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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, USA. It is about thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. It accumulated approximately between 1250 and 1104 Ma (1,104 million years ago, 1.1 billion). In ascending order, the Unkar Group is overlain by the Nankoweap Formation, about thick; the Chuar Group, about thick; and the Sixtymile Formation, about thick. The Nankoweap and Sixtymile formations together with the Chuar and Unkar groups comprise the Grand Canyon Supergroup.〔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.〕〔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.〕〔Timmons, JM, J. Bloch, K. Fletcher, KE Karlstrom, M Heizler, and LJ Crossey (2012) ''The Grand Canyon Unkar Group: Mesoproterozoic basin formation in the continental interior during supercontinent assembly.'' In JM Timmons and KE Karlstrom, eds., pp. 25-47, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado.〕 In general, the strata comprising 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). Within the central Grand Canyon, these strata occur in small, rotated, downfaulted blocks or slivers where they commonly are only partially exposed. Within this part of the Grand Canyon, the Unkar Group is incomplete because pre-Tonto Group erosion has removed strata above the level of the middle part of the Dox Formation. The missing part of the Unkar Group and the remainder of the overlying Grand Canyon Supergroup are preserved in a prominent syncline and fault block that is exposed in the eastern Grand Canyon. Examples of these fault blocks can be seen at the Isis Temple prominence, "Cheops Pyramid," and the intersection of Phantom Creek with the Bright Angel Canyon, (North Kaibab Trail). The Unkar Group contains thick basaltic sills and a number of small, dark dikes. In the area of Desert View and west of Palisades of the Desert, the basaltic sills form very prominent, dark gray cliffs.〔〔〔Lucchitta, I (2001) ''Hiking Arizona's Geology.'' Mountaineers's Books. (softcover, ISBN 0-89886-730-4)〕 Using gravity and aeromagnetic data, combined with gravity modeling, it was inferred that Proterozoic grabens, and half-grabens filled with strata of the Unkar Group – lie buried beneath Phanerozoic rocks in northern Arizona that surround the Grand Canyon. The grabens and half grabens filled with strata of the Unkar group are associated with northwest–southeast trending Mesoproterozoic fault systems that have curving, southwest-dipping traces. These fault systems were later reactivated during the Neoproterozoic, to form basins in which the following Chuar Group accumulated, and during the Cenozoic, to form geologic structures, i.e., faults, anticlines, synclines, and monoclines, that are exposed at the surface.〔Seeley, JM, and KG Randy (2003) '' Delineation of subsurface Proterozoic Unkar and Chuar Group sedimentary basins in northern Arizona using gravity and magnetics., implications for hydrocarbon source potential.'' American Association of Petroleum Geologists Bulletin. 87(8):1299-1321.〕 Major unconformities separate the Unkar Group from the strata overlying and underlying it. First, the Unkar Group, as the bottom unit of the Grand Canyon Supergroup, lies directly upon deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks. Second, an angular unconformity, with a dip of less than 10°, separates the base of the Nankoweap Formation from the underlying Unkar Group. Finally, a well-defined angular unconformity at the base of the relatively flat-lying Tonto Group separates it from the underlying faulted and folded strata of the Unkar Group and the rest of the Grand Canyon Supergroup that are typically tilted at angles of 10°-30°.〔〔 ==Nomenclature== The Unkar Group was first recognized and named by Charles D. Walcott as the "Unkar terrane" in 1894.〔 It and his “Chuar terrane” comprised what was then named the “Grand Canyon series” of "Proterozoic (Algonkian)" age. He regarded the Cardenas Basalt, unnamed at that time, as the uppermost unit in his "Unkar terrane." In 1910 and 1914, Levi F. Noble later divided what he called the "Unkar Group" into five subunits, which were the Hotauta Conglomerate, Bass Limestone, Hakatai Shale, Shinumo Quartzite, and Dox Sandstone.〔〔 The still unnamed Cardenas Basalt is only briefly noted as exposures of it are absent in the Shinumo 15-minute quadrangle. Although a recognized part of the Unkar Group, the basalt lava flows that overlie the Dox Formation were generally ignored and simply described as "basalt and diabase." In 1938, Charles R. Keyes applied the name "Cardenesan Series" to the basaltic volcanic rocks within the Unkar Group.〔Keyes, C (1938) ''Basement complex of the Grand Canyon: Pan American Geologist.'' 20:91-116.〕 In 1973, the current definition of the Unkar Group developed when the Nankoweap Formation, which had been earlier added to the Unkar Group was formally removed from it and the unconformity that separates the Nankoweap Formation from the Unkar Group was recognized.〔''The Development of Geological Studies in the Grand Canyon.'' Tyrona, Miscellaneous Publications no. 17. Department of Malacology, The Academy of, Natural Sciences of Philadelphia, Philadelphia, Pennsylvania.〕 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「Unkar Group」の詳細全文を読む スポンサード リンク
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