Unkar Group
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 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 (1,104 million years ago, 1.1 billion). In ascending order, the Unkar Group is overlain by the Nankoweap Formation, about thick; the Chuar Group, about thick; and the Sixtymile Formation, about 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.

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, Unkar strata occur in small, rotated, downfaulted blocks or slivers where they commonly are only partially exposed. Within this center 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 also 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.

Using gravity and aeromagnetic data, combined with gravity modeling, it was inferred that Proterozoic grabens, and half-grabens filled with strata of the Unkar Group – lie buried beneath Phanerozoic rocks in northern Arizona that surround the Grand Canyon. The grabens and half grabens filled with strata of the Unkar group are associated with northwest–southeast trending Mesoproterozoic fault systems that have curving, southwest-dipping traces. These fault systems were later reactivated during the Neoproterozoic, to form basins in which the following Chuar Group accumulated, and during the Cenozoic, to form geologic structures, i.e., faults, anticlines, synclines, and monoclines, that are exposed at the surface.
Major unconformities separate the Unkar Group from the strata overlying and underlying it. First, the Unkar Group, as the bottom unit of the Grand Canyon Supergroup, lies directly upon deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks. Second, an angular unconformity, with a dip of less than 10°, separates the base of the Nankoweap Formation from the underlying Unkar Group. Finally, a well-defined angular unconformity at the base of the relatively flat-lying Tonto Group separates it from the underlying faulted and folded strata of the Unkar Group and the rest of the Grand Canyon Supergroup that are typically tilted at angles of 10°–30°.
The western section of the Unkar group can be highlighted in a photo of three Unkar units below Isis Temple sitting on Vishnu Basement Rocks of Granite Gorge.
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strata
Concept
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Proterozoic
TIME
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geologic formation
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Grand Canyon
LOC
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Arizona
LOC
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Southwestern United States
LOC
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Grand Canyon Supergroup
LOC
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Bass Formation
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Shinumo Quartzite
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Dox Formation
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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Unkar Group
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