Article

Grand Canyon’s Three Sets of Rocks

Photo of Colorado River and the cliffs and buttes of Grand Canyon. — A view of Grand Canyon showing all three sets of Grand Canyon rocks.
NPS photo.

Introduction

Beginning with John Wesley Powell, the geologist who led the pioneering river expedition through Grand Canyon in 1869, geologists have recognized three main packages of rocks exposed in Grand Canyon:

The Vishnu Basement Rocks: The igneous and metamorphic rocks of the three Granite Gorges.
The Grand Canyon Supergroup: Tilted, mostly sedimentary rocks.
The Layered Paleozoic Rocks: The flat-lying strata in the upper two-thirds of the canyon.

The informal term “set” is used to refer to Powell’s three main packages of rocks. This term is not part of the formal stratigraphic naming system of supergroups, groups, formations, and members, but is closest to the supergroup level and is convenient and very visual for Grand Canyon.
Within these three main groupings of rocks are:

Six packages at the group stratigraphic level;
35 individual formations;
At 53 least members.

A total of more than 180 stratigraphic units have been named in Grand Canyon Park, including Paleozoic units exposed in places in the western Grand Canyon, Mesozoic rocks, and Cenozoic deposits that weren’t included in the three main sets of rocks.

The 3 Sets of Rocks

Set	Rock Types	Environment	Age (Ma)
Layered Paleozoic Rocks	Horizontal sedimentary rock layers	Low coastal plain and shallow seas of the continental shelf along the proto-Pacific coast	Paleozoic Era 270–530
Grand Canyon Supergroup	Tilted sedimentary and igneous rock layers	Rivers and shallow seas far from the active plate margin as the supercontinent Rodinia assembled (Unkar Group) and rifted apart (Chuar Group)	Meso- and Neoproterozoic Eras (Precambrian) 729–1255
Vishnu Basement Rocks	Metamorphic and igneous rocks with vertical folds and foliation	Originally in volcanic island chains that collided with ancestral North America to form the southwest United States; rocks were metamorphosed and invaded by magmas in the deep crust.	Paleo- and Mesoproterozoic Eras (Precambrian )1375–1840

Table 1.
Ma = mega annum = million years ago

Diagram showing stratigraphic column of Grand Canyon rocks. — Figure 24. Stratigraphic column of rocks of the Grand Canyon region showing the three sets of rocks and major unconformities: Great Nonconformity (white line), Great
Angular Unconformity (red line) and Great Unconformity (black line). Fm = Formation; Ss = Sandstone; Ls = Limestone.

Vishnu Basement Rocks

The Vishnu Basement Rocks consist of the ancient metamorphic rocks that formed and were intruded by igneous rocks in the deep crust nearly 2,000 million (2 billion) years ago. They have undetermined thickness because the original sedimentary succession has been intensely folded so the originally horizontal layering is now subvertical.

Grand Canyon Supergroup

The Grand Canyon Supergroup is only exposed in the eastern Grand Canyon. They are late Precambrian sedimentary and volcanic rocks predominantly deposited in rift basins from about 729 to 1,255 million years ago. These strata are about 12,000 feet (3,600 m) thick.

Layered Paleozoic Rocks

The Layered Paleozoic include the flat-lying sedimentary rocks in the “stair-step” upper canyon walls throughout Grand Canyon. These strata are 3,000–5,000 feet (900–1,500 m) thick. The layers record many changing environments from shallow oceans to large sand dunefields, and the history of life on Earth about 530 to 270 million years ago.

Numeric Ages of Grand Canyon Rocks

Learn More

Tiny image of the cover of a report titled Telling Time at Grand Canyon National Park.

To learn more about the age of Grand Canyon’s rocks, please see:

Karlstrom, K., L. Crossey, A. Mathis, and C. Bowman. 2021. Telling time at Grand Canyon National Park: 2020 update. Natural Resource Report NPS/GRCA/NRR—2021/2246. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/nrr-2285173. [IRMA Portal]

References

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Glossary

Absolute age: a numeric age in years. Numeric age is the preferred term.
Accuracy: measure of how close a numeric date is to the rock’s real age.
Angular unconformity: a type of unconformity or a gap in the rock record where horizontal sedimentary layers (above) were deposited on tilted layers (below). At Grand Canyon, horizontal layers of the Layered Paleozoic Rocks lie on top of the tilted rocks of the Grand Canyon Supergroup.
Basalt: a dark, fine-grained volcanic (extrusive igneous) rock with low silica (SiO2) content.
Biochron: length of time represented by a fossil biozone.
Carbonate: sedimentary rock such as limestone or dolostone largely composed of minerals containing carbonate (CO3^-2) ions.
Contact: boundary between two bodies of rock or strata.
Daughter isotope: the product of decay of a radioactive parent isotope.
Detrital: pertaining to grains eroded from a rock that were transported and redeposited in another.
Dike: a wall-like (planar) igneous intrusion that cuts across pre-existing layering.
Diabase: a dark igneous rock similar in composition to basalt but with coarser (larger) grain size.
Disconformity: a type of unconformity or gap in the rock record between two sedimentary layers caused by erosion or nondeposition where the layers are parallel to one another.
Dolomite: the mineral calcium magnesium carbonate CaMg(CO3)2 that usually forms when magnesium-rich water alters calcium carbonate (CaCO3).
Dolostone: a rock predominantly made of dolomite.
Eon: longest subdivision of geologic time in the Geologic Timescale; for example, the Proterozoic Eon.
Era: second-longest subdivision of geologic time below eon in the Geologic Timescale; for example, the Paleozoic Era.
Epoch: fourth-longest subdivision of geologic time, shorter than a period and longer than a stage in the Geologic Timescale; for example, the Pleistocene Epoch.
Faunal succession: the change in fossil assemblages through time which has a specific, reliable order.
Foliation: tectonic layering in metamorphic rocks caused by parallel alignment of minerals due to compression.
Formation: the fundamental unit in stratigraphy and geologic mapping that consists of a set of strata with distinctive rock characteristics. Formations may consist of a single rock type (e.g., Tapeats Sandstone or Redwall Limestone), or a mixture of rock types (e.g. Hermit Formation, which includes sandstone, mudstone, and shale).
Fossil: evidence of life in a geologic context usually consisting of the remains or traces of ancient life.
Fossil biozone: stratigraphic unit defined by a distinctive assemblage of fossils.
Ga: giga annum: billion years; in this paper, our usage implies billion years before present (or ago) when used for numeric ages.
Gneiss: a high-grade metamorphic rock with strong foliation and light and dark bands of minerals.
Granite: a high silica (SiO2) pink to white intrusive igneous rock composed mainly of feldspar and quartz.
Granodiorite: a gray intrusive igneous rock composed of feldspar, quartz, biotite, and hornblende with less silica (SiO2) than granite.
Group: a sequence of two or more related formations, with a stratigraphic rank higher than formation; for example, the Chuar Group is made up of the Nankoweap, Galeros, and Kwagunt formations.
Igneous rock: a rock that solidified from molten material (magma or lava), either within the Earth (as an intrusive or plutonic rock) or after eruption onto the Earth’s surface (as an extrusive or volcanic rock).
Inclusion: a fragment of an older rock within a younger rock.
Index fossil: a fossil or assemblage of fossils that is diagnostic of a particular time in Earth history.
Intrusion: an igneous rock body that crystallized underground. Intrusions may have any size or shape; large ones are known as plutons, thin ones parallel to layering are known as sills, and thin ones that cut across layering are called dikes.
Isotope: one of the forms of a chemical element (with the same atomic number) that contains a different number of neutrons.
Lateral continuity: a geologic principle that sedimentary rocks extend laterally, and that if they are now separated due to erosion, they were once laterally continuous; for example, the Kaibab Formation on the South Rim is laterally continuous with the Kaibab Formation on the North Rim.
Lava: molten rock erupted onto the Earth’s surface.
Ma: mega annum: million years; in this paper, our usage implies million years before present (or ago) when used for numeric ages.
Magma: molten or partially molten rock material formed within the Earth.
Member: a subdivision of a formation, usually on the basis of a different rock type or fossil content; for example, the Hotauta Conglomerate is a member of the Bass Formation.
Metamorphic rock: a rock formed by recrystallization under intense heat and/or pressure, generally in the deep crust.
Monadnock: a bedrock island that sticks above the general erosion level.
Nonconformity: an unconformity or gap in the rock record where sedimentary layers directly overlie older and eroded igneous or metamorphic rocks.
Numeric age: age of a rock in years (sometimes called absolute age).
Numeric age determination: measurement of the age of a rock in years, often through the use of radiometricdating techniques.
Orogeny: mountain building event, usually in a collisional tectonic environment.
Parent isotope: the radioactive isotope that decays to a daughter isotope.
Pegmatite: a type of intrusive igneous rock usually of granitic composition with large crystal size.
Period: third-longest subdivision of geologic time shorter than an era and longer than an epoch in the Geologic Timescale; for example, the Permian Period.
Plate tectonics: theory that describes the Earth’s outer shell as being composed of rigid plates that move relative to each other causing earthquakes, volcanism, and mountain building at their boundaries.
Pluton: large intrusion of magma that solidified beneath the Earth’s surface.
Precambrian: the period of time before the Cambrian Period that includes the Proterozoic, Archean, and Hadean eons and represents approximately 88% of geologic time.
Precision: measure of the analytical uncertainty or reproducibility of an age determination.
Proterozoic: geologic eon dominated by single-celled life extending from 2,500 to 541 million years ago; divided into the Paleoproterozoic (1,600–2,500 Ma), Mesoproterozoic (1,000–1,600 Ma), and Neoproterozoic (541–1,000 Ma) eras.
Radioactive decay: the process by which the nuclei of an unstable (radioactive) isotope lose energy (or decay) by spontaneous changes in their composition which occurs at a known rate for each isotope (expressed as a half life); for example, the parent uranium (238U) isotope decays to the daughter lead (206Pb) isotope with a half life of 4.5 billion years.
Radiometric dating: age determination method that uses the decay rate of radioactive isotopes and compares the ratio of parent and daughter isotopes within a mineral or rock to calculate when the rock or mineral formed.
Regression: geologic process that occurs when the sea level drops relative to the land level; for example, by sea level fall and/or uplift of the land, causing the withdrawal of a seaway from a land area.
Relative time: the chronological ordering of a series of events.
Rift basin: a basin formed by stretching (extension) of the Earth’s crust. Rift basins are linear, fault-bounded basins that can become filled with sediments and/or volcanic rocks.
Rodinia: a Neoproterozoic supercontinent that was assembled about 1.0 Ga (during Unkar Group time) and rifted about 750 Ma (during Chuar Group time).
Sedimentary rock: a rock composed of sediments such as fragments of pre-existing rock (such as sand grains), fossils, and/or chemical precipitates such as calcium carbonate (CaCO3).
Schist: a metamorphic rock with platy minerals such as micas that have a strong layering known as foliation or schistosity.
Silica: silicon dioxide (SiO2), a common chemical “building block” of most major rock-forming minerals, either alone (i.e., as quartz) or in combination with other elements (in clays, feldspars, micas, etc.).
Sill: a sheet-like igneous intrusion that is parallel to pre-existing layering.
Snowball Earth: a hypothesis that the Earth’s surface became completely or mostly frozen between 717 and 635 million years ago.
Stage: a short subdivision of geologic time in the Geologic Timescale often corresponding to the duration of a fossil assemblage.
Stratigraphic age: the era, period, epoch, or stage a rock is assigned to based on its fossil biozones or numeric age.
Stratigraphy: the study of layered rocks (strata), which usually consist of sedimentary rock layers, but may also include lava flows and other layered deposits.
Stromatolite: a fossil form constructed of alternating layers (mats) of microbes (algal or bacterial) and finegrained sediment.
Subduction zone: a plate boundary where two plates converge and one sinks (subducts) beneath the other.
Supergroup: a sequence of related groups, with a higher stratigraphic rank than group; for example, the Grand Canyon Supergroup consists of the Unkar and Chuar groups.
Superposition: principle of geology that the oldest layer in a stratigraphic sequence is at the bottom, and the layers get progressively younger upwards.
Tectonics: large-scale processes of rock deformation that determine the structure of Earth’s crust and mantle.
Trace fossil: a sign or evidence of past life, commonly consisting of fossil trackways or burrows.
Transgression: a movement of the seaway across a land area, flooding that land area because of a relative sea level rise and/or land subsidence.
Travertine: calcium carbonate (CaCO3) precipitated by a spring; most travertine deposits also contain some silica.
Unconformity: a rock contact across which there is a time gap in the rock record formed by periods of erosion and/or nondeposition.
Volcanic ash: small particles of rock, minerals, and volcanic glass expelled from a volcano during explosive eruptions. Volcanic ash may be deposited great distances (even hundreds of miles or kilometers) from the volcano in especially large eruptions.
Yavapai orogeny: mountain building period that occurred approximately 1,700 million years ago when the Yavapai volcanic island arc collided with proto-North America.
Zircon: a silicate mineral (ZrSiO4) that often forms in granite and other igneous rocks and incorporates uranium atoms, making it useful for radiometric dating.

Grand Canyon’s Three Sets of Rocks

Introduction