page_title: Bass Formation
hierarchical_section_title: Bass Formation
caption_attribution_description: gc 228
context_page_description: 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 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. 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 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 thick; the Chuar Group, about 1,900 m thick; and the Sixtymile Formation, about 60 m thick.
context_section_description: 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.
  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.
  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.
caption: From Komo Point / Komo Point Trail, a few sections near Granite Gorge of black Vishnu Basement Rocks and a section of the layered Bass Limestone (note horizontal layering & dk blackish-brown color) —- visible below the cliffs of Tapeats Sandstone.
  (high resolution expandable photo)
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