| Bass Formation | |
|---|---|
| Stratigraphic range: Mesoproterozoic, | |
 Stromatolites in Bass Limestone, Grand Canyon | |
| Type | Geological formation |
| Unit of | Unkar Group |
| Sub-units | Hotauta Member |
| Underlies | Hakatai Shale |
| Overlies | Vishnu Basement Rocks on major erosion unconformity |
| Thickness | 121 to 341 feet (37 to 104 m) |
| Lithology | |
| Primary | dolomite |
| Other | sandstone, conglomerate, argillite, and limestone |
| Location | |
| Region | Grand Canyon |
| Country | United States |
| Type section | |
| Named for | Bass Canyon |
| Named by | Noble (1914)[1] |
Cheops Pyramid (prominence), Grand Canyon (Isis Temple region, Bright Angel Canyon)
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 in the Grand Canyon region 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.[2]
The Bass Formation is the basal part of the Unkar Group.
(high resolution expandable photo)
The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) 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 113 to 150 m (371 to 492 ft) thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) 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.[2][3][4]
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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.[1][2][5]
- ^ a b Noble, LF (1914) The Shinumo quadrangle, Grand Canyon district, Arizona. Bulletin no. 549, US Geological Survey, Reston, Virginia.
- ^ a b c 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.
- ^ 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):681–99.
- ^ 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.
- ^ 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):1573–95.