Fossil Mountain Member of Kaibab Limestone

Usage of Geologic Unit Name:
Fossil Mountain Member*
Fossil Mountain Formation
Permian, Early*
Early Middle Permian - 270 million years (age used by National Park Service;
Mathis and Bowman, 2005)
Geologic Province:
Plateau sedimentary province*
Basin and Range province*
Paradox basin
Great Basin province*
Areal Extent:

Type Locality:
Bass Trail on Fossil Mountain, Coconino Co, AZ (Sorauf, Billingsley, 1991).

Unit Name History:
Assigned to Kaibab Group (Welsh and others, 1979). Named the basal member, Kaibab Formation (Sorauf, Billingsley, 1991). Reference section (Sorauf, Billingsley, 1991). Areal limits (Anderson, Hintze, 1993).
Description from Grand Canyon Area (from Billingsley, George H., 2000)

Fossil Mountain Member—Light-gray, cliff-forming, fine- to medium-grained, thin to medium-bedded [1–6 ft (0.3–2 m)], fossiliferous, sandy, cherty limestone. In general, unit weathers dark gray. Unit characterized by gray to white fossiliferous chert nodules and white chert lenses parallel to bedding; chert weathers dark gray to black. Some chert nodules contain concentric black and white bands. Includes brecciated chert beds 4–10 ft (1–3 m) thick in upper part at contact of thin limestone or gypsifereous siltstone of Harrisburg Member. Chert in central and western parts of map area makes up about 20 percent of unit; unit becomes sandy in northeastern quarter of map area. Generally forms cliff at rim of the Grand Canyon. Weathers into pinnacles or “pillars” detached from cliff in western half of map area. Unconformable contact with underlying Woods Ranch Member of Toroweap Formation attributed to solution erosion and channel erosion; average relief about 10 ft (3 m). Some channels have eroded as much as 150 ft (45 m) into the Woods Ranch in western half of map area. Erosion channels were filled with sandy cherty limestone typical of the Fossil Mountain, providing an extra thickness of the Fossil Mountain. Thickness about 230–350 ft (70–107 m).


Welsh, J.E., Stokes, W.L. and Wardlaw, B.R., 1979, Regional stratigraphic relationships of the Permian "Kaibab" or Black Box Dolomite of the Emery high, central Utah, IN Baars, D.L., ed., Permianland: Four Corners Geological Society Field Conference Guidebook, 9th Field Conference, Moab, UT, September 27-30, 1979, p. 143-149.

Sorauf, J.E. and Billingsley, G.H., 1991, Members of the Toroweap and Kaibab Formations, Lower Permian, northern Arizona and southwestern Utah: The Mountain Geologist, v. 28, no. 1, p. 9-24.

Anderson, R.E. and Hintze, L.F., 1993, Geologic map of the Dodge Spring quadrangle, Washington County, Utah and Lincoln County, Nevada: U.S. Geological Survey Geologic Quadrangle Map, GQ-1721, 1 sheet, scale 1:24,000.

Mathis, A., and Bowman, C., 2005, What's in a number? Numeric ages for rocks exposed within the Grand Canyon , Part 2: Nature Notes ( Grand Canyon National Park ), v. 21, no. 2, p. 1-5.

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Note that data on this page contains information that has been partly revised from bibliographic resources available via the National Geologic Map Database GEOLEX:
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Last modified: 1/7/2011