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The Neoproterozoic Chuar Group consists of of fossiliferous, 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 mudrock is 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. 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 stromatolites or microbially influenced carbonate precipitation. The gray and black mudrocks often contain an abundance of microfossils, including vase-shaped microfossils (VSMs), 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.〔〔 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, ISBN 978-0-87842-147-3)〕 (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 * * 5--Cardenas Basalt, youngest, Meso-Proterozoic 1070 ± 70 million years * * 4--Dox Formation * * 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) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chuar Group」の詳細全文を読む スポンサード リンク
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