Sixtymile Formation

Sixtymile Formation
Stratigraphic range: Cambrian,
527–509 Ma [1][2]
Sixtymile Formation in Grand Canyon
TypeGeological formation
Unit ofTonto Group[1][2]
UnderliesTapeats Sandstone
OverliesChuar Group
Thickness60 m (200 ft), at maximum
Lithology
Primarysiltstone and sandstone
Otherintraformational breccia
Location
Region Arizona, Northern Arizona)
Country United States
Extentthe Chuar syncline, Grand Canyon
Type section
Named byFord et al. (1972),[3] Ford and Breed (1973),[4] and Elston (1979)[5]

The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone that is exposed in only four places in the Chuar Valley. These exposures occur atop Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon, Arizona. The maximum preserved thickness of the Sixtymile Formation is about 60 m (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion prior to deposition of the Tapeats Sandstone.[6]

Sixtymile Formation and the underlying Chuar Group are preserved only in a broad asymmetric fold comprising both units, called the Chuar syncline. The Chuar Syncline is a doubly plunging fold, which means that along the hingeline within the axis of the syncline, beds in some areas (Nankoweap Canyon) dip toward the south, and in other areas (Lava Chuar Canyon), beds dip toward the north. The Sixtymile Formation, Chuar Group, and Chuar Syncline is bounded on the east by the Butte fault zone and on all other sides by the overlying Tapeats Sandstone. The hingeline of the Chuar Syncline parallels the trace of the Butte fault, suggesting a genetic relationship between the syncline and the fault.[6]

In descending order, the Sixtymile Formation is underlain by the Chuar Group, Nankoweap Formation, and the Unkar Group. The Unkar Group lies unconformably upon deeply eroded granites, gneisses, pegmatites, and schists of the Vishnu Basement Rocks. The Nankoweap and Sixtymile formations together with the Chuar and Unkar groups comprise the Grand Canyon Supergroup.[6]

  1. ^ a b Cite error: The named reference KarlstromOthers2018a was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference KarlstromOthers2020a was invoked but never defined (see the help page).
  3. ^ Ford, TD, WJ Breed, and JW Mitchell (1972) Name and age of the upper Precambrian basalts in the eastern Grand Canyon. Geological Society of America Bulletin. 83(1):223–226.
  4. ^ Ford, TD, and WJ Breed (1973) Late Precambrian Chuar Group, Grand Canyon, Arizona. Geological Society of America Bulletin. 84(4):1243–1260.
  5. ^ Elston, DP (1979) Late Precambrian Sixtymile Formation and orogeny at the top of the Grand Canyon Supergroup, northern Arizona. Professional Paper no. 1092, U.S. Geological Survey, Reston, Virginia. 20 pp.
  6. ^ a b c Dehler, CM, SM Porter, and JM Timmons (2012) The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon., in JM Timmons and KE Karlstrom, eds., pp. 49–72, Grand Canyon Geology: Two Billion Years of Earth's History. Special Paper no. 489. Geological Society of America, Boulder, Colorado.