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The Neoproterozoic Nankoweap Formation (pronounced Nan' coe weep), is a thin sequence of distinctive red beds that consist of reddish brown and tan sandstones and subordinate siltstones and mudrocks that unconformably overlie basaltic lava flows of the Cardenas Basalt of the Unkar Group and underlie the sedimentary strata of the Galeros Formation of the Chuar Group. The Nankoweap Formation is slightly more than 100 m in thickness. It is informally subdivided into informal lower and upper members that are separated and enclosed by unconformities. Its lower (ferruginous) member is 0 to 15 m thick. The Grand Canyon Supergroup, of which the Nankoweap Formation is part, unconformably overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.〔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.〕〔Ford, TD, and CM Dehler (2003) ''Grand Canyon Supergroup; Nankoweap Formation, Chuar Group, and Sixtymile Formation. Grand Canyon geology.'' In SS Beus and M Morales, eds., pp. 53-75, Grand Canyon Geology, 2nd ed. Oxford University Press, New York. ISBN 0195122984〕〔Timmons, JM, KE Karlstrom, MT Heizler, SA Bowring, GE Gehrels, and LJ Crossey, (2005) ''Tectonic inferences from the ca. 1254-1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis.'' Geological Society of America Bulletin. 117(11-12):1573-1595.〕 The strata of the Nankoweap Formation are exposed in a small area that occupies an area from just south of Carbon Canyon to Basalt Canyon on the west bank of the Colorado River to around Comanche Creek and Tanner Canyon on the east bank of the Colorado River within the eastern Grand Canyon, Coconino County, Arizona. It and associated strata of the Unkar and Chuar groups are preserved in a prominent syncline and fault block. The most complete, readily accessible, and easily studied, exposure of the Nankoweap Formation occurs in Basalt Canyon.〔〔 The nomenclature of the Nankoweap Formation has changed over time. Originally, the strata of the Nankoweap Formation was included in-part in the top of the "Unkar terrane" (Group) and in-part in the basal “Chuar terrane” (Group) by Walcott in 1894.〔Walcott, CD (1894) ''Precambrian igneous rocks of the Unkar terrane, Grand Canyon of the Colorado.'' 14th Annual Report for 1892/3, part 2, pp. 492-519, United States Geological Survey, Reston, Virginia.〕 Van Gundy first recognized the thin sequence of red beds unconformably overlying basaltic flows of the Unkar Group as a separate stratigraphic unit, which he called the “Nankoweap Group.“〔 Although more extensive outcrops exist in Basalt Canyon, Comanche Creek, and Tanner Canyon, it is named for a small, fault-bounded block of the Nankoweap Formation in Nankoweap Canyon. It was argued that these redbeds belonged neither to the underlying Unkar Group nor overlying Chuar Group.〔 Maxson in his geological mapping reduced the Nankoweap Group to formational rank because the Nankoweap had not been subdivided into formations.〔 Later, a locally preserved lower member (ferruginous unit and weathered zone) that unconformably overlies the Cardenas Basalt and is unconformably overlain by the upper member of the Nankoweap – was recognized.〔〔 ==Description== The Nankoweap Formation is slightly more than 100 m in thickness. Currently, it is informally divided into two informal members, the lower (ferruginous) member and the upper member. These members are separated and enclosed by unconformities.〔〔〔〔 The lower (ferruginous) member of the Nankoweap Formation consists of thin, 15 m or less thick, erosional remnants that overlie deeply eroded and often deeply weathered basaltic lava flows of the Cardenas Basalt. Within the Grand Canyon, the lower member of the Nankoweap Formation outcrops at only two locations adjacent to the trace of the north-south-trending Butte Fault. First, it outcrops just north of the Colorado River in the eastern side of the Basalt graben at Tanner Canyon Rapids. At Tanner Canyon Rapids, the lower member consists of red, highly resistant, hematite-cemented, quartzitic sandstones and siltstones that have a maximum thickness of about 15 m and disappears within about 200 m west of the Butte Fault between the upper member of the Nankoweap Formation and the eroded surface of the Cardenas Basalt. Finally, it outcrops 2 km due south of the Colorado River near and at the southern limit of preservation of the Cardenas Basalt and Nankoweap Formation. At this location, it consists of a thin layer of highly ferruginous sandstone that is composed of sediments derived from the underlying Cardenas Basalt and a 10 m thick ferruginous weathered zone (paleosol) developed in the eroded surface of the Cardenas Basalt.〔〔〔〔Elston, DP, and GR Scott (1976) ''Unconformity at the Cardenas-Nankoweap contact (Precambrian), Grand Canyon Supergroup, northern Arizona.'' Geological Society of America Bulletin. v. 87(12):1763-1772〕 Upper member of the Nankoweap Formation consists of about 100 m of generally reddish-purple, fine-grained, quartzitic sandstones. They are shaley and silty towards the top. The sandstones sometimes contain conglomeritic horizons composed of rounded, disc-shaped pebbles of Dox Sandstone and occasional pebbles of basalt lava. A lag deposit of gravel composed of white to yellowish, small pebble to cobble size chert gravel occurs locally at the base of the upper member. Typically, the sandstone sections are thin-to-medium bedded and exhibit planar tabular and trough cross-bedding, ripple marks, mudcracks, numerous soft-sediment deformation structures, and rare salt pseudomorphs. Towards the top of the upper member, massive, meter-scale sandstone beds become common.〔〔〔〔 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「Nankoweap Formation」の詳細全文を読む スポンサード リンク
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