The

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

Chuar Group consists of of exceptionally well-preserved, unmetamorphosed sedimentary

strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...

that is composed of about 85%

mudrock Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

. 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 An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

of the Chuar Group strata is limited to exposures along the western bank of the Colorado River in a 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.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.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.

Description

2021 Revised NPS Geologic Stratigraphic Column of the Grand Canyon

The Galeros Formation consists of a series of meter-scale sedimentary cycles composed of interbedded

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

s, and

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s, frequently capped by dolomite beds. These

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 Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...

bioherms. The Kwagunt Formation is composed of sandstones, siltstones,

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s, carbonates,

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

s, and

ironstone Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be c ...

s. 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 in diameter.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. The mudrocks of the Galeros and Kwagunt formations are normally interbedded with meter-thick

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 Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of wa ...

percentages that are as much 9.39 weight percent organic carbon. The sandstone beds often exhibit symmetrical

ripple mark In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (Ocean current, current or wind wave, waves) or directly by wind. Defining ripple cross-laminae and asymmetric ripples ...

s. These ripple marks are commonly draped with a thin veneer of mudstone with

mudcrack Mudcracks (also known as mud cracks, desiccation cracks or cracked mud) are sedimentary structures formed as muddy sediment dries and contracts.Jackson, J.A., 1997, ''Glossary of Geology'' (4th ed.), American Geological Institute, Alexandria, VA ...

Fossils

The Chuar Group contains a variety of Neoproterozoic fossils. The dolomite beds contain at least six different types of

s and microbially induced sedimentary structures. 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.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. 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 Testate amoebae (formerly thecamoebians, Testacea or Thecamoeba) are a polyphyletic group of unicellular amoeboid protists, which differ from naked amoebae in the presence of a test (biology), test that partially encloses the cell, with an apert ...

acritarch Acritarchs () are organic microfossils, known from the Precambrian to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecolog ...

s, or some of both. Finally, organic chemicals,

biomarker In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, ...

s, characteristic of

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

s have been reported.

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.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. 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.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. Finally, an U–Pb age obtained from CA-ID-TIMS analysis of

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

s recovered from a tuff at the top of the Walcott Member is 729.0 ± 0.9 Ma. These ages also indicate that the vase-shaped microfossils-bearing Walcott Member is between 751 and 729 Ma.

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.

References

Popular Publications

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

External links

U.S. Geological Survey, Reston, Virginia. *

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* Mathis, A., and C. Bowman (2007
''The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon''
National Park Service, Grand Canyon National Park, Arizona. * Share, J. (2102a

* Share, J. (2102a) [http://written-in-stone-seen-through-my-lens.blogspot.com/2012/02/great-unconformity-of-grand-canyon-part.html ''The Great Unconformity and the Late Proterozoic-Cambrian Time Interval: Part II – The Rifting of Rodinia and the "Snowball Earth" Glaciations That Followed.''] * Timmons, M. K. Karlstrom, and C. Dehler (1999
''Grand Canyon Supergroup Six Unconformities Make One Great Unconformity A Record of Supercontinent Assembly and Disassembly''Boatman's Quarterly Review. vol. 12, no. 1
pp. 29–32. * Timmons, S. S. (2003

National Park Service, Grand Canyon National Park, Arizona. {{Geology of the Grand Canyon area Grand Canyon Geologic groups of Arizona Neoproterozoic North America Mudstone groups Dolomite groups Sandstone groups Geologic formations of Utah