Bass Limestone

Bass Formation Stratigraphic range: Meso-Proterozoic, 1,250 Ma
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	Arizona, Grand Canyon Isis Temple region, southwest Bright Angel Canyon, at north side, Granite Gorge, and along Colorado River
Country	United States of America
Type section
Named for	Bass Canyon
Named by	Noble (1914)

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.

The Bass Formation is the basal part of the Unkar Group. 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.

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