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. The 5-unit Unkar Group is the basal member of the 8-member Grand Canyon Supergroup. The Unkar is about 1,600 to 2,200 m thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. Units 4 & 5 are found mostly in the eastern region of Grand Canyon. Units 1 through 3 are found in central Grand Canyon. The Unkar Group accumulated approximately between 1250 and 1104 Ma. In ascending order, the Unkar Group is overlain by the Nankoweap Formation, about 113 to 150 m thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick. These are all of the units of the Grand Canyon Supergroup. The Unkar Group makes up approximately half of the thickness of the 8-unit Supergroup.
Units 5 & 4 of the Unkar Group Reddish (unit 4)–Dox Formation below blackish (5)–Cardenas Basalt, upstream from Tanner Graben
View due-north, from Lipan Point, East Rim: Lava Butte, Temple Butte, Cardenas Basalt, Dox Formation, Palisades of the Desert – (on East Rim), etc. on Colorado River.
Close-up of Apollo Temple
(units 1-2-3 of 5 units) Shinumo Quartzite cliffs upon colorful Hakatai Shale upon horizontal thin beddings of Bass Formation (upon Vishnu Basement Rocks of Granite Gorge) (expandable high-resolution photo)
The Grand Canyon Supergroup is a Mesoproterozoic to a Neoproterozoic sequence of sedimentary strata, partially exposed in the eastern Grand Canyon of Arizona. This group comprises the Unkar Group, Nankoweap Formation, Chuar Group and the Sixtymile Formation, which overlie Vishnu Basement Rocks. Several notable landmarks of the Grand Canyon, such as the Isis Temple and Cheops Pyramid, and the Apollo Temple, are surface manifestations of the Grand Canyon Supergroup.
A view looking north. Unkar Group units lying below the Isis Temple/Cheops Pyramid landforms: (3). Shinumo Quartzite cliffs (lower part shown on top of photo), above (2). Hakatai Shale (red-orange-purple slopes), above (1). Bass Formation(finely-layered, dark slopes), sitting on top of (base units). Vishnu Basement Rocks (black stratum) within the Granite Gorge and side canyons.
Buried & dipped sections of the 8 named sections of the Grand Canyon Supergroup.
Stromatolite example
Basalt dike in orange-red Hakatai Shale along Colorado River at Hance Rapid, river mile 76.5, Grand Canyon.