The Great Unconformity at Grand Canyon

USA

Two major unconformities of the Great Unconformity: 500 million years are “missing” below the Unkar Group (green arrow) and up to 1.3 billion years are “missing” where Cambrian strata overlie Vishnu Basement rocks. (Photo: Michael Quinn, NPS).

Geological Period

Mesoproterozoic, Neoproterozoic and Cambrian

Main geological interest

Stratigraphy and sedimentology

Location

Grand Canyon National Park in Arizona, USA.
36°05’13.0″N, 112°07’07.0″W

One of the most profound unconformities on Earth, with up to 1.3 billion years of Earth’s history removed by erosion.

The Great Unconformity has multiple erosion surfaces, two shown in the photo above: a nonconformity where 1.25 Ga rocks of the Unkar Group rest on the 1.75 Ga igneous and metamorpohic rocks of the Vishnu Basement (green arrow) with 500 million years of Earth’s history “missing” (not recorded); and an angular unconformity where flat-lying 500 Ma Cambrian strata overlie tilted 1.25 to 0.73 Ga strata (red arrow, see Figure at lower right on opposite page). Important concepts for public science education: unconformities are the missing rock record that frames the rock record to provide an understanding of geologic time (Karlstrom and Crossey, 2019). The site is part of the Grand Canyon National Park UNESCO World Heritage Site.

Geologist’s hand spans 1.3 billion years of missing rock record across the erosional contact called the Great Unconformity: 1.84 Ga Vishnu Basement rocks below and 0.5 Ga Cambrian strata above. (Photo: Laura Crossey).

The vertical mile of rock revealed in Grand Canyon looks like a spectacularly complete rock record (and it is), but more time is missing than is preserved. Grand Canyon is the type location for a profound unconformity, found on many continents, that has Paleozoic sedimentary strata overlying the unroofed igneous and metaphoric core of ancient mountain belts (a nonconformity). Across this contact at Grand Canyon, up to 1.3 billion years of Earth History has been removed by erosion in several stages. As noted by Grand Canyon’s preeminent stratigrapher and former park naturalist Eddie McKee (1969), this unconformity is a composite erosion surface. McKee summarized: “These unconformities were discussed by Powell (1875), who pointed out that each represents a sequence of events of tremendous importance in Earth history, including the formation of mountains by tectonic forces, the erosion of these mountains to a condition of base level, and finally, the burial of the erosion surface by sediments of advancing seas”.

Recognition of great unconformities by John Wesley Powell led Clarence Dutton (1882) to name the Great Unconformity and identify Grand Canyon as its type area. This geologic feature remains of research importance as new geochronological and thermochronological tools are developed to quantify timing, magnitude, and causes of deep basement erosion (Karlstrom et al., 2018).

Grand Canyon has one of the world’s most complete geologic records, yet more time is missing (black) than preserved in the rock column at right. Red letters are approximate numeric ages for the time “missing” (not recorded) along each unconformity.

Dutton, C.E. (1882) Tertiary history of the Grand Canyon District, with atlas. Volume II: 263. Department of the Interior, p. Atlas, 23 Sheets. Available at: https://doi.org/10.3133/m2.

Karlstrom, K. et al. (2018) ‘Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons’, Nature Geoscience, 11(6), pp. 438–443. Available at: https://doi.org/10.1038/s41561-018-0131-7.

Karlstrom, K. and Crossey, L. (2019) The Grand Canyon Trail of Time Companion: Geology essentials for your canyon adventure. Albuquerque, NM: Trail of Time Press.

Karlstrom, K.E. and Timmons, J.M. (2013) ‘Many unconformities make one “Great Unconformity”’, in Grand Canyon Geology: 2 Billion Years of Earth History. Timmons, J.M., and Karlstrom, K.E. Geological Society of America Special Paper, pp. 73–80. Available at: https://pubs.geoscienceworld.org/gsa/books/book/647/chapter/3806403/Many-unconformities-make-one-Great-Unconformity (Accessed: 22 July 2022).

McKee, E.D. (1969) Stratified rocks of the Grand Canyon. USGS Numbered Series 669-B. Washington, D.C.: U.S. Government Printing Office, p. 2358. Available at: http://pubs.er.usgs.gov/publication/pp669B.

Powell, J.W. (1875) Exploration of the Colorado River of the West and its tributaries. USGS Unnumbered Series. Washington, D.C.: Government Printing Office, p. 368. Available at: http://pubs.er.usgs.gov/publication/70039238

Karl E. Karlstrom
Distinguished Professor, Department of Earth and Planetary Sciences, University of New Mexico, USA

Laura J. Crossey
Distinguished Professor, Department of Earth and Planetary Sciences, University of New Mexico, USA

The Great Unconformity at Grand Canyon

USA

Geological Period

Main geological interest

Location

One of the most profound unconformities on Earth, with up to 1.3 billion years of Earth’s history removed by erosion.

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