Nankoweap Formation

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Nankoweap Formation
Stratigraphic range: Neoproterozoic, <770 Ma
View from Lipan Point.jpg
Venus Temple landform, [1] [2] NNW of Lipan Point (East Rim); tilted (striped & multilayered) Nankoweap Formation above tilted (blackish) Cardenas Basalt, above (tilted and reddish) Dox Formation.
Type Geological formation
Unit of Grand Canyon Supergroup
Underlies Chuar Group and, as part of the Great Unconformity, the Tapeats Sandstone
Overlies Cardenas Basalt
Thickness370 feet (110 m) approximate maximum
Lithology
Primary sandstone, siltstone & shale (red beds)
Location
Region Arizona east Grand Canyon Lava Butte region on Colorado River, near Lipan Point
Country United States
(Southwestern United States)
Extent(eastern) Grand Canyon, Colorado River region
Type section
Named forNankoweap Canyon
Named byVan Gundy (1934), [3] Van Gundy (1951), [4] and Maxson (1961) [5]

The Neoproterozoic Nankoweap Formation (pronounced Nan' coe weep), 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. [6] [7] [8]

Contents

The strata of the Nankoweap Formation are exposed in a small area that occupies an area from just south of Carbon Canyon to Basalt Canyon on the west bank of the Colorado River around Comanche Creek and Tanner Canyon on the east bank of the Colorado River within the eastern Grand Canyon, Coconino County, Arizona. It and associated strata of the Unkar and Chuar Groups are preserved in a prominent syncline and fault block. The most complete, readily accessible, and easily studied, exposure of the Nankoweap Formation occurs in Basalt Canyon. [6] [7]

The nomenclature of the Nankoweap Formation has changed over time. Originally, the strata of the Nankoweap Formation was included in-part in the top of the "Unkar terrane" (Group) and in-part in the basal “Chuar terrane” (Group) by Walcott in 1894. [9] Van Gundy first recognized the thin sequence of red beds unconformably overlying basaltic flows of the Unkar Group as a separate stratigraphic unit, which he called the “Nankoweap Group.“ [3]

Close-up of Apollo Temple Precambrian - Cambrian Unconformity in Grand Canyon.jpg
Close-up of Apollo Temple

Although more extensive outcrops exist in Basalt Canyon, Comanche Creek, and Tanner Canyon, it is named for a small, fault-bounded block of the Nankoweap Formation in Nankoweap Canyon. It was argued that these redbeds belonged neither to the underlying Unkar Group nor overlying Chuar Group. [4] Maxson in his geological mapping reduced the Nankoweap Group to formational rank because the Nankoweap had not been subdivided into formations. [5] Later, a locally preserved lower member (ferruginous unit and weathered zone) that unconformably overlies the Cardenas Basalt and is unconformably overlain by the upper member of the Nankoweap – was recognized. [6] [7]

Description

The Nankoweap Formation is slightly more than 100 m in thickness. Currently, it is informally divided into two informal members, the lower (ferruginous) member and the upper member. These members are separated and enclosed by unconformities. [3] [4] [6] [7]

The lower (ferruginous) member of the Nankoweap Formation consists of thin, 15 m or less thick, erosional remnants that overlie deeply eroded and often deeply weathered basaltic lava flows of the Cardenas Basalt. Within the Grand Canyon, the lower member of the Nankoweap Formation outcrops at only two locations adjacent to the trace of the north-south-trending Butte Fault. First, it outcrops just north of the Colorado River in the eastern side of the Basalt graben at Tanner Canyon Rapids. At Tanner Canyon Rapids, the lower member consists of red, highly resistant, hematite-cemented, quartzitic sandstones and siltstones that have a maximum thickness of about 15 m and disappears within about 200 m west of the Butte Fault between the upper member of the Nankoweap Formation and the eroded surface of the Cardenas Basalt. Finally, it outcrops 2 km due south of the Colorado River near and at the southern limit of preservation of the Cardenas Basalt and Nankoweap Formation. At this location, it consists of a thin layer of highly ferruginous sandstone that is composed of sediments derived from the underlying Cardenas Basalt and a 10 m thick ferruginous weathered zone (paleosol) developed in the eroded surface of the Cardenas Basalt. [6] [7] [8] [10]

The upper member of the Nankoweap Formation consists of about 100 m of generally reddish-purple, fine-grained, quartzitic sandstones. They are shaley and silty towards the top. The sandstones sometime contain conglomeritic horizons composed of rounded, disc-shaped pebbles of Dox Sandstone and occasional pebbles of basalt lava. A lag deposit of gravel composed of white to yellowish, small pebble to cobble size chert gravel occurs locally at the base of the upper member. Typically, the sandstone sections are thin-to-medium bedded and exhibit planar tabular and trough cross-bedding, ripple marks, mudcracks, numerous soft-sediment deformation structures, and rare salt pseudomorphs. Towards the top of the upper member, massive, meter-scale sandstone beds become common. [6] [7] [8] [10]

Nature of contacts

Unconformities mark both the base and, possibly, the top of the Nankoweap Formation. The red beds of the Nankoweap Formation unconformably overlie the Cardenas Lavas of the Unkar Group. At most places, the upper member of the Nankoweap Formation directly overlies the Cardenas Basalt. In Basalt Canyon, an angular discordance in this lower contact reflects the erosion of 60 m of Cardenas Basalt has been recognized. West of Tanner Canyon, erosion has locally removed as much as 300 m of Cardenas Basalt before the deposition of the Nankoweap Formation. As demonstrated by a 10 m thick ferruginous weathered zone (paleosol), deep chemical weathering of the exposed lava flows of the Cardenas Basalt occurred before deposition of the Nankoweap Formation west of Tanner Canyon. The deeply weathered lavas retain their original textures but have been pervasively stained and altered to earthy hematite and siderite. [10]

The upper contact of the Nankoweap Formation with the base of the Galeros Formation of the Chuar Group appears to be quite sharp. The upper part of the Nankoweap Formation consists of cliff-forming sandstones that grade irregularly upward – from red to white. This change in color has been interpreted to reflect bleaching beneath an unconformity that separates red beds of the Nankoweap Formation from dark-gray dolomites of the Galeros Formation. Because this contact lies in the middle of sheer cliffs formed by the combined sandstones of the Nankoweap Formation and the overlying dolomites of the Galeros Formation, it is not easily accessible and little is known about it. The inaccessibility of the upper contact of the Nankoweap Formation has prevented a close examination of it. Except for wide shallow channels cut in the Nankoweap Formation and filled with dolomite, the upper contact lacks any stratigraphic evidence that indicates a large erosional or temporal hiatus. [10] [11] The contact between the overlying Tapeats Sandstone and the folded and faulted Nankoweap Formation is a prominent angular unconformity that is part of the Great Unconformity. [7]

Uranium-lead dating of detrital zircons from the Nankoweap Formation indicate the presence of zircon grains in approximately the 800–770 Ma age range. This finding indicates that the Nankoweap Formation is closely related in age and stratigraphy to the Chuar Group – and any unconformity associated with its upper contact is relatively minor in duration and significance. Thus, the Nankoweap Formation is most likely the basal unit of the Chuar Group. [12]

Depositional environments

The depositional environments of the Nankoweap Formation remain unclear and poorly defined. The deeply weathered paleosol developed in the Cardenas Basalt represents an extended period of subaerial weathering prior to the deposition of the Nankoweap Formation. The sandstones of its lower member are also quite weathered and, thus, appear to have accumulated subaerially. The sedimentary structures in the upper member of the Nankoweap Formation are interpreted to indicate that it accumulated beneath moderate- to low-energy, shallow waters, either in a shallow sea or lake. Strata within the upper part of the Nankoweap Formation is inferred to have accumulated in sand and mudflats. Also, cross-bedded sandstone layers within the upper member of the Nankoweap Formation are argued to be beach deposits. [6] [7] [10]

Fossils

Van Gundy [3] [4] identified a structure found in a sandstone bed of the Nankoweap Formation in Basalt Canyon as a trace fossil impression of a stranded jellyfish. This structure is approximately 12 cm in diameter and consists of a series of radiating lobes, rounded at their ends. Some of these lobes have a median groove radiating from a small, irregular hollow. Initially, later studies also considered this to be a jellyfish impression and it was eventually named “Brooksella canyonensis” by Bassler. [13] Later, Cloud [14] [15] obtained a partial second specimen and argued that both specimens were of inorganic origin and formed by "compaction of fine sands deposited over a compressible but otherwise unidentifiable structure, possibly a small gas blister." Paleontologists, who were unconvinced by Cloud's interpretation, reinterpretated this structure to be a burrow (trace fossil), known as Asterosoma,“ made by a sediment feeding, worm-like organism. Other paleontologists, who later reexamined both specimens, argued that they are inorganic, sedimentary structures similar to small "sand-volcanoes" formed by the upward expulsion of gas or fluid from sediments as more sediment is loaded on top or as the sediment is shaken during seismic activity. Both specimens are very similar in morphology to sedimentary structures initially interpreted to be fossil jellyfish and named “Astropolithon.” Like Brooksella canyonensis,Astropolithon is now regarded to be the result of the venting of fluidized sand into surficial sediments blanketed by microbial mats that were typical of Precambrian sea- and lake-bottoms. Very similar sedimentary structures have been observed in a thick tsunami deposit related to the asteroid impact at the Cretaceous–Paleogene boundary [7] [16]

Age (basal Chuar Group theory)

The Nankoweap Formation has not been directly dated using radiometric dating techniques. It is younger than the age of the Cardenas Basalt, which erupted about 1,104 million years ago. [8] [17] The Nankoweap Formation was presumed to be older than the 800 to 740 Ma strata that comprises the overlying Chuar Group. [18] However, uranium-lead dating of detrital zircons from the Nankoweap Formation found it contains detrital zircons that are approximately 800–770 Ma in age. This finding indicates that the Nankoweap Formation is younger than 770 Ma; it is closely related in age to the Chuar Group – and quite likely is the basal member of this group. [12]

Barite deposits

Within Nankoweap Canyon, prospect pits have been dug into numerous patches of sandstone that have been replaced by hematite. In these prospect pits, barite cemented sandstone, and thin barite veins, have been found. A few of these patches exceed 10 feet in width. The sandstones containing these patches are highly deformed and cut by numerous small faults. [19]

Nankoweap Formation above dark black Tanner Graben (riverside above Tanner Rapid) In the canyon.jpg
Nankoweap Formation above dark black Tanner Graben (riverside above Tanner Rapid)

Nankoweap Creek and Tanner Graben

Nankoweap Creek is located upstream from the Lipan Point–Basalt Creek section. The creek is west of Marble Canyon, and enters the canyon near Lower Marble Canyon.

The Tanner Graben, a downdropped graben of Dox Formation and Cardenas Basalt is topped by a bright, multilayered section of the Nankoweap Formation. It lies opposite Tanner Creek-(Tanner Canyon) which is the source for the Tanner Rapid, at the foot of Tanner Graben.

See also

Related Research Articles

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Great Unconformity Gap in geological strata

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Moenkopi Formation Geologic feature in the Southwestern United States

The Moenkopi Formation is a geological formation that is spread across the U.S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a group in Arizona. Part of the Colorado Plateau and Basin and Range, this red sandstone was laid down in the Lower Triassic and possibly part of the Middle Triassic, around 240 million years ago.

Tonto Group Landform in the Grand Canyon region, Arizona

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Muav Limestone Geologic strata in Southwestern United States

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 Geologic unit of the Grand Canyon, Arizona

The Cambrian Tapeats Sandstone is the lower geologic unit, about 230 feet (70 m) thick, at its maximum, of the 3-member Tonto Group. It is famous for being the highly-resistant mostly-horizontal unit above the 1,000 my, Great Unconformity expressed areally in the Grand Canyon of Arizona; also in other areas of Arizona and adjacent Nevada.

Cardenas Basalt Rock formation in the Grand Canyon, Arizona

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 Sequence of geologic strata of Proterozoic age

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.

Grand Canyon Supergroup Sequence of sedimentary strata

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.

Tanner Graben Landform in the Grand Canyon, Arizona

Located directly downstream of the Little Colorado River confluence with the Colorado River, the Tanner Graben, in the Grand Canyon, Arizona, US 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.

Hakatai Shale Mesoproterozoic rock formation

The Hakatai Shale is a Mesoproterozoic rock formation with important exposures in the Grand Canyon, Coconino County, Arizona. It consists of colorful strata that exhibit colors varying from purple to red to brilliant orange. These 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. 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. 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.

Bass Formation 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.

Shinumo Quartzite 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.

Dox Formation 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.

Vishnu Basement Rocks 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.

Bright Angel Shale Geologic feature

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 Cambrian geologic accumulation found in Arizona

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.

Malgosa Crest Landform in the Grand Canyon, Arizona

Malgosa Crest is a 5,446-foot-elevation summit located in the eastern Grand Canyon, in Coconino County of northern Arizona, US. The ridgeline-crest is situated adjacent to the East Rim, being a middle, and minor prominence,, along the Butte Fault. From south to north, and bordering the due-south flowing Colorado River, are Chuar Butte, Awatubi Crest, Kwagunt Butte, Malgosa Crest, and the Nankoweap Mesa. All the prominences are near the end of Marble Canyon,, Marble Canyon being the start of the Grand Canyon.

Ochoa Point Landform in the Grand Canyon, Arizona

Ochoa Point is a 4,761-foot-elevation cliff-summit located in the eastern Grand Canyon, in Coconino County of northern Arizona, US. The landform is on a southeast ridgeline from Apollo Temple, with the Ochoa Point prominence on its southeast terminus. Ochoa Point is 1.0 mi from Apollo Temple, 1.5 mi northwest from the southeast-flowing Colorado River, and 3.5 miles due-west from the south terminus of the East Rim, Grand Canyon.

Ochoa Point’s southwest cliff-flank, and Apollo Temple’s southwest arm, contain the dp-black Basalt Cliffs ; the Cardenas Basalt lies upon brilliantly colored reddish Dox Formation low-angle, erosion-slopes of five Unkar Group members. What makes Ochoa Point distinctive, the next rock unit above is the colorful, layered (banded), Nankoweap Formation. These rock layers all slope at approximately 15 degrees, and are topped by the short-cliff, horizontal Tapeats Sandstone.

References

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