Hakatai Shale

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Hakatai Shale
Stratigraphic range:
Meso-Proterozoic,
1,200±50 Ma?
Grand Canyon with Snow 4.JPG
Three approximate equal-thickness formations outcropping below Isis Temple-prominence at north perimeter, Granite Gorge
(Unkar Group)-Shinumo Quartzite (vertical cliffs), upon basalt sills, upon orange-red Hakatai Shale, upon Bass Formation.
Dk blackish canyons are the Granite Gorge Vishnu Basement Rocks.
Type Geological formation
Unit of Unkar Group (5 units)
Underlies Shinumo Quartzite-(unit 3)
Overlies Bass Formation-(unit 1)
Thickness985 feet (300 m) approximate maximum
Lithology
Primary shale
Other siltstone, sandy siltstone, and sandstone (subarkose, arkose and quartz arenite)
Location
Region Arizona, Grand Canyon
Isis Temple region, southwest Bright Angel Canyon, at north side, Granite Gorge, and along Colorado River, also,
numerous side canyons to Granite Gorge / Colorado River
Country United States
(Southwest United States)
Type section
Named forHakatai Canyon
Named byNoble (1914) [1]
Basalt dike in orange-red Hakatai Shale along Colorado River at Hance Rapid, river mile 76.5, Grand Canyon. Grand Canyon Supergroup Hakatai Shale with Basalt Dike.jpg
Basalt dike in orange-red Hakatai Shale along Colorado River at Hance Rapid, river mile 76.5, Grand Canyon.
Many layers of rock. Tonto Group and Unkar units. Shows the Vishnu rocks; but also the orange Hakatai Shale, Shinumo Quartzite, some Bass Formation?, etc. The upper massive layers show the white cliffs of Coconino Sandstone over the Supai Group redbeds that sit upon the Redwall Limestone. El canon grandioso, 9-15 (30597718082).jpg
Many layers of rock. Tonto Group and Unkar units. Shows the Vishnu rocks; but also the orange Hakatai Shale, Shinumo Quartzite, some Bass Formation?, etc. The upper massive layers show the white cliffs of Coconino Sandstone over the Supai Group redbeds that sit upon the Redwall Limestone.
Stratigraphy of the Grand Canyon Stratigraphy of the Grand Canyon.png
Stratigraphy of the Grand Canyon

The Hakatai Shale is a Mesoproterozoic rock formation that outcrops in the Grand Canyon, Coconino County, Arizona. It consists of colorful strata that exhibit colors that vary from purple to red to brilliant orange on outcrop. The colors are the result of the oxidation of iron-bearing minerals in the Hakatai Shale. It consists of lower and middle members that consist of bright-red, slope-forming, highly fractured, argillaceous mudstones and shale and an upper member composed of purple and red, cliff-forming, medium-grained sandstone. Its thickness, which apparently increases eastwards, varies from 137 to 300 m (449 to 984 ft). In general, the Hakatai Shale and associated strata of the Unkar Group rocks dip northeast (10°-30°) toward normal faults that dip 60° or more 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). In addition, thick, prominent, and dark-colored basaltic sills and dikes cut across the purple to red to brilliant orange strata of the Hakatai Shale. [2] [3] [4]

Contents

The bright orange-red slopes of the Hakatai Shale contrasts sharply against the grayish outcrops of the Bass Formation. The outcrop of the Hakatai Shale also contrasts greatly with the steep cliffs formed by Shinumo Quartzite as seen at the base of Isis Temple. In the central Grand Canyon north of Grand Canyon Village and viewed from the south at the South Rim, the bright orange-red unit can be seen below the Isis Temple and Cheops Pyramid landforms at the intersection of Bright Angel Canyon and Granite Gorge; the Bright Angel Trail from the South Rim traverses through the geographic region to the north, the North Kaibab Trail in Bright Angel Canyon. [2] [4]

The Hakatai Shale is part of a conformable sequence of sedimentary strata that comprise the Unkar Group. The Unkar Group 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. 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 Grand Canyon Supergroup, of which the Unkar Group is the lowermost part, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks. [2] [5] [6]

Description

~closeup of bedding in Hakatai Shale
(view downstream, ~west/northwest from Grandview Point, near East Rim, Grand Canyon) Desert View Grandview 3 (15358762918).jpg
~closeup of bedding in Hakatai Shale
(view downstream, ~west/northwest from Grandview Point, near East Rim, Grand Canyon)

Typically, the Hakatai Shale is subdivided into three informal members. In ascending order they are the Hance Rapids (lower), Cheops Pyramid (middle), and Stone Creek (upper) members. The total thickness of the Hakatai Shale varies from 135 m (443 ft) at Hance Rapids to nearly 300 m (980 ft) at the type section in Hakatai Canyon in the Shinumo Creek area. [2] [7]

First, the Hance Rapids (lower) member consists of purple to reddish-purple mudstone, interbedded sandy siltstone, and rare occurrences of thin-bedded subarkose- to quartz arenite. Beds of the lower member of the Hakatai Shale grade upward into the fine-grained middle member, with a boundary drawn at or very near their change in color. Second, the Cheops Pyramid (middle) member consists of mudstone, siltstone, and subordinate sandy siltstone that exhibit a striking reddish-orange color. These distinctively red-colored strata are commonly mottled with small to large (up to 10 cm and more across), non-red, spherical to spheroidal reduction spots. These mottles have a dark-gray to greenish-gray central nuclei that contain very dark gray to black central cores. Finally, the Stone Creek (upper) member of the Hakatai Shale consists of pale purple or lavender, fine- to coarse-grained, crossbedded coarse arkose. [7] [8] [9]

Basaltic sills and dikes intrude all of the Unkar Group below the Cardenas Basalt. Sills intruded only the Bass Formation and Hakatai Shale. Dikes intrude the Hakatai Shale, Shinumo Quartzite, and Dox Formation. The sills range in thickness from 23 m (75 ft) at Hance Rapids, eastern Grand Canyon, to 300 m (980 ft) in Hakatai Canyon in the Shinumo Creek area. The feeder dikes to these sills are not exposed. However, the feeder dikes for the Cardenas Basalt can be traced, discontinuously, to within a few meters of its base. Adjacent to the sills, the Hatakai Shale has been altered to knotted hornfels containing porphyroblasts of andalusite and cordierite that have been replaced by muscovite and green chlorite, respectively. [2]

Contacts

In the eastern part of the Grand Canyon, the contact between the Hatakai Shale with underlying Bass Formation is typically gradational over an interval of a meter or so. For example, in Red Canyon, the contact consists of an interval in which stromatolitic limestone of the Bass Formation is intimately interbedded with coarse deposits of the overlying Hakatai Shale. In the eastern part of the Grand Canyon, the contact is sharp, but conformable. [2] [10]

The contact between the Hatakai Shale and overlying Shinumo Quartzite is a distinct, significant disconformity. This disconformity is sharp and locally truncates cross-beds, and channel forms, within the sandstones of the underlying Hatakai Shale. Within the lower subarkose of the Shinumo Sandstone, a basal lag of conglomerate, which contains basement clasts up to 5 cm (2.0 in) across, lies on the eroded surface that forms this disconformity. This basal conglomerate contains quartzite clasts that lack any known equivalents in the Grand Canyon region. As documented by the dating of detrital zircons, this disconformity is estimated to represent a period of about 75 million years. [2] [7]

The contact between the Tapeats Sandstone and Hatakai Shale and the rest of the folded and faulted Unkar Group is a prominent angular unconformity, which is part of the Great Unconformity. The differential erosion of the Unkar Group left resistant beds of the Cardenas Basalt and Shinumo Quartzite as topographic highs, ancient monadnocks, that are now buried by sandstones, shales, and conglomerates of the Tapeats Sandstone. These monadnocks served locally as sources of coarse-grained sediments during the marine transgression that deposited the Tapeats Sandstone and other members of the Tonto Group. The contact between the Hatakai Shale and the Tapeats Sandstone forms part of a relative flat surface that lies between the monadnocks. [2]

Fossils

Stromatolites have been found within spectacular exposures of the Hatakai Shale in Rodgers Canyon. At this outcrop, they occur in the transitional zone between it and the Bass Formation where stromatolitic carbonate beds are intimately interbedded with coarse deposits of the Hakatai Shale. The base of these deposits contains stromatolite onchaloids, small algal mounds, that are built upon coarse clasts of sandstone-conglomerate. [10] In addition, the dark gray to black central cores of the reduction mottles found in the Cheops Pyramid (middle) member are possibly of organic (stromatolitic?) origin. [7]

A number of spurious and discredited reports about other types of fossils found in the Hatakai Shale have been made. In one case, apparent trails of metazoans were reported from the Hatakai Shale. [11] However, these reported trace fossils are now regarded to be nonbiogenic pseudofossils. [12]

Depositional environments

The strata comprising the Hance Rapids (lower) member is interpreted to have accumulated beneath shallow, low-energy waters. The change from Bass Formation to the lower member likely represent the waning stages of marine deposition during which the Bass Formation accumulated. The Cheops Pyramid (middle) member is regarded to have accumulated either in a shallow, near-shore marine environment or in coastal plain mudflats, or deltas. The greenish gray reduction mottles are similar in nature to reduction mottles observed in the lower middle and upper members of the Dox Formation, which are regarded as having accumulated in nonmarine environments. The Stone Creek (upper) member is of probable marine deltaic origin. [2] [8] [9]

Age

The Hatakai Shale is between 1,253 and 1,104 million years old. It is younger than a layer of volcanic ash in the underlying Bass Formation that has been dated at 1,253 million years. In addition, the 40Ar/39Ar dating of detrital muscovite from the basal, Escalante Creek Member of the overlying Dox Formation indicates that it is younger than 1,140 million years. The overlying Dox Formation is older than the age of the Cardenas Basalt, which is 1,104 million years. [13] [14]

Isis Temple-Cheops Pyramid horst

Isis Temple-Cheops Pyramid is a small horst, located close to the geographic center of the Grand Canyon. It is bordered on the east by the Bright Angel Fault and on the south by Granite Gorge; the Bright Angel Fault continues to the south-southwest across Granite Gorge and up Garden Creek, up the Bright Angel Trail to the South Rim. The north side of Isis Temple-Cheops Pyramid is bordered by the approximate east-west, Grandview Fault.

See also

Related Research Articles

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The geology of the Grand Canyon area includes one of the most complete and studied sequences of rock on Earth. The nearly 40 major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from about 200 million to nearly 2 billion years old. Most were deposited in warm, shallow seas and near ancient, long-gone sea shores in western North America. Both marine and terrestrial sediments are represented, including lithified sand dunes from an extinct desert. There are at least 14 known unconformities in the geologic record found in the Grand Canyon.

Great Unconformity the huge gap in geology

Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland, or that observed by John Wesley Powell in the Grand Canyon in 1869. Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods of geologic time sufficiently long to raise great mountains and then erode them away.

Sierra Ancha

The Sierra Ancha is a mountain range in Gila County, in central Arizona. It lies between Roosevelt Lake to the south, the Tonto Basin to the west, Cherry Creek to the east, and Pleasant Valley to the north. The range is one of several, including the Bradshaw Mountains, Mingus Mountain of the Black Hills, and the Mazatzal Mountains, which form a transitional zone between the lowland deserts of southern Arizona and the Colorado Plateau of northeastern Arizona. The highest point in the range is Aztec Peak, at an elevation of 2345 m (7694 ft).

Frenchman Mountain mountain in United States of America

Frenchman Mountain is located east of Las Vegas in the U.S. state of Nevada. Made up of rocks similar to those found on the bottom of the Grand Canyon, Frenchman Mountain formed when faulting elevated and tilted the rocks followed by erosion, giving it its sharp triangular profile. The mountain provides an example of the Great Unconformity with the tilted Paleozoic Tapeats Sandstone underlain by Paleoproterozoic Vishnu Schist, which is some of the oldest rock on the North American continent, having been created about two billion years ago.

Tonto Group

The Cambrian Tonto Group is the three-member sequence of geologic formations that represent the basal section of Paleozoic rocks in the Grand Canyon. The group is about 1,250 feet (381 m) thick. The base unit, the Cambrian Tapeats Sandstone was deposited upon the erosion surface of the Vishnu Basement Rocks, which is found in Granite Gorge. The erosion resistant Tapeats Sandstone forms the platform, called Tonto Platform, that the two less erosion resistant upper layers, the Bright Angel Shale and Muav Limestone, rest upon.

Muav Limestone

The Cambrian Muav Limestone is the upper geologic unit of the 3-member Tonto Group. It is about 650 feet (198 m) thick at its maximum. It is a resistant cliff-forming unit. The Muav consists of dark to light-gray, brown, and orange red limestone with dolomite and calcareous mudstone. The Muav is overlain in some areas by the Devonian Temple Butte Formation, but the major unit above are the vertical cliffs of Mississippian Redwall Limestone. The Muav is located in the lower elevations of the Grand Canyon, Arizona.

Tapeats Sandstone

The Cambrian Tapeats Sandstone is the lower geologic unit, about 230 feet (70 m) thick, at its maximum, of the 3-member Tonto Group.

Cardenas Basalt

The Cardenas Basalt, also known as either the Cardenas Lava or Cardenas Lavas, is a rock formation that outcrops over an area of about 310 km2 (120 mi2) in the eastern Grand Canyon, Coconino County, Arizona. The lower part of the Cardenas Basalt forms granular talus slopes. Its upper part forms nearly continuous low cliffs that are parallel to the general course of the Colorado River. The most complete, readily accessible, and easily studied exposure of the Cardenas Basalt lies in Basalt Canyon. This is also its type locality.

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 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.

Nankoweap Formation

The Neoproterozoic Nankoweap Formation, is a thin sequence of distinctive red beds that consist of reddish brown and tan sandstones and subordinate siltstones and mudrocks that unconformably overlie basaltic lava flows of the Cardenas Basalt of the Unkar Group and underlie the sedimentary strata of the Galeros Formation of the Chuar Group. The Nankoweap Formation is slightly more than 100 m in thickness. It is informally subdivided into informal lower and upper members that are separated and enclosed by unconformities. Its lower (ferruginous) member is 0 to 15 m thick. The Grand Canyon Supergroup, of which the Nankoweap Formation is part, unconformably overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.

Isis Temple mountain in United States of America

Isis Temple, in the Grand Canyon, Arizona, USA, is a prominence below the North Rim, and adjacent Granite Gorge. The prominence lies north of the north bank of the west-flowing Colorado River, and is just north of Middle Granite Gorge. The Trinity Creek and canyon flow due-south at its west border; its north, and northeast border/flank is formed by Phantom Creek and canyon, a west tributary of Bright Angel Creek; the creeks intersect about 3 mi southeast, and 1 mi north of Granite Gorge. The peak of Isis Temple is only ~202 ft lower than Grand Canyon Village.

Grand Canyon Supergroup

The Grand Canyon Supergroup is a Mesoproterozoic to Neoproterozoic sequence of sedimentary strata, mostly exposed in the eastern Grand Canyon of Arizona. This group is composed of 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.

Tanner Graben mountain in United States of America

Located directly downstream of the Little Colorado River confluence with the Colorado River, the Tanner Graben, in the Grand Canyon, Arizona, USA is a prominence and cliffside rock formation below the East Rim. Tanner Graben is located riverside, on the Colorado River, on a north-northwest bank at Mile 68.5, and lies opposite Tanner Canyon. The Tanner Rapid, created by Tanner Creek lies at the riverside foot of the graben. The graben is a pronounced feature because of the black Cardenas Basalt that forms the middle section of the graben, presumably free of debris accumulation by its cliff face steepness, and winds, and airflow drainage that course through the Colorado River's canyons; unprotected side canyons of Cardenas Basalt show accumulations as a slope-forming geologic unit, with little showing of black basalt.

Bass Formation lithostratigraphic unit

The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the stair-stepped topography, resistant dolomite layers form risers and argillite layers form steep treads. In general, the Bass Formation in the Grand Canyon region and associated strata of the Unkar Group-rocks 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. In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation.

Shinumo Quartzite

The Shinumo Quartzite also known as the Shinumo Sandstone, is a Mesoproterozoic rock formation, which outcrops in the eastern Grand Canyon, Coconino County, Arizona. It is the 3rd member of the 5-unit Unkar Group. The Shinumo Quartzite consists of a series of massive, cliff-forming sandstones and sedimentary quartzites. Its cliffs contrast sharply with the stair-stepped topography of the underlying Hakatai Shale. Overlying the Shinumo, dark green to black, fissile, slope-forming shales of the Dox Formation create a well-defined notch. It and other formations of the Unkar Group occur as isolated fault-bound remnants along the main stem of the Colorado River and its tributaries in Grand Canyon. Typically, the Shinumo Quartzite and associated strata of 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.

Dox Formation

The Dox Formation, also known as the Dox Sandstone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The strata of the Dox Formation, except for some more resistant sandstone beds, are relatively susceptible to erosion and weathering. The lower member of the Dox Formation consists of silty-sandstone and sandstone, and some interbedded argillaceous beds, that form stair-stepped, cliff-slope topography. The bulk of the Dox Formation typically forms rounded and sloping hill topography that occupies an unusually broad section of the canyon.

Vishnu Basement Rocks lithostratigraphic unit

The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group. These basement rocks have also been called either the Vishnu Complex or Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the Granite Gorge Metamorphic Suite; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic granite, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.

Bright Angel Shale stratigraphic formation

The Cambrian Bright Angel Shale is the middle layer of the three member Tonto Group geologic feature. The 3-rock Tonto section famously sits upon the Great Unconformity because of the highly resistant cliffs of the base layer, vertical Tapeats Sandstone cliffs.

The Neoproterozoic Chuar Group consists of 5,250 feet (1,600 m) of fossiliferous, unmetamorphosed sedimentary strata that is composed of about 85% mudrock. The Group is the approximate upper half of the Grand Canyon Supergroup, overlain by the thin, in comparison, Sixtymile Formation, the top member of the multi-membered Grand Canyon Supergroup.

Sixtymile Formation

The Sixtymile Formation is the uppermost unit of the Grand Canyon Supergroup. It is a very thin accumulation of sandstone, siltstone, and breccia 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 meters (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion during the formation of the Great Unconformity and prior to deposition of the Tapeats Sandstone.

References

  1. Noble, LF (1914) The Shinumo Quadrangle, <!——>Grand Canyon District, Arizona. Bulletin. no. 549. US Geological Survey, Reston, Virginia. 100 pp.
  2. 1 2 3 4 5 6 7 8 9 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.
  3. Billingsley, GH (2000) Geologic Map of the Grand Canyon 30 × 60 Quadrangle, Coconino and Mohave Counties, Northwestern Arizona, Pamphlet to accompany Geologic Investigations Series I–2688 Version 1.0.U.S. Geological Survey, Reston, Virginia. 15 pp.
  4. 1 2 Beus, SS, RR Rawson, RO Dalton, GM Stevenson, VS Reed, and TM Daneker (1974) Preliminary report on the Unkar Group (Precambrian) in Grand Canyon, Arizona. In TNV Karlstrom, GA Swann, and RL Eastwood, ed., pp. 34-53, Geology of northern Arizona, with notes on archaeology and paleoclimate; Part 1, Regional studies. Geological Society of America, Rocky Mountain Section, Boulder, Colorado. 407 pp.
  5. Elton, DP, and EH McKee (1982) Age and correlation of the late Proterozoic Grand Canyon disturbance, northern Arizona. Geological Society of America Bulletin. 93(8):681-699.
  6. Karlstrom, KE, BR Ilg, Bradley, D Hawkins, ML Williams, G Dumond, KK. Mahan, and SA Bowring, Samuel (2012) Vishnu Basement Rocks of the Upper Granite Gorge: Continent formation 1.84 to 1.66 billion years ago. In JM Timmons and KE Karlstrom, eds., pp. 7-24, Grand Canyon geology: Two billion years of earth's history. Special Paper no 294, Geological Society of America, Boulder, Colorado.
  7. 1 2 3 4 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):1573-1595.
  8. 1 2 Reed VS (1976) Stratigraphy and depositional environment of the Hakatai Shale, Grand Canyon, Arizona. Unpublished Master's Thesis, Northern Arizona University, Flagstaff, Arizona. 163 pp.
  9. 1 2 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.
  10. 1 2 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. 24-47, Grand Canyon Geology: Two Billion Years of Earth's History. Special Paper no. 489, Geological Society of America, Boulder, Colorado.
  11. Glaessner, MF (1965) Pre-Cambrian life -Problems and perspectives Geological Society of London Proceedings. 1626:165-169.
  12. Cloud, P (1973) Pseudofossils: A Plea for Caution. Geology. 1(3):123-127.
  13. Heizler, MT., KE Karlstrom, and MJ Timmons (1999) Where have all the old micas gone? New Mexico Geology. 21(2):34.
  14. Timmons, JM, KE Karlstrom, CM Dehler, JW Geissman, and MT Heizler (2001) Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recorded in the Grand Canyon Supergroup and establishment of northwest- and north-trending tectonic grains in the southwestern United States. Geological Society of America Bulletin. 113(2):163– 180.

Further reading