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Grand Canyon Supergroup

Grand Canyon Supergroup
Stratigraphic range: Meso- and Neo-Proterozoic, between 1,250 to
650–700 Ma [1]
TypeSupergroup
Sub-unitsChuar Group
Nankoweap Formation
Unkar Group[1]
UnderliesTonto Group, which is separated from it by the Great Unconformity
OverliesVishnu Basement Rocks
Thickness11,500 ft (3,500 m) to 13,100 ft (4,000 m)
Location
Country United States of America
(Southwestern United States)
Type section
Named forGrand Canyon
Named by(Walcott, 1883, 1894)[2][3]
A view looking north. Unkar Group units lying below the Isis Temple/Cheops Pyramid landforms:
Shinumo Quartzite cliffs (lower part shown on top of photo), above
Hakatai Shale (red-orange-purple slopes), above
Bass Formation(finely-layered, dark slopes), sitting on top of (base units).
Vishnu Basement Rocks (black stratum) within the Granite Gorge and side canyons.

The Grand Canyon Supergroup is a Mesoproterozoic to a Neoproterozoic sequence of sedimentary strata, partially exposed in the eastern Grand Canyon of Arizona. This group comprises the Unkar Group, Nankoweap Formation, Chuar Group and the Sixtymile Formation, which overlie Vishnu Basement Rocks. Several notable landmarks of the Grand Canyon, such as the Isis Temple and Cheops Pyramid, and the Apollo Temple, are surface manifestations of the Grand Canyon Supergroup.

Geology

The Grand Canyon Supergroup uncomformably overlies the Vishnu Basement Rocks but was never subjected to metamorphism. The units of the Supergroup were faulted and then tilted from 10 to 15 degrees.[4]

A section of the Unkar Group members is visible north of Grand Canyon Village, on the South Rim; specifically at the intersection of the Bright Angel Fault and the Grandview–Phantom Fault (which cuts through Phantom Creek), on the North Rim (at the north side of Granite Gorge).

The majority of the Grand Canyon Supergroup geologic members are found in the south and southwest stretch of the Colorado River starting from Marble Canyon, traverses the southeast perimeter of the Kaibab Plateau, and flows northeasterly entering Granite Gorge and regions of the Vishnu Basement Rocks. Here, the river leaves the last of the Unkar Group's lower-unit members behind. Other sections of the Unkar Group reappear downstream due to the general faulting of the Grand Canyon.

Geologic Sequence of the Super Group
Geologic stratigraphic column of strata exposed in and near the Grand Canyon showing stratigraphic units and major unconformities

The units of the Grand Canyon Supergroup in increasing age:[1]

Chuar Group

Unkar Group

Fossils

Stromatolite from the Grand Canyon Supergroup exhibit at Trail of Time

The Chuar Group of the Grand Canyon Supergroup has yielded a few Neoproterozoic fossils. Its dolomite beds contain at least six different types of stromatolites. In some of the gray and black mudrocks, paleontologists have found an abundance of organic microfossils, including vase-shaped microfossils (VSMs) likely presentative of arcellinid testate amoebae,[5] acritarchs, "Sphaerocongregus variabilis", and organic chemicals characteristic of dinoflagellates. Finally, the enigmatic circular fossils of Chuaria circularis are found at various levels within the Chuar Group.[6][7]

The Unkar Group of the Grand Canyon Supergroup also contians fossil stromatolites. stromatolites have been found in the dolomite beds of Bass Formation; in the transitional zone between the Hakatai Shale and Bass Formation; and from the he Comanche Point Member of the Dox Formation.[8]

Specific locales

Various creeks, canyons, or landforms give clues to unit locales of the Grand Canyon Supergroup members.

Isis Temple and "Cheops Pyramid"

Basalt dike in orange-red Hakatai Shale along Colorado River at Hance Rapid, river mile 76.5, Grand Canyon.
Isis Temple and Cheops Pyramid on a fault-block (horst), sitting on "island" of Shinumo Quartzite, of the Unkar Group. Bottom of the Unkar above Granite Gorge, and Vishnu Basement Rocks. (3.5 mi by 2.0 mi block)
At Apollo Temple landform, the Cambrian Tapeats Sandstone lies above the angular Great Unconformity with strata of the Chuar Group, Nankoweap Formation (the striped), Cardenas Basalt (the dark), and Dox Formation (red) below. (The lower strata are dipping to the right (east), as well as a total regional-dip to the northeast.)

The Isis Temple and Cheops Pyramid formation is a part of the Shinumo Quartzite 'island' and made up of the bottom three Unkar Group members. They were overlain at the Great Unconformity, onto Tapeats Sandstone (named for the bottom unit of the marine transgression series consisting of sandstone–(conglomerates)–shale–limestone laid down by the Tapeats Sea), and are evident along the Colorado River on the Tonto Platform. The three units of the Tonto Group are Muav Limestone, upon Bright Angle Shale, upon Tapeats Sandstone. The Tapeats is a thin cliff-former unit found along Granite Gorge and its side canyons forming a flat surface in locales not dramatically affected by faulting, up-warping (domes), or down-warping.

The Isis Temple with Cheops Pyramid formation contains the bottom three members, a virtual quartzite 'mountain island' in the Tapeats Sea. (Tapeats Sandstone was deposited only around Shinumo Quartzite monadnocks.) The formation is a fault-block section located north of Granite Gorge and the intersection of Phantom Creek with the Bright Angel Canyon (which is also the intersection of the Bright Angel and Grandview–Phantom faults). The dimensions of this block are approximately 3.5 mi by 2.0 mi,[9] with slopes of multi-colored Hakatai Shale and Bass Formation (shown as reddish, yellowish, and lavender).

Apollo Temple

The Apollo Temple formation occurs[10] at the southeast of the Kaibab Plateau (Walhalla Plateau). The base of the largely flat-topped Redwall Limestone monolith sits upon cliffs of Muav Limestone, then Bright Angel Shale, and Tapeats Sandstone. The Nankoweap Formation is near the monolith's center of the base. The northeast side of the base merges into the overlying unit above the Nankoweap, the Chuar Group slopes.

Northeast Grand Canyon

In the northeast Grand Canyon, the Tapeats is still the defining cliff unit at the Great Unconformity between the paleozoic Tapeats Sandstone and the basement geology below. However, the entire east of the Kaibab Plateau is an uplifted side of the East Kaibab Monocline.[11] The down-dropped east side of the Kaibab Plateau is defined by the south-flowing Colorado River.

Colorado sits below the East Rim, where the Little Colorado River enters from the east-southeast. Other sub-faults intersect it north and south of Little Colorado. Even further to the extreme northeast, at the Marble Canyon (Marble Platform), four parallel faults (2 sets of 2 – including the Eminence Fault), run parallel to the side of the Paria Plateau, striking northeasterly, resulting in the Colorado River's sharp turn to the southwest. The river then turns due-south – running down the East Kaibab Monocline.

References

  1. ^ a b c Connors, T.B., Tweet, J.S., and Santucci, V.L., 2020. Stratigraphy of Grand Canyon National Park. In: Santucci, V.L., Tweet, J.S., ed., pp. 54–74, 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, 603 pp.
  2. ^ Walcott, CD (1883) Pre-Carboniferous strata in the Grand Canyon of the Colorado, Arizona. American Journal of Science, 3d ser., vol. 26, pp. 437–42, 484.
  3. ^ Walcott, CD (1894) Precambrian igneous rocks of the Unkar terrane, Grand Canyon of the Colorado. 14th Annual Report for 1892/3, part 2, pp. 492–519, United States Geological Survey, Reston, Virginia.
  4. ^ Timmons, M, KE Karlstrom, and C Dehler (1998) Grand Canyon Supergroup Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly Archived 2013-07-13 at the Wayback Machine. Boatman's Quarterly Review. vol. 12, no. 1 Archived 2013-09-28 at the Wayback Machine, pp. 29–32. online: accessed April 2014
  5. ^ Lahr, Daniel J.G.; Kosakyan, Anush; Lara, Enrique; Mitchell, Edward A.D.; Morais, Luana; Porfirio-Sousa, Alfredo L.; Ribeiro, Giulia M.; Tice, Alexander K.; Pánek, Tomáš; Kang, Seungho; Brown, Matthew W. (2019-03-18). "Phylogenomics and Morphological Reconstruction of Arcellinida Testate Amoebae Highlight Diversity of Microbial Eukaryotes in the Neoproterozoic". Current Biology. 29 (6): 991–1001.e3. doi:10.1016/j.cub.2019.01.078. hdl:10261/240246. ISSN 0960-9822. PMID 30827918. S2CID 72333352.
  6. ^ Ford, TD, and CM Dehler (2003) "Grand Canyon Supergroup: Nankoweap Formation, Chuar Group, and Sixtymile Formation." in: Beus, S.S., Morales, M., eds., pp. 49–72, Grand Canyon Geology. Oxford University Press, New York.
  7. ^ Dehler, CM, SM Porter, and JM Timmons (2012) "The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon", in JM Timmons and KE Karlstrom, eds., pp. 49–72, Grand Canyon Geology: Two Billion Years of Earth's History. Special Paper no. 489, Geological Society of America, Boulder, Colorado.
  8. ^ Timmons, JM, KE Karlstrom, MT Heizler, SA Bowring, GE Gehrels, and LJ Crossey, (2005) Tectonic inferences from the ca. 1254–1100 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): 1573–95.
  9. ^ Arizona Road & Recreation Atlas, Benchmark Maps, pp. 12–13.
  10. ^ Note: The Apollo Temple formation is at mile 72 from Lee's Ferry
  11. ^ Chronic, Halka. Roadside Geology of Arizona, from Section: "Grand Canyon National Park", pp. 279–87. Graphic, Faults of eastern Grand Canyon show an NW-SE trend superimposed on a NE-SW trend inherited from Precambrian time. In Grand Canyon, these faults can be studied in three-dimensional detail. (about 35 faults shown), p. 286.
Quelle: http://en.wikipedia.org/wiki/Grand_Canyon_Supergroup