Sixtymile Formation

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Sixtymile Formation
Stratigraphic range: Cambrian,
527–509  Ma
Sixtymile Formation in Grand Canyon.jpg
Sixtymile Formation in Grand Canyon
Type Geological formation
Unit of Tonto Group [1] [2]
Underlies Tapeats Sandstone
Overlies Chuar Group
Thickness60 meters (200 ft), at maximum
Lithology
Primary siltstone and sandstone
Otherintraformational breccia
Location
RegionFlag of Arizona.svg  Arizona, Northern Arizona)
CountryFlag of the United States.svg  United States
Extentthe Chuar syncline, Grand Canyon
Type section
Named byFord et al. (1972), [3] Ford and Breed (1973), [4] and Elston (1979) [5]

The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone 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 prior to deposition of the Tapeats Sandstone. [6]

Contents

Sixtymile Formation and the underlying Chuar Group are preserved only in a broad asymmetric fold comprising both units, called the Chuar syncline. The Chuar Syncline is a doubly plunging fold, which means that along the hingeline within the axis of the syncline, beds in some areas (Nankoweap Canyon) dip toward the south, and in other areas (Lava Chuar Canyon), beds dip toward the north. The Sixtymile Formation, Chuar Group, and Chuar Syncline is bounded on the east by the Butte fault zone and on all other sides by the overlying Tapeats Sandstone. The hingeline of the Chuar Syncline parallels the trace of the Butte fault, suggesting a genetic relationship between the syncline and the fault. [6]

In descending order, the Sixtymile Formation is underlain by the Chuar Group, Nankoweap Formation, and the Unkar Group. The Unkar Group lies unconformably upon deeply eroded granites, gneisses, pegmatites, and schists of the Vishnu Basement Rocks. The Nankoweap and Sixtymile formations together with the Chuar and Unkar groups comprise the Grand Canyon Supergroup. [6]

Nomenclature

The Sixtymile Formation of Chuar Group was first recognized by Ford and Breed (1972). [3] It was formally named the Sixty Mile Formation and was regarded to be the upper formation of Chuar Group of Grand Canyon Supergroup by Ford and Breed (1973). [4] They designated the type locality of the Sixtymile Formation to be outcrops of red to white sandstone and siltstone with chert and interformational breccia exposed by cliffs on the north side of the upper part of Sixtymile Canyon. Later, in 1979, its spelling was changed to Sixtymile. [5] In 2001, Timmons and others [7] reassigned what had been identified as the lowest part of the Sixtymile Formation to the underlying Kwagunt Formation. The age of the Sixtymile Formation was regarded to be Precambrian until 2018, when dating of detrital zircons from it determined it to be Cambrian in age and, as a result, it is assigned to the lowermost formation of the Tonto Group. [1] [2]

Description

The Sixtymile Formation is typically subdivided into three informal members. They are (1) a lower red siltstone, sandstone, and breccia member; (2) a middle cherty quartzite member; and (3) an upper breccia-bearing sandstone and conglomerate member. The lower member, which is 22 meters (72 ft) to 27 meters (89 ft) thick, consists of beds of a heterogeneous mixture of laminated hematitic sandstone; thin-bedded micaceous sandstone containing rock fragments; monomict and polymict breccia; crudely bedded sandstone; and thin-bedded soft sandy siltstone. Individual beds are discontinuous and local in extent and often grade into each other. Blocks of dolomite from the underlying Walcott Member often occur in the lower member. [5] [8]

The Middle Member. which is about 25 meters (82 ft) thick, of the Sixtymile Formation consists of thin bedded, finely laminated, very fine grained cherty quartzite. The laminated quartzite of the Middle Member is folded on a moderately large scale. The thin beds and laminations of it are crinkled in a manner that reflects slumping of the member toward the axis of the Chuar Syncline. Chert is common to abundant within the Middle Member. The middle of this member contains numerous thin discontinuous beds of chalky-white chert. The lower part of the Middle Member is purplish red, which grades upward to a creamy, mottled-and-streaked red. This member is resistant to erosion and characteristically forms cliffs. This member forms small hills within its upper contact on either side of the Chuar Syncline. The upper contact of the Middle Member with the overlying Upper Member is sharp and unconformable. The basal sandstones of the Upper Member cut about 1.5 meters (4.9 ft) into the beds of the middle member. In inaccessible exposures, conglomerate of the Upper Member appears to unconformably overly the middle member. [5] [8]

The Upper Member of the Sixtymile Formation, which is about 12 meters (39 ft), consists of fine-grained fluvial and fanglomeratic sandstone that grades abruptly into sandy conglomerate toward the axis of the syncline. It is only preserved in a narrow trough-shaped area in the center of the Chuar Syncline. The sandstone is pale red to brown, and contains scattered rock fragments. These rock fragments include chalky-white chert derived from the middle member. Some of the sandstone exhibits fluvial crossbedding and is fanglomeratic. Massive weathering maroon conglomerate is present in the lower part of the Upper Member. It grades laterally into the sandstone. [5] [8]

Contacts

The basal contact of the Walcott Member with the overlying Sixtymile Formation is conformable. Typically, the contact between the underlying black shale typical of the Walcott and overlying basal red sandstones of the Sixtymile Formation consists of a 1.5 meters (4.9 ft) interval that is gradational in nature. The transition beds and the basal laminated red sandstone of the Sixtymile Formation lack any fragmental or exotic debris, unlike the overlying strata. These field relations thus indicate that deposition was continuous across the Chuar Group–Sixtymile boundary. [5] [6] [8]

The upper contact of the Sixtymile Formation is a disconformity that laterally becomes an angular unconformity. Within the center of the Chuar Syncline, this upper contact is a disconformity. Away from the center, the strata of the Sixtymile Formation, an angular discordance between the formations of 6° to 10° can be seen. This unconformity has an irregular hilly surface. In case of the Sixtymile Formation, a small amount of relief, 20 meters (66 ft) to 30 meters (98 ft), in the Sixtymile Canyon area was the result of resistance to erosion of the Middle Member of the Sixtymile Formation (fig. 4). Local erosion removed all of the Upper Member of the Sixtymile from all but the central part of the Chuar syncline prior to the deposition of the Tapeats Sandstone [5] [6] [8]

Fossils

No fossils have been reported from the Sixtymile Formation. [5] [6] [8]

Depositional environment

The strata of the Sixtymile Formation records the accumulation of sediments adjacent to an active fault scarp. The sandstones and siltstones of the Lower Member are inferred to have accumulated within a lake occupying a basin formed by subsidence of the Chuar Syncline. The breccias and blocks of dolomite are regarded to be landslide deposits created by the collapse of an active fault scarp associated with the Butte fault zone. The sediments of the Middle Member are inferred to have accumulated in standing water, presumably a lake along the axis of the Chuar syncline as indicated by its very fine grain size, the thin regular bedding, and its bedded chert. The Upper Member consists of fine-grained fluvial and fanglomeratic sandstone and conglomerate that were deposited by a stream that once flowed along the trough of the Chuar Syncline. The Sixtymile Formation provides dramatic evidence of active faulting along the Butte fault system. [5] [6] [8]

Age

In 2000, the radiometric dating of volcanic ash within the uppermost Walcott Member of the Chuar Group, 1 meter (3.3 ft) below the base of the Sixtymile Formation provided a maximum age for its deposition. The U-Pb date of seven zircon fractions, including four single grains, yielded a date of 742±6 million years ago. Thus, the Sixtymile Formation accumulated after 742±6 Ma. [9]

However, in 2018, the dating of detrital zircons by Karl Karlstrom [1] established that the Sixtymile Formation as being Cambrian in age, between 520 and 509 million years old. Thus, the Sixtymile Formation accumulated in lacustrine, fluvial, and shallow marine environment and are preserved within narrow, fault-controlled basins contemporaneous with the accumulation strata of the lower Tonto Group in the western Grand Canyon and Lake Mead regions. [10] [2] As a result, the Sixtymile Formation can be considered a member of the Tonto Group. [1] [11]

See also

Related Research Articles

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<span class="mw-page-title-main">Tonto Group</span> Cambrian geologic unit in the Grand Canyon region, Arizona

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<span class="mw-page-title-main">Bass Formation</span> Lithostratigraphic unit found in Arizona, US

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.

<span class="mw-page-title-main">Shinumo Quartzite</span> Mesoproterozoic rock formation in the Grand Canyon, Arizona

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 typically brightly-colored strata of the underlying slope-forming 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.

<span class="mw-page-title-main">Dox Formation</span> Landform in the Grand Canyon, Arizona

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.

<span class="mw-page-title-main">Vishnu Basement Rocks</span> Lithostratigraphic unit in the Grand Canyon, Arizona

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.

<span class="mw-page-title-main">Bright Angel Shale</span> Cambrian geologic formation found in the Southwestern United States

The Bright Angel Shale is one of five geological formations that comprise the Cambrian Tonto Group. It and the other formations of the Tonto Group outcrop in the Grand Canyon, Arizona, and parts of northern Arizona, central Arizona, southeast California, southern Nevada, and southeast Utah. The Bright Angel Shale consists of locally fossiliferous, green and red-brown, micaceous, fissile shale (mudstone) and siltstone with local, thicker beds of brown to tan sandstone and limestone. It ranges in thickness from 57 to 450 feet. Typically, its thin-bedded shales and sandstones are interbedded in cm-scale cycles. They also exhibit abundant sedimentary structures that include current, oscillation, and interference ripples. The Bright Angel Shale also gradually grades downward into the underlying Tapeats Sandstone. It also complexly interfingers with the overlying Muav Limestone. These characters make the upper and lower contacts of the Bright Angel Shale often difficult to define. Typically, its thin-bedded shales and sandstones erode into green and red-brown slopes that rise from the Tonto Platform up to cliffs formed by limestones of the overlying Muav Limestone and dolomites of the Frenchman Mountain Dolostone.

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.

The geology of Lesotho is built on ancient crystalline basement rock up to 3.6 billion years old, belonging to the Kaapvaal Craton, a section of stable primordial crust. Most of the rocks in the country are sedimentary or volcanic units, belonging to the Karoo Supergroup. The country is notable for large fossil deposits and intense erosion due to high rainfall and a rare case of southern African glaciation during the last ice age. Lesotho has extensive diamonds and other natural resources and has the highest concentration of kimberlite pipes anywhere in the world.

References

  1. 1 2 3 4 5 Karlstrom, KE, Hagadorn, J, Gehrels, G, Matthews, W, Schmitz, MD, Madronich, L, Mulder, J, Pecha, M, Giesler, D and Crossey, L (2018) Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons. Nature Geoscience. 11(6):438–443.
  2. 1 2 3 4 Karlstrom, KE, Mohr, MT, Schmitz, MD, Sundberg, FA, Rowland, SM, Blakey, R, Foster, JR, Crossey, LJ, Dehler, CM and Hagadorn, JW (2020) Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale. Geology. 48(5):425–430.
  3. 1 2 Ford, TD, WJ Breed, and JW Mitchell (1972) Name and age of the upper Precambrian basalts in the eastern Grand Canyon. Geological Society of America Bulletin. 83(1):223–226.
  4. 1 2 Ford, TD, and WJ Breed (1973) Late Precambrian Chuar Group, Grand Canyon, Arizona. Geological Society of America Bulletin. 84(4):1243–1260.
  5. 1 2 3 4 5 6 7 8 9 Elston, DP (1979) Late Precambrian Sixtymile Formation and orogeny at the top of the Grand Canyon Supergroup, northern Arizona. Professional Paper no. 1092, U.S. Geological Survey, Reston, Virginia. 20 pp.
  6. 1 2 3 4 5 6 7 Dehler, CM, SM Porter, and JM Timmons (2012) The Neoproterozoic Earth system revealed from the Chuar Group of Grand Canyon., in JM Timmons and KE Karlstrom, eds., pp. 49–72, Grand Canyon Geology: Two Billion Years of Earth's History. Special Paper no. 489. Geological Society of America, Boulder, Colorado.
  7. ↑ Timmons, J. M., K. E. Karlstrom, C. M. Dehler, J. W. Geissman, and M. T. 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. 11(2): 163–181.
  8. 1 2 3 4 5 6 7 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.
  9. ↑ Karlstrom, KE, SA Bowring, CM Dehler, AH Knoll, SM Porter, DJ Des Marais, AB Weil, ZD Sharp, JW Geissman, MB Elrick, JM Timmons, LJ Crossey, and KL Davidek (2000) Chuar Group of the Grand Canyon: Record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma. Geology. 28(7):619–622.
  10. ↑ Santucci VL and Tweet JS (ed). 2020. Grand Canyon National Park: Centennial paleontological resource inventory (non-sensitive version). Natural Resource Report. NPS/GRCA/NRR—2020/2103. National Park Service. Fort Collins, Colorado. p. 601.
  11. ↑ Karlstrom, KE and Crossey, LJ (2019) Classic Rock Tours 3. Grand Canyon Geology, One Hundred and Fifty Years after John Wesley Powell: A Geology Guide for Visiting the South Rim of Grand Canyon National Park. Geoscience Canada. 46(4):163–193.
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