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The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the stair-stepped topography, resistant dolomite layers form risers and argillite layers form steep treads. In general, the Bass Formation and associated strata of the Unkar Group rocks 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). In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation.〔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 Bass Formation is the basal part 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 is overlain in ascending order by the Nankoweap Formation, about thick; the Chuar Group, about thick; and the Sixtymile Formation, about thick. The Grand Canyon Supergroup, of which the Bass Formation is the lowermost formation, 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.〕 There has been some discussion about the nomenclature of the Bass Formation. It was originally named the ''Bass Limestone'' in 1914 for Bass Canyon, where it is typically exposed. The ''Bass Limestone'' has been reclassified as the ''Bass Formation'' by geologists because it consists of heterogeneous mixtures of clastic and carbonate sedimentary strata of which dolomite is the predominant rock type and limestone is only a minor rock type. In addition, its Hotauta Member was originally designated as the ''Hotauta Conglomerate'' in 1914 as a separate unit of formation status for Hotauta Canyon. It was later included in the Bass Formation as a member where it has remained.〔〔〔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.〕 ==Description== The Bass Formation consists of interbedded sandstone (arkose), and silty sandstone, prominent interbeds of conglomerate and dolomite, and subordinate interbeds of argillite and limestone. Dolomite and sandy dolomite are the predominate lithologies. Beds of intraformational breccia are also found throughout the Bass Formation. The dolomites and limestones vary in color from gray to red-gray and sometimes contain biscuit-form and biohermal stromatolite beds. The conglomerates, breccias, sandstones, and argillites vary in color from purple-brown to dark red and reddish brown. Dolomite and limestone dominate the Bass Formation in the central part of the Grand Canyon, while the sandstone, conglomerate, and argillite dominate the Bass Formation in the eastern part of the Grand Canyon. The Bass Formation typically becomes generally finer grained toward the top.〔〔〔Dalton, Russell O., Jr. (1972) Stratigraphy of the Bass Formation (Late Precambrian, Grand Canyon, Arizona). Unpublished Master’s thesis, Northern Arizona University : Flagstaff, Arizona. 140 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.〕 Volcanic ash layers occur in the Bass Formation. They consist of layers of white, very fine-grained tephra, that are interbedded with dolomite and argillite toward the base of the formation. These layers are characterized by sharp basal contacts, some graded-bedding, and a diverse silicate mineral assemblage that differs from the carbonate or calcsilicate dominated mineral assemblages of adjacent strata. Zircons from one of these volcanic ash beds has been dated using Uranium-lead dating techniques.〔〔Bloch, JD, JM Timmons, GE Gehrels, LJ Crossey, and KE Karlstrom (2006) ''Mudstone petrology of the Mesoproterozoic Unkar Group, Grand Canyon, USA: provenance, weathering and sediment transport on intracratonic Rodinia.'' Journal of Sedimentary Petrology. 76(9):1106-1119.〕 The base of both the Bass Formation and Unkar Group in eastern Grand Canyon is marked by the Hotauta Member of the Bass Formation. It is a prominent discontinuous, conglomerate, initially named the Hotauta Conglomerate and later redesignated the Hotauta Member. This conglomerate consists of rounded, pebble to cobble-sized clasts of chert, granite, quartz, plagioclase crystals, and micropegmatites in a quartz sand matrix. About 80% of the gravel-size clasts consist of granite and quartzite. The quartzite gravel lacks any local equivalents in the Grand Canyon, indicating a distant source. Excellent exposures of the Hotauta Member occur at the Colorado River level near Hance Rapids (river mile 77) and along the South Kaibab and North Kaibab Trails.〔〔〔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.〕 A variety of sedimentary structures have been reported from the Bass Formation. They include ripple marks; mudcracked surfaces; cone-in-cone structures; interformational breccias/conglomerates; both normal and reversed small-scale, graded bedding; and local channel fills. The dolomite and limestone within the Bass formation have been affected by post-depositional alteration including: dolomitization, recrystallization, stylolitization, and silicification.〔〔〔 Basaltic sills and dikes intrude all strata of the Unkar Group that underlie the Cardenas Lava. Only sills, whose feeder dikes are not exposed, can be observed intruding the Bass Formation. Where the sills have intruded siliceous dolomites in the Bass Formation, metasomatism and recrystallization have produced chrysotile asbestos, both above and below the sills. Veins of asbestos with fibers up to in length, often commonly occur within of the upper and lower contacts of the sills.〔 The Bass Limestone shows generally, an increase in thickness to the northwest ranging from 37 (121 ft) to thick. Its average thickness is . It is thick at Phantom Creek (north side of Isis Temple, Cheops Pyramid, & Utah Flats), and thick at Crystal Creek. The thin section of Bass Formation at Crystal Creek likely reflects the presence of a Vishnu Basement Rocks topographic high in the paleosurface on which it accumulated.〔〔 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「Bass Formation」の詳細全文を読む スポンサード リンク
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