks, two main groups of younger Early Paleoproterozoic igneous intrusive (plutonic) rocks have been distinguished within the Vishnu Basement Rocks. One group, which dates between 1.74 and 1.71 billion years ago, consists of large plutons such as the Zoroaster pluton, the Ruby pluton, and the Diamond Creek pluton. There is no noticeable baking and metamorphism of the

country rock Country rock is a music genre that fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal sty ...

adjacent to them. Because of this, they were likely shallowly emplaced beneath the

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc ...

in which the metavolcanics and metasediments of the Granite Gorge Metamorphic Suite accumulated. In addition, these intrusive rocks have undergone all the deformation that has also affected their adjacent country rock. This further indicates that they are just slightly younger than the metavolcanic and metasedimentary rocks they intrude. This and their calc-alkaline granitic composition, which is similar to plutons forming in modern

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

-zone-related volcanic arcs, indicates that they are remnants of early volcanic arc systems associated with Early Paleoproterozoic subduction zones. Comparable volcanic arc systems, which are associated with subduction zones, are active today in the

Aleutian Islands The Aleutian Islands ( ; ; , "land of the Aleuts"; possibly from the Chukchi language, Chukchi ''aliat'', or "island")—also called the Aleut Islands, Aleutic Islands, or, before Alaska Purchase, 1867, the Catherine Archipelago—are a chain ...

and

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

(e.g.,

Sumatra Sumatra () is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the list of islands by area, sixth-largest island in the world at 482,286.55 km2 (182,812 mi. ...

and

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

). The second group of younger Early Paleoproterozoic igneous intrusive rocks is quite different in style, age, and significance. These igneous intrusive rocks consist of granitic and pegmatitic dike swarms, i.e. the Cottonwood, Cremation, Sapphire, and Garnet pegmatite complexes, that cut the Granite Gorge Metamorphic Suite from east to west. They formed as granite

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

, and related pegmatite fluids, filled crack-systems as magma migrated through the crust. The chemical composition of the granite and pegmatite comprising these dike swarms is indicative of the partial melting of the metasedimentary and metavolcanic rocks of the Granite Gorge Metamorphic Suite both in-place and at greater depth in the crust. These dikes exhibit a wide variability in the degree that they have been deformed from straight and nearly undeformed – to varying degrees of folding, stretching, and shearing. The variable degree of the deformation of these structures is interpreted to indicate that these dike swarms were emplaced during a period of significant mountain building and crustal thickening that was possibly associated with continental collision.

Ultramafic rocks

Also present within the Vishnu Basement Rocks are thin, discontinuous, and unnamed lenses of

ultramafic rock Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

s. They are found in several places within the Inner Gorge, such as at River Miles 81, 83, and 91, Salt Creek, Granite Park, and Diamond Creek. These ultramafic rocks occur typically as tectonic fault-bounded slivers, which are often associated with tectonic shear zones and exhibit coarse-grained relict cumulate textures. These rocks are interpreted to be the tectonically dismembered parts of the bases of large 1.74-to-1.71-billion-years-old plutons that have intruded the Granite Gorge Metamorphic Suite. This interpretation is based upon the abundance of

phlogopite Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica. Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3 ...

and geochemistry of light

rare-earth element The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set o ...

s that imply a geochemical contribution from subducting slab material. The composition of these ultramafic rocks is consistent with their origin by simple fractional crystallization within a pluton.Seaman, SJ, KE Karlstrom, ML Williams, and AJ Petruski (1997) ''Proterozoic Ultramafic Bodies in the Grand Canyon.'' Geological Society of America, Abstracts with Programs. 29(6):A-89.

Upper contact

The upper contact of the Vishnu Basement Rocks is a major unconformity between it and either the Tonto Group or Unkar Group that resulted from uplift and the deep erosion, by at least , of the Vishnu Basement Rocks and any overlying strata. In the case of the unconformity between the Vishnu Basement Rocks and the Unkar Group, studies of the underlying Vishnu Basement Rocks indicate they were uplifted from a depth of about to a depth of about , between 1.75 and 1.66 billion years ago, and from a depth of about to the weathered surface on which the Bass Formation of the Unkar Group accumulated, between 1.66 and 1.25 billion years ago.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):1573–95.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. 25–47, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado.

References

External links

* Anonymous (nd
''Vishnu Basement Rocks.''
U. S. Geological Survey, Reston, Virginia. * Babcock, R.S., E.H. Brown, M.D. Clark and D.E. Livingston (1979). ''Geology of the Older Precambrian Rocks of the Grand Canyon:'' Part I. Petrology and Structure of the Vishnu Complex. Part II. The Zoroaster Plutonic Complex and Related Rocks. Part III. Petrology of Mafic Schists and Amphibolites. Precambrian Research, 8, pp. 219–302. * Babcock, R.S. (1990). ''The Precambrian Crystalline Core'' ''(of the Grand Canyon)''. In S.S. Beus and M. Morales, eds., ''Grand Canyon Geology'' (1st ed.), pp. 11–28. Oxford University Press/Museum of Northern Arizona. * Billingsley, G.H., W.J. Breed, M.D. Clark, P.W. Huntoon and others (1976). ''Geologic Map of the Grand Canyon National Park, Arizona''.

Museum of Northern Arizona The Museum of Northern Arizona is a museum in Flagstaff, Arizona, United States, established as a repository for Indigenous material and natural history specimens from the Colorado Plateau. The museum was founded in 1928 by zoologist Dr. Harol ...

and Grand Canyon Natural History Association. {{ISBN? * Billingsley, GH (2000
''Geologic Map of the Grand Canyon 30' by 60' Quadrangle, Coconino and Mohave Counties, Northwestern Arizona.''
Geologic Investigation Series no. I-2688, U.S. Geological Survey, Reston, Virginia. * Mathis, A, and C Bowman (2007
''The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon''Grand Canyon National Park, Arizona
National Park Service, Grand Canyon National Park, Arizona. * Noble, FL (1914

ttp://ngmdb.usgs.gov/ngmdb/ngmdb_home.html National Geologic Map Database U.S. Geological Survey, Reston, Virginia. * Share, J (2102a
''The Great Unconformity of the Grand Canyon and the Late Proterozoic-Cambrian Time Interval: Part I – Defining It.''
last accessed September 22, 2013. * Share, J (2102a

last accessed September 22, 2013. * Timmons, JM, MK Karlstrom, and C Dehler (1999) ttp://www.gcrg.org/bqr/12-1/supergroup.html ''Grand Canyon Supergroup Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly''br>Boatman's Quarterly Review. vol. 12, no. 1
pp. 29–32. * Timmons, S.S (2003

National Park Service, Grand Canyon National Park, Arizona. Natural history of the Grand Canyon Geologic formations of Arizona Geologic formations of Nevada Precambrian Arizona Paleoproterozoic geology Metamorphic complexes