Layered Paleozoic Rocks

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.

Supai Group's rock layers as seen along Hermit Trail. The rocks of the Supai Group are red sandstones and siltstones, deposited 315-285 million years ago during the Paleozoic Era- Early Pennsylvanian Period.

NPS Photo by Kristen M. Caldon.

Introduction

Rock Type

Horizontal sedimentary rock layers

Environment

Low coastal plain and shallow seas of the continental shelf along the proto-Pacific coast

Age (Ma; mega annum = million years ago)

Paleozoic Era
270–530

The Layered Paleozoic Rocks consist of the classic sedimentary strata that make up the upper portion of Grand Canyon’s rock walls (Figure 15; Table 8); this is the stratigraphic sequence that most people think of when they consider the canyon’s geology.

Figure 15. The topmost (youngest) of the Layered Paleozoic Rocks seen from the Grand Canyon's South Rim.

Photo by Laurie Crossey.

Best numeric ages of the Layered Paleozoic Rock

Group	Formation	Stratigraphic Age	Numeric Age (Ma)	Age Range (Ma)
–	Kaibab Formation	Early Middle Permian	270	269–273
–	Toroweap Formation	Late Early Permian	275	273–278
–	Coconino Sandstone	Early Permian	280	276–282
–	Hermit Formation	Early Permian	285	284–290
Supai Group	Esplanade Sandstone	Early Permian	290	290–294
	Wescogame Formation	Late Pennsylvanian	300	299–303
	Manakacha Formation	Early Pennsylvanian	315	314–317
	Watahomigi Formation	Early Pennsylvanian	320	318–323
–	Surprise Canyon Formation	Late Mississippian	325	324–326
–	Redwall Limestone	Late Early – Middle Mississippian	340	335–338
–	Temple Butte Formation	Middle – Late Devonian	385	375–385
Tonto Group	Frenchman Mountain Dolostone	Late Middle Cambrian	500	497–503
	Muav Formation	Late Middle Cambrian	504	503–505
	Bright Angel Formation	Middle Cambrian	506	505–507
	Tapeats Sandstone	Middle Cambrian	508	507–509
	Sixtymile Formation	Early Middle Cambrian	510	509–530

Table 8.
Ma = mega annum = million years ago.

Tonto Group

Tapeats sandstone ledges as seen while looking down into Salt Creek from along the Tonto Trail in Grand Canyon National Park.

NPS Photo by Michael Quinn.

The Tonto Group consists of five formations in Grand Canyon

Sixtymile Formation

Sandy beaches and river bottoms, similar to modern Atlantic sandy coastlines

Tapeats Sandstone

Sandy beaches and river bottoms, similar to modern Atlantic sandy coastlines

Bright Angel Formation

Muddy sea floor, similar to modern Gulf of Mexico

Muav Formation

Tropical sea, similar to modern Bahama Banks

Frenchman Mountain Dolostone

Tropical sea, similar to modern Bahama Banks

Revisions to the Stratigraphy of the Tonto Group

Significant changes in the stratigraphy of the Tonto Group include:

• Addition of the Sixtymile Formation to the base of the Tonto Group.

• Formal redesignation of the Bright Angel Shale to the Bright Angel Formation because it contains a variety of sedimentary rock types.

• Formal redesignation of the Muav Limestone to the Muav Formation because it contains a variety of sedimentary rock types.

• Formal designation of the “undifferentiated dolomites” as the Frenchman Mountain Dolostone and assignment to the top of the Tonto Group.

These changes to the Tonto Group are the first major change to the stratigraphic nomenclature of the Paleozoic units at Grand Canyon since USGS Geologist and former Grand Canyon National Park Naturalist Edwin McKee defined the Supai Group and Geologists Geologist Billingsley and Stanley Beus identified the Surprise Canyon Formation.

The Cambrian versus the Proterozoic age and the style of deposition of the Sixtymile Formation resulted in its formal addition to the base of the Tonto Group. Furthermore, because the youngest detrital zircon grains in the upper Sixtymile Formation are less than 508 million years old, the rest of the overlying Tonto Group must be less than 508 million years old, younger than previously thought (Table 8).

Fossils from the Frenchman Mountain Dolostone are more than 497 million years old. The deposition and paleoenvironment of the Frenchman Mountain Dolostone were continuations of the marine environments of the underlying Muav Formation, so it was also added to the Tonto Group. In just 10 million years (remarkably fast), the marine transgression that deposited most of the Tonto Group advanced from Nevada across Arizona, into eastern Colorado, and beyond into the mid-continent (Figure 33).

Temple Butte Formation

Shallow sea in the west and tidal channels in the east

Redwall Limestone

Tropical sea, similar to modern Java Sea

The Redwall Limestone forms distinct red cliffs about mid-level in the canyon. It was deposited 340 million years ago.

NPS Photo by Kristen M. Caldon.

Surprise Canyon Formation

Limestone plateau, similar to modern Yucatan Peninsula

Supai Group

The Supai Group consists of four formations in Grand Canyon

Watahomigi Formation

Coastal lowlands and shallow sea, similar to modern US Gulf Coast

Manakacha Formation

Arid coast and dunefields, similar to modern Namibia

Wescogame Formation

Arid coast and dunefields, similar to modern Namibia

Esplanade Sandstone

Arid coast and dunefields, similar to modern Namibia

Hermit Formation

Rivers and swamps in arid environment, similar to modern Nile Delta

Coconino Sandstone

Desert sand dunefields along coast, similar to modern Arabian Desert

Coconino Sandstone as seen along the Bright Angel Trail.

NPS Photo by Kristen M. Caldon.

Toroweap Formation

Near the coast of a shallow sea, similar to modern Persian Gulf

Kaibab Formation

Shallow sea, similar to modern Persian Gulf

Numeric Ages for the Layered Paleozoic Rocks

Assigning numeric ages for the units of the Layered Paleozoic Rocks is difficult because there are no directly datable volcanic beds, and existing detrital zircon data do not have enough young grains to refine depositional ages. Nevertheless, their rock type, age, and overall geologic setting have been extensively studied and their ages are well constrained by index fossils.

The International Stratigraphic Chart was to better constrain the numeric ages for Paleozoic rocks in Grand Canyon NP. Units with richer fossil records have more precise age constraints, and global calibration of fossil biozones is becoming more precise in the v 2020/01 International Stratigraphic Chart. A related challenge for the Layered Paleozoic Rocks was identifying the best central age for the age of each unit.

Figure 34. A fallen boulder of Coconino Sandstone located adjacent to the Dripping Springs Trail shows trackways of a tetrapod, or mammal-like reptile, that walked on the sand dune and predated the dinosaurs. The tracks are enhanced by a false-color depth map (depth in mm).

(TOP: FRANCISCHINI ET AL. (2019); BOTTOM: SPENCER LUCAS).

Sedimentary deposition was nearly continuous between the Hermit Formation, Coconino Sandstone, Toroweap Formation, and Kaibab Formation during the Permian with the different formations being designated based on lithology resulting from distinct depositional environments. The only fossils known from the Coconino Sandstone are trace fossils (tracks of both invertebrates and vertebrates) (Figure 34), with some tetrapod traces indicating that they are from the Early Permian biochron (approximately 280 Ma). The Kaibab Formation forms the rim of Grand Canyon, and is the youngest Paleozoic rock in Grand Canyon. It is early Middle Permian based on microfossils and invertebrate fossils.

Future revisions to the age of units in the Layered Paleozoic Rocks may come as additional detrital zircon dates are derived across key faunal transitions within them.

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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• Billingsley, G. H., and S. S. Beus. 1985. The Surprise Canyon Formation, an Upper Mississippian and Lower Pennsylvanian (?) rock unit in the Grand Canyon, Arizona. U.S. Geological Survey, Washington, D.C. Bulletin 1605: A27–A33.

• Breed, W.J., and Ford, T.D., 1973, Chapter Two-and-a-half of Grand Canyon history, or the Sixty Mile Formation: Plateau, v. 46, no. 1, p. 12-18.

• Cohen, K. M., S. C. Finney, P. L. Gibbard, and J.-X. Fan. 2013; updated in 2020. The ICS International Chronostratigraphic Chart. Episodes 36: 199–204. Available at https://stratigraphy.org/icschart/ChronostratChart2020-01.pdf (accessed 01 Jun 2020).

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• Gehrels, G. E., R. Blakey, K. E. Karlstrom, J. M. Timmons, S. Kelley, B. Dickinson, and M. Pecha. 2011. Detrital zircon U-Pb geochronology of Paleozoic strata in the Grand Canyon, Arizona. Lithosphere 3: 183–200. doi: 10.1130/L121.1.

• Hawkins, D. P., S. A. Bowring, B. R. Ilg, K. E. Karlstrom, and M. L. Williams. 1996. U-Pb geochronologic constraints on Proterozoic crustal evolution. Geological Society of America Bulletin 108: 1167–1181.

• Hintze, L. F. and B. J. Kowallis. 2009. Geologic History of Utah. Brigham Young University Geology Studies Special Publication 9, Provo, Utah.

• Holland, M. E., K. E. Karlstrom, G. Gehrels, O. P. Shufeldt, G. Begg, W. L. Griffin, and E. Belousova. 2018. The Paleoproterozoic Vishnu basin in southwestern Laurentia: implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave province: Precambrian Research 308: 1–17.

• Ilg, B. R., K. E. Karlstrom, D. Hawkins, and M. L. Williams. 1996. Tectonic evolution of Paleoproterozoic rocks in Grand Canyon, Insights into middle crustal processes. Geological Society of America Bulletin 108: 1149–1166.

• Karlstrom, K. E., S. A. Bowring, C. M. Dehler, A. H. Knoll, S. Porter, Z. Sharp, D. Des Marais, A. Weil, J. W. Geissman, M. Elrick, M. J. Timmons, K. Keefe, and L. Crossey. 2000. Chuar Group of the Grand Canyon: Record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma. Geology 28: 619–622.

• Karlstrom, K. E., and L. J. Crossey. 2019. The Grand Canyon Trail of Time companion: Geology essentials for your canyon adventure. Trail of Time Press, Albuquerque, New Mexico.

• Karlstrom, K. E., J. Hagadorn, G. G. Gehrels, W. Mathews, M. Schmitz, L. Madronich, J. Mulder, M. Pecha, D. Geisler, and L. J. Crossey. 2018. U-Pb dating of Sixtymile and Tonto Group in Grand Canyon defines 505–500 Ma Sauk transgression. Nature Geoscience 11: 438–443.

• 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.

• Karlstrom, K. E., M. T. Mohr, M. Schmitz, F. A. Sundberg, S. Rowland, J. Hagadorn, J. R. Foster, L. J. Crossey, C. Dehler, and R. Blakey. 2020. Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian timescale. Geology 48: 425–430. doi: 10.1130/G46755.1.

• 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.

• Karlstrom, K. E., and J. M. Timmons. 2012. Many unconformities make one “Great Unconformity” in J. M. Timmons and K. E. Karlstrom, editors. Grand Canyon geology: 2 billion years of Earth history. Geological Society of America Special Paper 489, Boulder, Colorado.

• Lillie, Robert J. 2005. Parks and plates: The geology of our national parks, monuments, and seashores. W. W. Norton, New York, New York.Mathis, A. 2006. Grand Canyon yardstick of geologic time: A guide to the canyon’s geologic history and origin. Grand Canyon Association, Grand Canyon, Arizona, USA.

• Mathis, A. and C. Bowman. 2005a. What’s in a number?: Numeric ages for rocks exposed within Grand Canyon. Nature Notes XXI(1). Grand Canyon National Park, Arizona.

• Mathis, A. and C. Bowman. 2005b. What’s in a number?: Numeric ages for rocks exposed within Grand Canyon. Part 2. Nature Notes, v. XXI(2). Grand Canyon National Park, Arizona.

• Mathis, A. and C. Bowman 2006a. The grand age of rocks Part 1— Numeric ages for rocks exposed within Grand Canyon. Boatman’s Quarterly Review 19(1):27–29. Grand Canyon River Guides, Flagstaff, Arizona. Available at https://www.gcrg.org/bqr/pdfs/19-1.pdf (accessed 01 Jun 2020).

• Mathis, A. and C. Bowman 2006b. The grand age of rocks Part 2—Grand Canyon’s three sets of rocks. Boatman’s Quarterly Review: Grand Canyon River Guides. 19(2):19–23. Grand Canyon River Guides, Flagstaff, Arizona. Available at https://www.gcrg.org/bqr/pdfs/19-2.pdf (accessed 01 Jun 2020).

• Mathis, A. and C. Bowman 2006c. The grand age of rocks Part 3—Geologic dating techniques. Boatman’s Quarterly Review 19(4):19–23. Grand Canyon River Guides, Flagstaff, Arizona. Available at https://www.gcrg.org/bqr/pdfs/19-4.pdf (accessed 01 Jun 2020).

• Mathis, A. and Bowman, C. 2007. Telling Time at Grand Canyon National Park. Park Science, 24(2): 78–83. Available at https://irma.nps.gov/Datastore/DownloadFile/615879 (accessed 07 May 2020).

• McKee, Edwin D. 1931. Ancient landscapes of the Grand Canyon region. Northland Press, Flagstaff, Arizona.

• McKee, Edwin D. 1969. Stratified rocks of the Grand Canyon. In The Colorado River Region and John Wesley Powell. USGS Professional Paper 669-B. Washington DC. Available at https://pubs.usgs.gov/pp/0669/report.pdf (accessed 11 May 2020).

• McKee, E. D. 1975. The Supai Group — Subdivision and nomenclature. U.S. Geological Survey Bulletin 1395-J, Washington, D.C , Washington, D.C.McKee, E. D., and C. E. Resser. 1945. Cambrian history of the Grand Canyon region. Carnegie Institution of Washington Publication 563.

• Mohr, M.T., M.D. Schmitz, N.L. Swanson-Hysell, K.E. Karlstrom; F.A. Macdonald, M.E. Holland, Y. Zhang, N. Anderson. In review. High-Precision U-Pb geochronology links magmatism in the SW Laurentia Large Igneous Province and Midcontinent Rift.

• Mulder, J. A., K. E. Karlstrom, K. Fletcher, M. T. Heizler, J. M. Timmons, L. J. Crossey, G. E. Gehrels, and M. Pecha. 2017. The syn-orogenic sedimentary record of the Grenville Orogeny in southwest Laurentia. Precambrian Research 294: 33–52.

• National Park Service. 2017. Foundation Statement. Grand Canyon National Park. Grand Canyon, Arizona.

• Perry, D. L. 2012. What makes learning fun? Principles for the design of Intrinsically motivating museum exhibits. AltaMira Press, Lanham, Maryland.

• Powell, J. W. 1875. Exploration of the Colorado River of the West and its tributaries. Explored in 1869, 1870, 1871, and 1872, under the direction of the Secretary of the Smithsonian Institution. U.S. Government Printing Office, Washington, D.C., USA.

• Rooney, A. D., J. Austermann, E. F. Smith, Y. Li, D. Selby, C. M. Dehler, M. D. Schmitz, K. E. Karlstrom, and F. A. Macdonald. 2018. Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon. Geological Society of America Bulletin 130(7–8): 1085–1098.

• Santucci, V. L., and J. S. Tweet, editors. 2020. 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.

• Schuchert, Charles. 1918. The Cambrian of the Grand Canyon of Arizona. American Journal of Science (Series 4) 45: 368.

• Shufeldt, O. P., K. E. Karlstrom, G. E. Gehrels, and K. Howard. 2010. Archean detrital zircons in the Proterozoic Vishnu Schist of the Grand Canyon, Arizona: Implications for crustal architecture and Nuna reconstructions. Geology 38: 1099–1102.

• Spamer, Earle E. 1989. The development of geological studies in the Grand Canyon. Tryonia. Miscellaneous Publications of the Department of Malacology. The Academy of Sciences of Philadelphia No. 17. Philadelphia, Pennsylvania.

• Timmons, J. M., and K. E. Karlstrom, editors. 2012. Grand Canyon geology: 2 billion years of Earth history. Geological Society of America Special Paper 489. Boulder, Colorado.

• Timmons, J. M., K. E. Karlstrom, M. T. Heizler, S. A. Bowring, G. E. Gehrels, and L. J. Crossey. 2005. Tectonic inferences from the ca. 1255–1100 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.

• Whitmeyer, S., and K. E. Karlstrom. 2007. Tectonic model for the Proterozoic growth of North America.Geosphere 3(4): 220–259.

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