Grand Canyon Supergroup

Telling Time at Grand Canyon

This article is part of a series adapted from the Natural Resources Report "Telling Time at Grand Canyon: 2020 Update". All of the articles, including numeric ages for rocks exposed at Grand Canyon, are available on the [article series] index page.

Figure 14. Chuar Group shales in the Chuar Valley. The Grand Canyon Supergroup cannot all be seen in one view.

Photo by Laurie Crossey.

Introduction

Rock Type

Tilted sedimentary and igneous rock layers

Environment

Rivers and shallow seas far from the active plate margin as the supercontinent Rodinia assembled (Unkar Group) and rifted apart (Chuar Group)

Age (Ma; mega annum = million years ago)

Meso- and Neoproterozoic Eras (Precambrian)729–1255

The rocks of the Grand Canyon Supergroup are primarily sedimentary strata, are divided into the lower Unkar Group and the upper Chuar Group, and range in age from 1,255 Ma to 729 Ma (Figures 23 and 24; Table 5). These rocks formed in fault-bounded continental rift basins.A slight angular unconformity separates the Unkar Group from the overlying Chuar Group.

^{Figure 23. Rock and Time. Grand Canyon has one of the world’s most complete geologic records, yet more time is missing (black = time not recorded in the column on the right) than preserved. We assign approximate numeric ages to the time missing along the unconformities based on the age range of rocks directly above versus below these erosion surfaces. Note that the modern erosion surface on top of the Kaibab Formation has 270 million years missing. Diagram does not show all formations and unconformities in the Layered Paleozoic Rocks because of space considerations.}

^{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.}

Best numeric ages of the Grand Canyon Supergroup

Group	Formation	Stratigraphic Age	Numeric Age (Ma)	Precision (Ma)	Duration (Ma)
Chuar Group	Kwagunt Formation Walcott Member	Neoproterozoic	729	± 1	729–745
	Kwagunt Formation Awatubi Member	Neoproterozoic	750	± 8	745–751
	Kwagunt Formation Carbon Butte Member	Neoproterozoic	753	–	751–755
	Galeros Formation Duppa Member	Neoproterozoic	755	–	755–757
	Galeros Formation Carbon Canyon Member	Neoproterozoic	757	± 7	757–760
	Galeros Formation Jupiter Member	Neoproterozoic	765	–	760–765
	Galeros Formation Tanner Member	Neoproterozoic	770	–	765–770
	Nankoweap Formation	Neoproterozoic	775	–	770–775
Unkar Group	Cardenas Basalt	Mesoproterozoic	1,082	± 1.25	1,081–1,083
	Dox Formation	Mesoproterozoic	1,120	–	1,105–1,140
	Shinumo Sandstone	Mesoproterozoic	1,130	–	1,140–1,150
	Hakatai Shale	Mesoproterozoic	1,230	–	1,230–1,245
	Bass Formation	Mesoproterozoic	1,255	± 2	1,245–1,250
	Bass Formation Hotauta Conglomerate Member	Mesoproterozoic	1,255	–	1,250–1,255

Table 5.

Table Notes

• Ma = mega annum = million years ago
• Precision is indicated when radiometric age determinatons are available. The Bass Formation and Walcott Member of the Kwagunt Formation of the Chuar Group have U-Pb zircon dates on interbedded ash deposits; Cardenas has an U-Pb date (1082.18 ± 1.25 Ma) on the basalt; Awatubi and Carbon Canyon dates are Re-Os dates. Ar-Ar dating uses the K-Ar decay scheme, and is more accurate and precise than K-Ar dating since it provides ways to internally cross-check the results.
• Members of the Kwagunt and Galeros Formations are presented because modern geologic investigations have focused on these mappable members. The members are also displayed individually along the Trail of Time.
• Unconformities are present between the Chuar and Unkar Groups, and between the Shinumo Sandstone and Hakatai Shale

Unkar Group

The Unkar Group (Figures 29 and 30) consists of sediments that were shed off mountains in the region now part of west Texas that formed during the assembly of the Rodinia supercontinent. The Unkar Group was deposited between 1,255 Ma and 1,082 Ma.

^{Figure 29. Units of the Unkar Group as seen from the South Rim at Lipan Point. The members of the Dox Formation are also labeled.
Photo by Laurie Crossey.}

Figure 30. The Unkar Group exposed near Unkar Rapid.

NPS photo.

Age Range (Ma; mega annum = million years ago)

1,082–1,255

The age of the Bass Formation at the base of the Unkar Group is constrained by a volcanic ash bed that was dated to 1,255 ± 2 Ma. The ages of the overlying Hakatai Shale, Shinumo Sandstone, and Dox Formation are constrained by detrital zircon data (Tables 5 and 7). A 100-million-year unconformity separates the Hakatai Shale from the overlying Shinumo Sandstone.

Stacked basalt lava flows of the Cardenas Basalt that were erupted at 1,082 Ma are the youngest preserved units in the Unkar Group. The lava flows were fed by dikes and sills of diabase magma of similar age like the one shown in Figure 8C. These basalts formed at a time of incipient, but failed, rifting of ancient North America that caused the tilting of the Unkar Group.

Lower Unkar Group

Tectonic and Depositional Environment

Continental collisions taking place far to the south (in modern coordinates) formed the supercontinent Rodinia and squeezed southwestern North America. This compression caused NE-trending folds that formed basins flooded by shallow seas.

Age Range (Ma; mega annum = million years ago)

1,230–1,255

Formations and Age

Bass Formation Hotauta Conglomerate Member 1,255 Ma

Interlayered with the carbonate and sandstone of the rest of the Bass Formation so it is the same age

Bass Formation 1,255 Ma

Radiometric U-Pb zircon date is 1,255 ± 2 Ma from volcanic ash bed

Hakatai Shale 1,230 Ma

Youngest zircons are 1,255–1,230 Ma

Upper Unkar Group

Tectonic and Depositional Environment

NW-SE compression from continental collisions to the south (present coordinates) caused the continent to rift along NW-trending basins that filled with sediments from rivers and floodplains. Finally, these basins were intruded by molten rock forming dikes and sills that fed eruptions of the Cardenas Basalt lava flows.

Age Range (Ma; mega annum = million years ago)

1,082–1,150

Formations and Age

Shinumo Sandstone 1,130 Ma

Youngest zircons indicate <1,150, but is greater than 1,082 Ma Cardenas

Dox Formation 1,120 Ma

Youngest zircons indicate <1,130 but is greater than 1,082 Ma Cardenas

Cardenas Basalt 1,082 Ma

Radiometric U-Pb dating of dikes that fed the basalts gives 1082.18 ± 1.25 Ma

Chuar Group

Figure 31. Stromatolites were formed by single-celled cyanobacteria that formed
colonies in shallow oceans.

Photo by Laurie Crossey.

The Chuar Group is made up of mudstone with interbeds of sandstone and carbonate (Figure 14). It contains a rich diversity of single-celled organisms (Figure 31). Its fossils include the first heterotrophic predators (organisms that ate each other). Tectonically, the Chuar Group records the breakup and rifting of the Rodinia supercontinent and the formation of the proto-Pacific ocean. Chemically, the Chuar Group shows wild oscillations in ocean chemistry as the expanding biosphere was interacting with the atmosphere and hydrosphere during the interval leading to the Snowball Earth (717 to 635 million years ago, a time not recorded by Grand Canyon rocks).

The Chuar Group consists of three formation:

• Nankoweap Formation
• Galeros Formation
• Kwagunt Formation

Age Range (Ma; mega annum = million years ago)

729–755

Formations and Age

Nankoweap Formation

Age Range (Ma; mega annum = million years ago)
770–775
Tectonic and Depositional Environment
Sandstones that formed near the shore of an intracontinental seaway.

Galeros Formation

Age (Ma; mega annum = million years ago)
755–770
Tectonic and Depositional Environment
As the Chuar basin subsided, muds and limes were deposited in a seaway that intermittently dried out and reflooded.

Members and Age

Formation	Stratigraphic Age	Numeric Age (Ma)
Galeros Formation Duppa Member	Neoproterozoic	755
Galeros Formation Carbon Canyon Member	Neoproterozoic	757
Galeros Formation Jupiter Member	Neoproterozoic	765
Galeros Formation Tanner Member	Neoproterozoic	770

Numeric ages for members were assigned by:

• Tanner Member: Stratigraphic fit between Carbon Canyon Member and Nankoweap Formation

• Juniper Member: Stratigraphic fit between Carbon Canyon Member and Nankoweap Formation

• Carbon Canyon Member: Organic-rich carbonates give a date of 757 ± 7 Ma based on Re-Os radiometric date

• Duppa Member: Stratigraphic fit between 751–757

Kwagunt Formation

Age (Ma; mega annum = million years ago)
729–755
Tectonic and Depositional Environment
As the supercontinent Rodinia broke up, rift basins were filled with sediments. The Chuar sediments were deposited in shallow inland seaways that extended to Death Valley, northern Utah, and beyond.
Members and Age

Formation	Stratigraphic Age	Numeric Age (Ma)
Kwagunt Formation Walcott Member	Neoproterozoic	729
Kwagunt Formation Awatubi Member	Neoproterozoic	750
Kwagunt Formation Carbon Butte Member	Neoproterozoic	753

Numeric ages for members were assigned by:

• Carbon Butte Member: Stratigraphic fit between Duppa Member and Awatubi Member
• Awatubi Member: Marcasite nodules at the base of this member give a Re-Os date of 751 ± 8 Ma
• Walcott Member: Zircons give a U-Pb age of 729 ± 1 Ma (Rooney et al. 2018), a refinement of a previous 742 ± 6 Ma U-Pb zircon age

Revisions to Supergroup Stratigraphy and Age

Major revisions to the stratigraphy of the Grand Canyon Supergroup occurred in 2017 with the publication of new research on the age and stratigraphic relationships of units in the supergroup.

• The Nankoweap Formation was assigned to the Chuar Group.
• The Sixtymile Formation was moved from the Supergroup to the base of the Tonto Group in the Layered Paleozoic Rocks.

New zircon data has indicated that the Nankoweap Formation is younger than previously thought, with a maximum age of 775 Ma instead of the 900 Ma date previously assigned to it. As a result of the new date, as well as it having a similar depositional history to the units above it, the Nankoweap was incorporated into the Chuar Group.

The Sixtymile Formation was previously placed in the Grand Canyon Supergroup because of its flexure in the Chuar syncline. The previous age of 650 Ma based on the incorrect interpretation that it was related to sea level drawdown during glaciations of the Snowball Earth. The Sixtymile Formation was moved to Cambrian Tonto Group based on the age of the youngest detrital zircons that are as young as 530 Ma at the base of the section and 508 Ma at the top.

New age determinations have also refined the age of several formations within the Grand Canyon Supergroup (Table 7). Numeric ages for members in the Kwagunt and Galeros formations in the Chuar Group are given because geologists have studied each member extensively and treat them with the attention generally given to formations.

Direct ages have also been obtained by application of the rhenium-osmium radiogenic dating method on carbonates in the 757 Ma Carbon Canyon Member, and the 751 Ma Awatubi Member where nodules of iron-sulfide (marcasite) were dated (Table 5). Because the relative sequence of layers is so clear (Fig. 14), the durations of most Chuar members is estimated to be just 3–5 million years each (Table 5).

The formal names of several formations have also been changed to better reflect their composition.

• The Bass Limestone is now the Bass Formation,
• The Shinumo Quartzite was renamed the Shinumo Sandstone to clarify that the unit is not metamorphic, and
• The Dox Sandstone became the Dox Formation.

Learn More

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]

Meet the Authors

Authors

• Dr. Karl Karlstrom is a Distinguished Professor of Geology at the University of New Mexico with a specialty in tectonics; he has researched Grand Canyon rocks of all ages over the past 35 years.
• Dr.Laurie Crossey is a Professor of Geology and Geochemistry at the University of New Mexico who has worked on Grand Canyon rocks and water issues over the past 20 years.
• Allyson Mathis is a Research Associate with the Northern Rockies Conservation Cooperative, and a geologist by training. Allyson worked for the National Park Service at Grand Canyon National Park from 1999 to 2013.
• Carl Bowman is a retired air quality specialist, formerly of Grand Canyon National Park, where he worked in various positions between 1980 and 2013.

References Cited in Full Report

References

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• Karlstrom, K. E., S. S. Harlan, M. L. Williams, J. McLelland, J. W. Geissman, and K. I. Ahall. 1999. Refining Rodinia: Geologic evidence for the Australia–Western U.S. connection for the Proterozoic. GSA Today 9(10): 1–7.

• Karlstrom, K. E., B. R. Ilg, M. L. Williams, D. P. Hawkins, S. A. Bowring, and S. J. Seaman. 2003. Paleoproterozoic rocks of the Granite Gorges. Pages 9–38 in S. S. Beus and M. Morales, editors. Grand Canyon geology, second edition. Oxford University Press, Oxford, United Kingdom.

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• Karlstrom, K., S. Semken, L. Crossey, D. Perry, E. D. Gyllenhaal, J. Dodick, M. Williams, J. Hellmich-Bryan, R. Crow, N. Bueno Watts, and C. Ault. 2008. Informal geoscience education on a grand scale: The Trail of Time exhibition at Grand Canyon. Journal of Geoscience Education 56: 354–361.

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Glossary of Geologic Terms

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.

Photos and Illustrations

Related Links

• Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA npshistory.org]

• NPS Geology—Geologic Time Scale

• NPS—Geologic Resources Division