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
Thickness110 m (360 ft) 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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<span class="mw-page-title-main">Grand Canyon Supergroup</span> Sequence of sedimentary strata

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<span class="mw-page-title-main">Hakatai Shale</span> 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.

<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 Dox Formation comprises the bulk of the Unkar Group, the older subdivision of the Grand Canyon Supergroup. The Unkar Group is about 1,600 to 2,200 m thick and composed of, in ascending order, the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. The Unkar Group is overlain in ascending order 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. The entire Grand Canyon Supergroup overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.

<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 ft. 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.

<span class="mw-page-title-main">Sixtymile Formation</span> Cambrian geologic formation found in Grand Canyon, 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 m (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion prior to deposition of the Tapeats Sandstone.

<span class="mw-page-title-main">Ochoa Point</span> 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

  1. ↑ Blakey, R, and W Ranney (2008) Ancient Landscapes of the Colorado Plateau. Grand Canyon Association, Grand Canyon, Arizona. 176 pp, ISBN   978-1934656037
  2. ↑ DeLorme Mapping Company (2002) Arizona atlas & gazetteer : detailed topographic maps, back roads, BLM, state lands, GPS grids, 5th ed. DeLorme Mapping Company, Yarmouth, Maine.
  3. 1 2 3 4 Van Gundy, C.E. (1934) Some observations of the Unkar Group of the Grand Canyon Algonkian. Grand Canyon Nature Notes. 9(8):338–349.
  4. 1 2 3 4 Van Gundy, CE (1951) Nankoweap group of the Grand Canyon Algonkian of Arizona. Geological Society of America Bulletin 62(8):953–959.
  5. 1 2 Maxson J.H. (1961) Geologic history of the Bright Angel quadrangle. in J.H. Maxson and F. Matthes, compilers, Geologic map of the Bright Angel quadrangle. Canyon Natural History Association, Grand Canyon, Arizona.
  6. 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.
  7. 1 2 3 4 5 6 7 8 9 Ford, TD, and CM Dehler (2003) Grand Canyon Supergroup; Nankoweap Formation, Chuar Group, and Sixtymile Formation. Grand Canyon geology. In SS Beus and M Morales, eds., pp. 53–75, Grand Canyon Geology, 2nd ed. Oxford University Press, New York. ISBN   0195122984
  8. 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.
  9. ↑ Walcott, CD (1894) Precambrian igneous rocks of the Unkar terrane, Grand Canyon of the Colorado. 14th Annual Report for 1892/3, part 2, pp. 492–519, United States Geological Survey, Reston, Virginia.
  10. 1 2 3 4 5 Elston, DP, and GR Scott (1976) Unconformity at the Cardenas-Nankoweap contact (Precambrian), Grand Canyon Supergroup, northern Arizona. Geological Society of America Bulletin. v. 87(12):1763–1772
  11. ↑ Ford, TD, and WJ Breed (1973) Late Precambrian Chuar Group, Grand Canyon, Arizona. Geological Society of America Bulletin, Vol. 84, Issue 4, pp. 1243–1260.
  12. 1 2 Dehler, CM, KE Karlstrom, GE Gehrels, JM Timmons, and LJ Crossey (2012) Stratigraphic revision, provenance, and new age constraints of the Nankoweap Formation and Chuar Group, Grand Canyon Supergroup, Grand Canyon, Arizona. Geological Society of America, Abstracts with Programs, 44(6):82.
  13. ↑ Bassler, RS, (1941) A supposed jellyfish from the pre-Cambrian of the Grand Canyon. Proceedings of the United States National Museum. 89(3104):519–522.
  14. ↑ Cloud, PE, Jr. (1960) Gas as a sedimentary and diagenetic agent. American Journal of Science. 258-A:35–45.
  15. ↑ Cloud, PE, Jr. (1968) Pre-metazoan evolution and the origins of the Metazoa. In ET Drake, ed., pp. 1–72, Evolution and environment. Yale University Press New Haven, Connecticut.
  16. ↑ Seilacher, A (2007) Trace Fossil Analysis. Springer-Verlag, Heidelberg, Germany. 226 pp. ISBN   978-3-540-47225-4
  17. ↑ 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.
  18. ↑ 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 294, Geological Society of America, Boulder, Colorado.
  19. ↑ Ford, TD, WJ Breed, and C. Downie (1969) Preliminary geologic report of the Chuar Group, Grand Canyon, Arizona. In DL Baars, ed., pp. 114–121, Geology and natural history of the Grand Canyon region – 5th Field Conference, Powell Centennial River Expedition. Four Corners Geological Society, Durango, Colorado.
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