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Chuar Group
Chuar Group
Stratigraphic range: Neoproterozoic,; 782 to 739 Ma Pha. Proterozoic Archean Had.
Type	Geological formation
Unit of	Grand Canyon Supergroup
Sub-units	Kwagunt Formation ; Galeros Formation
Underlies	Sixtymile Formation and, as part of the Great Unconformity, the Tapeats Sandstone
Overlies	Nankoweap Formation
Thickness	1,600 m (5,200 ft) at maximum
Lithology
Primary	mudstone
Other	dolomite and sandstone
Location
Region	(eastern) Grand Canyon-(southwest) Colorado Plateau ; Arizona-(north) ; Utah-(southern)
Country	United States
Type section
Named for	Chuar Canyon
Named by	Walcott (1894) and Noble (1910, 1914)

The Neoproterozoic Chuar Group consists of 1,600 m (5,200 ft) of exceptionally well-preserved, unmetamorphosed sedimentary strata that is composed of about 85% mudrock. The Group is the approximate upper half of the Grand Canyon Supergroup, overlain by the thin, in comparison, Sixtymile Formation, the top member of the multi-membered Grand Canyon Supergroup. The outcrop of the Chuar Group strata is limited to exposures along the western bank of the Colorado River in a 150 km² (58 sq mi) area of the eastern Grand Canyon, Arizona. The strata of the Chuar Group have been subdivided into the Galeros Formation (lower) and the Kwagunt Formation (upper) using the base of the prominent, thick sandstone unit.^[4]^[5]

Description

The Galeros Formation consists of a series of meter-scale sedimentary cycles composed of interbedded mudrocks, siltstones, and sandstones, frequently capped by dolomite beds. These strata have been subdivided, in ascending order, into the Tanner, Jupiter, Carbon Canyon, and Duppa members. Dolomite is dominate in all of these members. Thick (meter-scale) basal dolomite beds define the Tanner and Jupiter members. In case of the Jupiter Member, the basal dolomite bed is stromatolitic. The Carbon Canyon Member contains stromatolite bioherms.^[4]^[5]

The Kwagunt Formation is composed of sandstones, siltstones, shales, carbonates, cherts, and ironstones. These strata have been subdivided, in ascending order, into the Carbon Butte, Awatubi, and Walcott members. The Carbon Butte Member consists largely of sandstone interbedded with carbonates and, locally, ironstones. The Awatubi Member consists of a distinctive zone of stromatolitic bioherms at its base overlain by grey, green, and black organic-rich siltstones and shales, some of which contain marcasite nodules. The base of the Walcott Member consists of a distinctive meter-scale laminated dolomite bed (the Flakey dolomite). The Flakey dolomite is overlain by silicified oolites, chert beds, black shales. A pair of meter-scale dolomite beds (upper and lower dolomite couple) occur in the upper Walcott along with early diagenetic dolomite nodules up to 1 m (3.3 ft) in diameter.^[4]^[5]^[7]

The mudrocks of the Galeros and Kwagunt formations are normally interbedded with meter-thick sandstone and dolomite beds. The mudrocks are typically gray to black when freshly exposed and weather to reddish or greenish colors. The fresh gray to black colors of the mudrocks are due to a high organic content. Some samples of these mudrocks contain high total organic carbon percentages that are as much 9.39 weight percent organic carbon. The sandstone beds often exhibit symmetrical ripple marks. These ripple marks are commonly draped with a thin veneer of mudstone with mudcracks.^[4]^[5]

Fossils

The Chuar Group conatins a variety of Neoproterozoic fossils. The dolomite beds contain at least six different types of stromatolites and microbially induced sedimentary structures.^[4]^[5] The gray and black mudrocks of the Duppa, Carbon Canyon, Jupiter, and Tanner members of the Chuar Group often contain organic-walled microfossils, including filaments, colonial forms, and both smooth-walled and ornamented vesicles.^[7]^[8] A morphological group of organic microfossil, called vase-shaped microfossils, occur within the Walcott and Awatubi members of the Kwagunt Formation along with the enigmatic, circular, macroscopic, organic-walled fossil Tawuia (Churnia circularis), and phosphatic scale microfossils. The vase-shaped microfossils are likely presentatives of either arcellinid testate amoebae, acritarchs, or some of both.^[4]^[5] Finally, organic chemicals, biomarkers, characteristic of dinoflagellates have been reported.^[8]

Age

The age of the Chuar Group is well constrained between as being 729 and 782 Ma by Re-Os and U-Pb ages. First, U–Pb dating of detrital zircons from basal beds of the underlying Nankoweap Formation indicates it to be less than 782 Ma.^[9] Second, Re–Os ages from organic-rich carbonates in the upper Galeros Formation and marcasite nodules in the lower Kwagunt Formation yielded ages of 757.0 ± 6.8 and 751.0 ± 7.6 Ma, respectively.^[10] Finally, an U–Pb age obtained from CA-ID-TIMS analysis of zircons recovered from a tuff at the top of the Walcott Member is 729.0 ± 0.9 Ma.^[10] These ages also indicate that the vase-shaped microfossils-bearing Walcott Member is between 751 and 729 Ma.^[7]

Depositional Setting

The types of fossils found and sedimentary strata comprising the Chuar Group are indicative of its deposition within a low-energy marine embayment. During the deposition of the Chuar Group, this embayment was influenced by tidal and wave processes, infrequent large storms, microbial activity and carbonate precipitation, and the accumulation of mud and organic matter in quiet water. The sediments and fossils suggest that the Chuar Group accumulated in relatively shallow water (tens of meters or less), possibly, with times of intermittent exposure on a tidal flat.^[5]

References

^ ^a ^b 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.
^ Noble, LF (1910) Contributions to the geology of the Grand Canyon, Arizona; the geology of the Shinumo area (continued). American Journal of Science. Series 4, vol. 29, pp. 497–528.
^ Noble, LF (1914) The Shinumo quadrangle, Grand Canyon district, Arizona. Bulletin no. 549, US Geological Survey, Reston, Virginia.
^ ^a ^b ^c ^d ^e ^f 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.
^ ^a ^b ^c ^d ^e ^f ^g 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.
^ Karlstrom, K., Crossey, L., Mathis, A., and Bowman, C., 2021. Telling time at Grand Canyon National Park: 2020 update. Natural Resource Report NPS/GRCA/NRR—2021/2246. National Park Service, Fort Collins, Colorado. 36 pp.
^ ^a ^b ^c Tingle, K.E., Porter, S.M., Raven, M.R., Czaja, A.D., Webb, S.M. and Bloeser, B., 2023. Organic preservation of vase‐shaped microfossils from the late Tonian Chuar Group, Grand Canyon, Arizona, USA. Geobiology, 21(3), pp.290-309.
^ ^a ^b Lahr, D.J., Kosakyan, A., Lara, E., Mitchell, E.A., Morais, L., Porfirio-Sousa, A.L., Ribeiro, G.M., Tice, A.K., Pánek, T., Kang, S. and Brown, M.W., 2019. Phylogenomics and morphological reconstruction of Arcellinida testate amoebae highlight diversity of microbial eukaryotes in the Neoproterozoic. Current Biology, 29(6), pp.991-1001.
^ Dehler, C.M., Gehrels, G., Porter, S.M., Heizler, M., Karlstrom, K.E., Cox, G., Crossey, L.J., and Timmons, J.M., 2017. Synthesis of the 780–740 ma Chuar, Uinta Mountain, and Pahrump (ChUMP) groups, western USA: Implications for Laurentia-wide cratonic marine basins. Geological Society of America Bulletin, 129, pp.607– 624.
^ ^a ^b Rooney, A.D., Austermann, J., Smith, E F., Li, Y., Selby, D., Dehler, C.M., Schmitz, M.D., Karlstrom, K.E., and Macdonald, F.A. 2018. Coupled Re-Os and U-Pb geochronology of the Tonian Chuar Group, Grand Canyon. Geological Society of America Bulletin, 130, pp.1085–1098.

Popular Publications

Blakey, Ron and Wayne Ranney, Ancient Landscapes of the Colorado Plateau, Grand Canyon Association (publisher), 2008, 176 pages, ISBN 978-1934656037
Chronic, Halka. Roadside Geology of Arizona, Mountain Press Publishing Co., 1983, 23rd printing, pp. 229–232, ISBN 978-0-87842-147-3
Keller, B.; 2012; Overview of the Grand Canyon Supergroup; Grand Hikes; accessed .
Lucchitta, Ivo, Hiking Arizona's Geology, 2001, Mountaineers's Books, ISBN 0-89886-730-4