The

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...

Chuar Group consists of of

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

iferous, 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 Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone that is exposed in only four places in the Chuar Valley. These exposures occur atop Nankoweap Butte and within Awatubi and S ...

, the top member of the multi-membered Grand Canyon Supergroup. The mudrock is interbedded with meter-thick

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

and

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

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 the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organi ...

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 (current or waves) or wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''unidi ...

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, ...

s. These strata 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. The Chuar Group is quite fossiliferous. The dolomite beds are associated with at least six different types of either

stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). T ...

s or microbially influenced carbonate precipitation. The gray and black mudrocks often contain an abundance of

microfossil A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

s, including vase-shaped microfossils (VSMs) likely presentative of 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 that partially encloses the cell, with an aperture from which the ...

acritarch Acritarchs are organic microfossils, known from approximately 1800 million years ago 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 refle ...

s, ''"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. 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.

Geologic sequence of Grand Canyon Supergroup

The units of the Grand Canyon Supergroup:Chronic, H (1983) Roadside Geology of Arizona. The Mountaineers Books, Seattle, Washington. (softcover, ) (1250 Ma to 700 Ma) * 4 – Sixtymile Formation (ends at ~700 Ma) * 3 – Chuar Group ** Kwagunt Formation ** Galeros Formation * 2 – Nankoweap Formation * 1 –

Unkar Group The Unkar Group is a sequence of strata of Proterozoic age that are subdivided into five geologic formations and exposed within the Grand Canyon, Arizona, Southwestern United States. The 5-unit Unkar Group is the basal member of the 8-member Gr ...

** 5 – Cardenas Basalt, youngest, Meso- Proterozoic 1070 ± 70 million years ** 4 –

Dox Formation The Dox Formation, also known as the Dox Sandstone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The strata of the Dox Formation, except for some more resistant sandstone beds, are re ...

** 3 – Shinumo Quartzite ** (2B) – Diabase sills (Unkar Group)-(4 sills, 25 ft thickness max; sources of Cardenas Basalt) ** 2 – Hakatai Shale ** 1 – Bass Formation, (begins at ~1250 Ma)

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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*

*

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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

Geologic sequence of Grand Canyon Supergroup

See also

References

Popular Publications

External links