Mitchell Jennings

GEOLOGIC EVOLUTION OF THE GRAND CANYON

MITCHELL JENNINGS

ABSTRACTThe Grand Canyon is truly a natural wonder. Although many theories exist about how

the canyon formed, an exact timeline and series of events that explain how it was formed are still unknown. This paper will cover stratigraphy of the canyon and geomorphology but will be focusing on the uplift of the Colorado Plateau and the incision of the Colorado River into the Grand Canyon. The pace and timing of plateau uplift is often debated and still unknown, but the arrival of the modern Colorado River is marked by a large and localized extinction event that occurred in a southern basin, what is now modern-day Lake Mead.

The most popular and widely accepted theories of the evolution of the Grand Canyon are the Headward Erosion Theory and the Spillover Theory. The Headward Erosion Theory is the older of the two and involves multiple drainage areas, a north flowing ancestral Colorado River, and a pre-incised canyon that eroded headward and captured the Ancestral Colorado River to form the Grand Canyon. The Spillover Theory suggests that a large basin was created to the north, filled up due to a change in flow direction and spilt over the Colorado Plateau to form the Colorado River which incised into the Grand Canyon.

Key Words: Grand Canyon, Colorado River, Laramide orogeny, geologic uplift

INTRODUCTION

The Grand Canyon is located in Northern Arizona and is situated between two large manmade reservoirs, Lake Powell to the north and Lake Mead to the south. The dimensions of the canyon are; 360 km long, 30 km wide (at the South Rim), and 1,830 m deep. The Grand Canyon rock sequence has preserved at least eight sea transgressional events over a time span of 1,500 Ma to 200 Ma. The driving force behind the formation of the Grand Canyon is the Colorado River, but the canyon could not have reached its depth without plateau uplift. This uplift was caused by a mountain building event known as the Laramide orogeny. This orogeny was caused by the subduction of the Farallon plate under the North American plate, which occurred around 70 to 80 Ma. The shallow subduction angle of the plate played a role in the unique uplift of the area, now known as the Colorado Plateau.

Orogenic events usually result in the tilting of rock formations. The Colorado Plateau exhibits significant uplift but minimal tilting, approximately 1.5 degrees, and the mechanics of which are still largely debated and unknown. Dr. Karl Karlstrom suggests that during the Miocene, highlands created by the Laramide orogeny collapsed to form a basin and range. This

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inversion of topography left the Colorado Plateau higher and reversed flow directions within multiple watersheds/drainages in the area. These new flow directions created new topographic valleys and drainage channels that are responsible for the flow path of the modern Colorado River. “But the Colorado River did not become integrated across the Kaibab Plateau and through western Grand Canyon until after deposition of the Hualapai Limestone (ending 5.97 ± 0.07 Ma; Spencer et al., 2001).” (Karlstrom et al., 2008)

BACKGROUND

The Grand Canyon is located in the arid Southwest United States and is world-renowned for its horizontal bedding which preserves its geologic history. The formation of the canyon began around 2.0 Ga. The Vishnu Schist is the oldest in the Grand Canyon sequence and is a metamorphosed igneous rock that was deposited as a result of the North American plate moving over a hot-spot. This sediment was later metamorphosed by the Yavapai-Mazatzal orogeny around 1.7 Ga (Fig.1).

Fig. 1 Regional map of Proterozoic provinces of western Laurentia (Karlstrom et al., 2004).

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At approximately 1.4 Ga, a large magma body intruded into the basement rock which formed dikes and massive granite bodies within the schist rock. This granite intrusion occurred in two phases, labeled I-1 and I-2 in (Fig. 2).

Figure 2. Stratigraphic Column of Grand Canyon (TASA Graphic Arts, Inc., 2000)

During the middle to late Precambrian, a transgressional event deposited sediment that makes up the Grand Canyon Supergroup. Over the next several hundred million years, orogenic events lifted, tilted, and metamorphosed these sediments. Between the late Precambrian and early Paleozoic eras, erosional events removed large portions of the Grand Canyon Supergroup and left a relatively flat landscape. These erosional events created a Great Angular Unconformity, which is a sequence of tilted strata that contacts horizontal strata within a vertical sequence. (Fig. 2) This change in bedding dip tells geologists that information/stratigraphic history has been erased. Continuing through the Paleozoic and Mesozoic eras, rising and falling sea levels deposited around 4,600 meters of sediments that form the renowned flat sedimentary sequence of the Grand Canyon.

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DETAILED STRATIGRAPHY

FORMATION NAME AVERAGE THICKNESS AGE DESCRIPTIONChinle Formation 1,700 to 1,000 feet

thickLate Triassic claystone, sandstone,

limestone, siltstone, and conglomerate

Moenkopi Formation 2,000 feet Triassic Red, slope-forming, fine-grained, thin-bedded shaley siltstone and sandstone.

Kaibab Formation 500-800 feet Upper Permian Reddish-gray and brownish-gray, slope-forming gypsum, siltstone, sandstone, and limestone

Toroweap Formation 300 feet Permian Includes, in descending order, Woods Ranch, Brady Canyon, and Seligman Members

Coconino Sandstone 200 feet Lower Permian sandstone Tan to white, cliff-forming, fine-grained, wellsorted, cross-bedded quartz.

Hermit Shale 850 feet Lower Permian Red, slope-forming, fine-grained, thin-bedded siltstone and sandstone. Contains poorly preserved plant fossils in channel fills in lower part of formation

Supai Group 550 feet Lower Permian, Pennsylvanian, and Upper Mississippian

well-sorted calcareous sandstone (upper unit),  dark-red siltstone, and gray limestone (lower unit)

Surprise Canyon Formation

50 feet Upper Mississippian

Dark-reddish-brown siltstone and sandstone, gray limestone and dolomite

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FORMATION NAME AVERAGE THICKNESS AGE DESCRIPTION

Redwall Limestone 650 feet Upper and Lower Mississippian

Light-olive-gray, ledge- and cliff-forming, thin-bedded, fine-grained limestone (upper), Yellowish-gray and brownish, fine-grained dolomite (lower)

Temple Butte Limestone

100 feet Upper and Middle Devonian

Purple, reddish-purple, and lightgray, fine- to coarse-grained, thin- to medium-bedded, ripple-laminated ledges of mudstone, sandstone, dolomite, and conglomerate

Tonto Group 1,500 feet Middle and Lower Cambrian

limestone and dolomite lithologies belong to the Muav; shale and siltstone lithologies belong to the Bright Angel; and sandstone and conglomerate lithologies belong to the Tapeats

Grand Canyon Supergroup

2,200 feet Middle Proterozoic

Includes, in descending order, unnamed diabase sills and dikes, Cardenas Basalt, Dox Formation, Shinumo Quartzite, Hakatai Shale, and Bass Formation

Zoroaster Granite Unknown Precambrian Granite plutons, stocks, and pegmatite and aplite dikes emplaced synchronously with peak metamorphism

Vishnu Schist Unknown Precambrian Quartz-mica schist, pelitic schist, and meta-arenites of metamorphosed, arc-basin, submarine sedimentary rocks

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HEADWARD EROSION THEORY

The Headward Erosion Theory suggests that the modern day Colorado River achieved its present course by a combination of headward erosion and stream capture. In this model a pre-incised canyon deeper than 600 m, formed on the western Hualapai Plateau by headward erosion, continued along a strike-valley drainage, and captured ancestral Colorado River flow. (Young, 2008) At this time, thought to be late Miocene, the ancestral Colorado River is projected to flow southeast toward the Gulf of Mexico. Near modern day Little Colorado River, the ancestral river turned northward toward the Gulf of California. This northward flow of the ancestral river is key for the present Colorado River’s interception of ancestral river flow. (Fig.3)

Fig.3 Headward erosion of modern Colorado River (www.answeringenesis.org)

Headward erosion of this pre-incised canyon, modern Colorado River, captured ancestral river flow and began directing the majority of water flow down the pre-incised canyon. After the ancestral Colorado River was captured uplift of the plateau not only lifted the land around the river but also directed the water flow of multiple watersheds and drainage areas into the modern Colorado River. This massive volume increase of water flow combined with plateau uplift was key

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for the incision of the Colorado River into the Grand Canyon. (Fig. 4)

Fig. 4 Uplift around river channel (http://www.dkimages.com/discover/previews/774/206778.JPG)

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SPILLOVER THEORY

Spillover Theory suggests that Kaibab uplift altered the flow of the Ancestral Colorado River, forcing it to flow southeast toward the Gulf of Mexico. At some point around 12 Ma the ancestral river’s path to the Gulf of Mexico was blocked. This blockage caused the river to back up and create a large basin. (Fig. 5)

Fig. 5 Overflow diagram (www2.pvc.maricopia.edu)

This basin, thought to be Lake Bidahochi, continued to fill until it overflowed across the plateau. The stream created by the overflow, followed topographic low areas across the plateau where it combined with drainage flow and began to increase its volume and energy. Once the river reached what is now southern Nevada, it started eroding massive amounts of sediment while working its way down through the stratigraphic column and upstream towards the large reservoir, it achieved this massive headward erosion via a series of water falls. These high-energy waterfall areas began to erode and incise into the landscape very rapidly to create the modern path of the Colorado River and eventually the Grand Canyon. (Fig. 6)

Fig. 6 Diagram of Lake Bidahochi (www.kaibab.org)

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DISSCUSION

Although Heardward Erosion Theory and Spillover Theory are the most widely accepted, they are not without their faults. Headward Erosion Theory requires a pre-incised canyon and a north flowing Ancestral Colorado River. Problems with these two requirements lie within age dating of the pre-incised canyon and evidence of a north flowing ancestral river. Lucchitta suggests that the interception/stream capture point of the Ancestral Colorado River occurred between the Shivwits Plateau and the Kaibab uplift where the ancestral Colorado River possibly turned northward. (Lucchitta, 1989) Contrary to the idea of headward erosion leading to the capture of a north flowing ancestral river, Spencer suggests that there is evidence to support insufficient headward erosion. He claims that the distance of headward travel, 270 km, is too far for accelerated down cutting to be transmitted upstream and across at least two drainage divides all as a result of a few hundred meters in subsidence. (Spencer, 2001)

Problems with the Spillover Theory begin with lack of evidence that supports a basin large enough to create the Grand Canyon. Meek and Douglass denote Lake Bidahochi as being the basin that overflowed across the plateau. Dickinson argues against Meek and Douglass suggesting that Lake Bidahochi’s water level never reached an elevation high enough to spill over the plateau. He stated that “Bidahochi paleogeography indicates that Hopi Lake was a playa system that never achieved appreciable depth”. (Dickenson, 2013) Dickenson also suggests that topographic profiles in northern Arizona are not compatible with the Spillover Theory and could not have happened without post-basin deformation and/or pre-canyon-cutting that altered the landscape in a way that in inconsistent with geologic evidence.

CONCLUSION

Findings suggest that Miocene topographic inversion left the Colorado Plateau higher, reversed some drainages, and created significant fault scarps at the western edge of the Colorado Plateau. (Karlstrom et al., 2008) These drainages and faulted areas played a large role in transporting water to the Colorado River but more information is needed to establish a timeline as to when and how the Colorado River trough was formed. Unconformities within the canyon’s stratigraphy combined with massive erosion of the landscape make determining the geologic evolution of the Grand Canyon a near impossible task. Though it is largely debated on how the Grand Canyon was formed, geologists are comfortable with the fact that the Colorado River was the driving force behind the canyon’s incision. Weather the Grand Canyon was formed due to the Headward Erosion Theory, Spillover Theory, or possibly even a combination of the two, at this time definitive evidence has not been found.

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REFERENCESBillingsley, G.H., and Elston, D.P. (1989), Geologic log of the Colorado River from Lees Ferry to Temple

Bar, Lake Mead, Arizona, in Elston, D.P., Billingsley, G.H., and Young, R.A. (editors), Geology of Grand Canyon, northern Arizona: Washington, D.C., American Geophysical Union, (p. 1-47).

Billingsley, George H., (2000), Geologic Map of the Grand Canyon 30' by 60' Quadrangle, Coconino and Mohave Counties, Northwestern Arizona: U.S. Geological Survey Geologic Investigation Series I-2688

Dexter, L. R. (2009). Grand Canyon: the puzzle of the Colorado River. In Geomorphological Landscapes of the World (pp. 49-58). Springer Netherlands.

Dickinson W.R., 2013, Rejection of the lake spillover model for initial incision of the Grand Canyon, and discussion of alternatives: Geosphere, v. 9, p. 1–20, doi:10.1130/GES00839.1.

Gray, R.. (1964). Late Cenozoic Geology of Hindu Canyon, Arizona. Journal of the Arizona Academy of Science, 3(1), 39–42. http://doi.org/10.2307/40021927

Holland, M. E., Karlstrom, K. E., Doe, M. F., Gehrels, G. E., Pecha, M., Shufeldt, O. P., ... & Belousova, E. (2015). An imbricate midcrustal suture zone: The Mojave-Yavapai Province boundary in Grand Canyon, Arizona. Geological Society of America Bulletin, 127(9-10), 1391-1410.

Karlstrom, K. E., Crow, R., Crossey, L. J., Coblentz, D., and Van Wijk, J. W. (2008). Model for tectonically driven incision of the younger than 6 Ma Grand Canyon. Geology, 36(11), 835-838.

Lucchitta, I. (1989). History of the Grand Canyon and of the Colorado River in Arizona. Geologic evolution of Arizona: Arizona Geological Society Digest,17, 701-715.

McKee, E. D., and Resser, C. E. (1945). Cambrian history of the Grand Canyon region (Vol. 563). Carnegie Institution.

McKee, E. D. (1954). Stratigraphy and history of the Moenkopi Formation of Triassic age. Geological Society of America Memoirs, 61, 1-126.

McKee, E.H., 1975, The Supai Group; subdivision and nomenclature, IN Contributions to stratigraphy: U.S. Geological Survey Bulletin, 1395-J, (p. J1-J7).

Meek, N., and Douglass, J. (2001). Lake overflow: An alternative hypothesis for Grand Canyon incision and development of the Colorado River. Colorado River: Origin and evolution: Grand Canyon, Arizona, Grand Canyon Association, 199-204.

Sorauf, J.E. and Billingsley, G.H., 1991, Members of the Toroweap and Kaibab Formations, Lower Permian, northern Arizona and southwestern Utah: The Mountain Geologist, v. 28, no. 1, (p. 9-24).

Spencer, J. E., and Pearthree, P. A. (2001). Headward erosion versus closed-basin spillover as alternative causes of Neogene capture of the ancestral Colorado River by the Gulf of California. The Colorado River: Origin and Evolution: Grand Canyon, Arizona, Grand Canyon Association Monograph, 12, 215-219.

Stewart, J. H., Poole, F. G., Wilson, R. F., Cadigan, R. A., Thordarson, W., and Albee, H. F. (1972). Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau region (No. 690). Geological Survey (US).

Young, R. A. (2008). Pre–Colorado River drainage in western Grand Canyon: Potential influence on Miocene stratigraphy in Grand Wash Trough.Geological Society of America Special Papers, 439, 319-333.

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