Unkar Group

Unkar Group Stratigraphic range: Meso-Proterozoic, between 1,254 and 1,100 Ma
Reddish Dox Formation below Cardenas Basalt, upstream from Tanner Graben
Type	Geologic group
Unit of	Grand Canyon Supergroup
Sub-units	Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt
Underlies	Nankoweap Formation and, as part of the Great Unconformity, the Tapeats Sandstone
Overlies	Vishnu Basement Rocks
Area	Arizona, east Grand Canyon, Lava Butte region, on Colorado River, near Lipan Point
Thickness	5,200 to 7,200 feet (1,600 to 2,200 m)
Lithology
Primary	sandstone, siltstone, shale, and basaltic volcanic rocks
Other	dolomite and limestone
Location
Region	northcentral Arizona into southcentral Utah in the subsurface
Country	United States of America
Type section
Named for	Unkar Valley
Named by	Walcott (1894) and Noble (1910, 1914)

The Unkar Group is a sequence of strata of Proterozoic age that are subdivided into five geologic formations and exposed within the Grand Canyon, Arizona, Southwestern United States. It is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. It accumulated approximately between 1250 and 1104 Ma (1,104 million years ago, 1.1 billion). In ascending order, the Unkar Group is overlain 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 Nankoweap and Sixtymile formations together with the Chuar and Unkar groups comprise the Grand Canyon Supergroup.

In general, the strata comprising the Unkar Group 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). Within the central Grand Canyon, these strata occur in small, rotated, downfaulted blocks or slivers where they commonly are only partially exposed. Within this part of the Grand Canyon, the Unkar Group is incomplete because pre-Tonto Group erosion has removed strata above the level of the middle part of the Dox Formation. The missing part of the Unkar Group and the remainder of the overlying Grand Canyon Supergroup are preserved in a prominent syncline and fault block that is exposed in the eastern Grand Canyon. Examples of these fault blocks can be seen at the Isis Temple prominence, "Cheops Pyramid," and the intersection of Phantom Creek with the Bright Angel Canyon, (North Kaibab Trail). The Unkar Group contains thick basaltic sills and a number of small, dark dikes. In the area of Desert View and west of Palisades of the Desert, the basaltic sills form very prominent, dark gray cliffs.