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The Shinumo Quartzite also known as the Shinumo Sandstone, is a Mesoproterozoic rock formation, which outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Shinumo Quartzite consists of a series of massive, cliff-forming sandstones and sedimentary quartzites. Its cliffs contrast sharply with the stair-stepped topography of the underlying Hakatai Shale. Overlying it, dark green to black, fissile, slope-forming shales of the Dox Formation create a well-defined notch. It and other formations of the Unkar Group occur as isolated fault-bound remnants along the main stem of the Colorado River and its tributaries in Grand Canyon. Typically, the Shinumo Quartzite and associated strata of the Unkar Group dip northeast (10°-30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim).〔Hendricks, JD, and GM Stevenson (2003) ''Grand Canyon Supergroup: Unkar Group.'' In SS Beus and M Morales, eds., pp. 39-52, Grand Canyon Geology, 2nd ed. Oxford University Press, New York.〕 The Shinumo Quartzite is a middle member of the Unkar Group. The Unkar Group is about thick and composed of, in ascending order, the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. The Unkar Group consists of a sequence of sedimentary rocks that accumulated in a variety of environments ranging from fluvial to shallow-marine. Overall, the strata within it are conformable with the exception of a significant disconformity between the Hakatai Shale and Shinumo Quartzite. The Unkar Group is overlain in ascending order by the Nankoweap Formation, about thick; the Chuar Group, about thick; and the Sixtymile Formation, about thick. The Unkar Group, as the base section of the Grand Canyon Supergroup, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.〔〔Elton, DP, and EH McKee (1982) ''Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona.'' Geological Society of America Bulletin. 93(8):681-699.〕〔Karlstrom, KE, BR Ilg, Bradley, D Hawkins, ML Williams, G Dumond, KK. Mahan, and SA Bowring, Samuel (2012) ''Vishnu Basement Rocks of the Upper Granite Gorge: Continent formation 1.84 to 1.66 billion years ago.'' In JM Timmons and KE Karlstrom, eds., pp. 7-24, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado.〕 == Description == In sharp contrast to argillaceous strata above and below it, the Shinumo Quartzite consists predominately of beds of red, brown or purple sedimentary quartzites that are firmly, to extremely well cemented by silica, and characterized by poorly developed bedding. It also contains beds of massive white, red or purple sandstone and conglomeratic sandstone. One conglomeratic sandstone layer that lies about above the base of the Shinumo Quartzite near the South Kaibab trail contains jasper pebbles.〔〔Daneker, TM (1974) ''Sedimentology of the Precambrian Shinumo Sandstome, Grand Canyon, Arizona.'' Unpublished Master’s thesis, Northern Arizona University : Flagstaff, Arizona. 390 pp.〕〔Elston, DP (1989) ''Middle and late Proterozoic Grand Canyon Supergroup, Arizona.'' In DP Elston, GH Billingsley, and RA Young, RA., eds., pp. 94-105, Geology of the Grand Canyon, Northern Arizona (with Colorado River Guides). American Geophysical Union Fieldtrip Guidebook T115/315 for International Geologic Congress, 28th. American Geophysical Union, Washington DC. 239 pp.〕 Typically, the Shinumo Quartzite has been subdivided into four poorly defined, unnamed members. First, the basal lower member consisting of purplish arkosic conglomeratic sandstone. Unlike the rest of the Shinumo Quartzite, it contains quartzite and granite gravels up to in diameter. As in the case of the Hotauta Conglomerate, the quartzite gravel of the lower member lacks any known equivalents in the Grand Canyon region. Second, the lower middle member is composed of purple cross-bedded quartzite, sometimes is subdivided into two units, and overlies the lower member. The purple quartzites of the lower middle member exhibit crisp, well-defined reduction spots. Third, the upper middle member is composed of "rusty red" quartzite and overlies the lower middle member. The "rusty-red" quartzite of the upper middle member grades downward into the lower middle member through an interval of interbedded purple and red-brown quartzite. Their colors and cementation appear to be the result of ground or connate water alteration after their deposition. Finally, the upper member of the Shinumo Quartzite is composed of reddish brown and locally purple sandstone and an overlying well-cemented gray quartzite The Shinumo Quartzite is characterized as a quartzite, but the gray quartzite within the upper member of the Shinumo Quartzite contains mudstone-rich intervals. In addition, the gray quartzite was subjected to cementation by silica and bleaching that removed its original reddish brown and purple hematite pigments, after its deposition.〔〔〔 Basaltic sills and dikes intrude all formations of the Unkar Group below the Cardenas Lava. The Hakatai Shale, Shinumo Quartzite, and Dox Formation are intruded by these dikes. They can be traced, discontinuously, to within a few meters of the base of the Cardenas Lava.〔〔〔 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「Shinumo Quartzite」の詳細全文を読む スポンサード リンク
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