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 Unkar Group is the basal formation of the 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. The Cardenas Basalt and Dox Formation are found mostly in the eastern region of Grand Canyon. The Shinumo Quartzite, Hakatai Shale, and Bass Formation 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.Hendricks, JD, and GM Stevenson (2003) Grand Canyon Supergroup: Unkar Group. In SS Beus and M Morales, eds., pp. 39–52, Grand Canyon Geology, 2nd ed. Oxford University Press, New York.Elston, DP (1989) Middle and late Proterozoic Grand Canyon Supergroup, Arizona. In DP Elston, GH Billingsley, and RA Young, RA., eds., pp. 94–105, Geology of the Grand Canyon, Northern Arizona (with Colorado River Guides). American Geophysical Union Fieldtrip Guidebook T115/315 for International Geologic Congress, 28th. American Geophysical Union, Washington DC. 239 pp.Timmons, JM, J. Bloch, K. Fletcher, KE Karlstrom, M Heizler, and LJ Crossey (2012) The Grand Canyon Unkar Group: Mesoproterozoic basin formation in the continental interior during supercontinent assembly. In JM Timmons and KE Karlstrom, eds., pp. 25–47, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado. The Unkar Group makes up approximately half of the thickness of 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, 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.Lucchitta, I (2001) Hiking Arizona's Geology. Mountaineers's Books. ()
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.Seeley, JM, and KG Randy (2003) Delineation of subsurface Proterozoic Unkar and Chuar Group sedimentary basins in northern Arizona using gravity and magnetics., implications for hydrocarbon source potential. American Association of Petroleum Geologists Bulletin. 87(8): 1299–1321.
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.
Nomenclature The Unkar Group was first recognized and named by Charles D. Walcott as the "Unkar terrane" in 1894. It and his “Chuar terrane” comprised what was then named the “Grand Canyon series” of "Proterozoic (Algonkian)" age. He regarded the Cardenas Basalt, unnamed at that time, as the uppermost unit in his "Unkar terrane." In 1910 and 1914, Levi F. Noble later divided what he called the "Unkar Group" into five subunits, which were the Hotauta Conglomerate, Bass Limestone, Hakatai Shale, Shinumo Quartzite, and Dox Sandstone. The still unnamed Cardenas Basalt is only briefly noted as exposures of it are absent in the Shinumo 15-minute quadrangle. Although a recognized part of the Unkar Group, the basalt lava flows that overlie the Dox Formation were generally ignored and simply described as "basalt and diabase." In 1938, Charles R. Keyes applied the name "Cardenesan Series" to the basaltic volcanic rocks within the Unkar Group.Keyes, C (1938) Basement complex of the Grand Canyon: Pan American Geologist. 20: 91–116. In 1973, the current definition of the Unkar Group developed when the Nankoweap Formation, which had been earlier added to the Unkar Group was formally removed from it and the unconformity that separates the Nankoweap Formation from the Unkar Group – was recognized.The Development of Geological Studies in the Grand Canyon. Tyrona, Miscellaneous Publications no. 17. Department of Malacology, The Academy of, Natural Sciences of Philadelphia, Philadelphia, Pennsylvania.
Formations of the Unkar Group The Bass Formation not only contains gray to red-gray dolomite and sandy dolomite but also interbedded purple-brown to dark red and reddish brown sandstone (arkose), and silty sandstone, prominent interbeds of conglomerate, and subordinate interbeds of argillite and limestone. A prominent conglomerate, the Hotauta Member, fills paleovalleys cut into the underlying Vishu basement complex at its base. The Bass Formation also contains stromatolite beds and thin volcanic ash layers. The Hotauta Member is regarded to be fluvial in origin. The remainder of the Bass Formation accumulated in relatively warm shallow marine waters.Timmons, JM, KE Karlstrom, MT Heizler, SA Bowring, GE Gehrels, and LJ Crossey, (2005) Tectonic inferences from the ca. 1254–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis. Geological Society of America Bulletin. 117(11–12): 1573–95.
The Hakatai Shale consists of purple, reddish-purple, reddish-orange, and pale purple or lavender mudstone, sandy siltstone, siltstone, and arkosic sandstone. The brightly colored slopes of the Hakatai Shale contrasts sharply against the grayish outcrops of the Bass Formation. The sloping exposures of the Hakatai Shale also contrast greatly with the steep cliffs formed by the overlying Shinumo Quartzite. Stromatolites occur in the transitional zone between the Hakatai Shale and Bass Formation. The Hakatai Shale accumulated in low-energy, shallow, near-shore, marine environments.
In sharp contrast to argillaceous strata above and below it, the Shinumo Quartzite consists characteristically of beds that are red, brown, or purple sedimentary quartzites and lesser massive white, red, or purple sandstone; also conglomeratic sandstone. Within these cliff-forming sandstones, mudstone-rich intervals occur. Some of these sandstone beds exhibit well-developed soft-sediment deformation structures. No fossils have been found in the Shinumo Quartzite. The lower and middle parts of the Shinumo Quartzite accumulated in coastal tidal flats and the upper part of it represents the deposits of river deltas. The gradational contact between the Shinumo Quartzite and Dox Formation above indicates a shift from deposition in coastal deltas – to fluvial deposition by a large river system. Of note, the 'soft-sediment deformation' seen in this Shinumo Quartzite formation indicates significant earthquake and tectonic activity during its deposition.
The Dox Formation consists of a heterogeneous mixture of light-tan to greenish brown, siliceous quartz sandstone; calcareous lithic and arkosic sandstone; dark-brown-to-green shale and mudstone; red mudstone, siltstone, and quartz sandstone; sandy argillite; micaceous mudstone; and red quartzose, silty sandstone. In ascending order, these sediments have been subdivided into the Escalante Creek, Solomon Temple, Comanche Point, and Ochoa Point Members. Stromatolites have been reported from the Comanche Point Member. The Dox Formation locally interfingers with, and is baked by, basalt lava flows of the overlying Cardenas Basalt. Within the central Grand Canyon, pre-Tapeats Sandstone erosion has removed parts of the Unkar Group above the level of the middle part of the Dox Formation. The missing part of the Dox Formation and overlying Cardenas Basalt and Chuar Group can be found in a prominent syncline and fault block in the eastern Grand Canyon. The Dox Formation accumulated in a variety of marine, coastal, estuarine, and fluvial environments.Lucchitta, I, and JD Hendricks (1983) Characteristics, depositional environment and tectonic interpretations of the Proterozoic Cardenas Lavas, eastern Grand Canyon, Arizona. Geology. 11(3): 177–81.
The Cardenas Basalt is composed mainly of thin discontinuous beds of pahoehoe lava flows of olivine-rich basalt. The lower part of this formation consists of complexly interbedded, thin, and discontinuous beds of basalt, hyaloclastite, and sandstone that form low, talus-covered slopes. The upper unit of the Cardenas Basalt consists of cliff-forming basaltic and andesitic lava flows that are interbedded with beds of breccia, sandstone, and lapillite. No fossils have been found in the Cardenas Basalt. The Cardenas Basalt was formed by the subaerial eruption of basaltic and andesitic magma in wet coastal environments such as river deltas or tidal flats. Angular unconformities of vastly differing magnitudes separate the Cardenas Basalt from the overlying Nankoweap Formation and Tonto Group.