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75-595: The Bass Formation is the basal part of the Unkar Group . The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) 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 (371 to 492 ft) thick;

150-604: A bivalve of unknown affinity, which were reported from the Bass Formation, are now judged to be rounded mud-flakes or pellets that are likely oncolites of algal origin. The lithology and sedimentary structures observed in the Bass Limestone indicate that, except for the Hotauta Member, it accumulated beneath a sea that transgressed from the west. The Hotauta Member consists of fluvial sands and gravels that accumulated within valleys on an eroded surface of

225-412: A marine transgression from the west. The dolomite of the Bass Formation likely was originally deposited as limestone and subsequently altered to dolomite by later diagenesis . This limestone largely accumulated in clear, relatively warm, shallow marine waters by both biological and abiological processes. During the maximum, deepest, incursion of marine waters, limestone and deep-water mud accumulated in

300-435: A shoreline is lacking. Various types of other fossils, i.e. jellyfish , sponges , worm trails, and bivalves have been reported from the Bass Formation. Critical examinations of these reported fossils have concluded that the fossil sponges are inorganic silica concretions ; the jellyfish are either gas escape structures or algal colonies; and the worm trails are inorganic sedimentary structures. In addition, fossils of

375-535: A common source with the pyroclastic deposits and lavas of the Cardenas Basalt. The isochron ages of these sills and dikes and the Cardenas Basalt lavas are basically identical. Only sills are exposed in outcrops of the Bass Formation and Hakatai Shale. These sills range in thickness from 23 m (75 ft) at Hance Rapids , eastern Grand Canyon, to 300 m (980 ft) in Hakatai Canyon in

450-522: A dark-reddish brown, structureless, ferruginous sediment that is usually a few centimeters to 30 cm (0.98 ft) thick. This contact is regarded as a classic example of an ancient peneplain . In the eastern part of the Grand Canyon, the contact between the Bass Formation and the overlying Hatakai Shale is typically gradational over an interval of a meter or so. For example, in Red Canyon,

525-607: A depth of 10 km (6.2 mi) to the Earth's surface between 1300 and 1250 Ma. Thus, the surface on which the Unkar Group accumulated, is about 1250 Ma old, and the Unkar Group that buried it, is younger. The maximum age of the Unkar Group is also established by uranium-lead (U-Pb) dating of zircons from an ash bed from the Bass Formation in the basal Unkar Group of 1254 Ma near river Mile 78 ( List of Colorado River rapids and features ). The dating of detrital zircons from

600-527: A heterogeneous mixture of light-tan to greenish brown, siliceous quartz sandstone; calcareous lithic and arkosic sandstone; dark-brown-to-green shale and mudstone; red mudstone, siltstone, and quartz sandstone; sandy argillite; micaceous mudstone; and red quartzose, silty sandstone. In ascending order, these sediments have been subdivided into the Escalante Creek, Solomon Temple, Comanche Point, and Ochoa Point Members. Stromatolites have been reported from

675-453: A number of small, dark dikes . In the area of Desert View and west of Palisades of the Desert, the basaltic sills form very prominent, dark gray cliffs. Using gravity and aeromagnetic data, combined with gravity modeling, it was inferred that Proterozoic grabens, and half-grabens filled with strata of the Unkar Group – lie buried beneath Phanerozoic rocks in northern Arizona that surround

750-581: A relief of about 6 m (20 ft) in the Shinuino 15-minute topographic quadrangle and 15 m (49 ft) in both the Bright Angel and Vishnu 15-minute topographic quadrangles. In Hotauta Canyon, and at Granite Narrows, this surface is extremely smooth with a relief of only a few meters. The greatest relief on this surface can be seen opposite the mouth of Shinumo Creek where low rounded hills of Vishnu Basement Rocks rise 6 m (20 ft) above

825-515: A relief of about 6 m (20 ft) in the Shinumo 15-minute topographic quadrangle and 15 m (49 ft) in the Bright Angel and Vishnu 15-minute topographic quadrangles. In Hotauta Canyon, and at Granite Narrows, this surface is extremely smooth with a relief of only a few meters. The greatest relief on this surface can be seen opposite the mouth of Shinumo Creek where low rounded hills of Vishnu Basement Rocks rise 6 m (20 ft) above

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900-512: A shallow, near-shore marine environment, coastal plain mudflats , and deltas that marked the beginning of Hakatai Shale deposition – dominated the Grand Canyon area. The uranium-lead dating of zircons from an ash bed in the Bass Formation, argon–argon dating of overlying igneous rocks, and thermochronologic studies of the underling Vishnu Basement Rocks – constrain the age of the Bass Formation and associated Unkar Group . Zircons from air-fallen, volcanic ash beds in its lower part yielded

975-404: A uranium-lead date of 1,254.8 ± 1.6 Ma. This date is consistent with radiometric ages of Precambrian strata interpreted to be correlative with the Bass Formation and its age estimated from earlier paleomagnetic studies. In addition, this date is consistent with the younger Cardenas Basalt having been erupted about 1,104 Ma. Finally, the uranium-lead dating of zircons is consistent with studies of

1050-403: Is 80 m (260 ft). It is 100 m (330 ft) thick at Phantom Creek (north side of Isis Temple , Cheops Pyramid, & Utah Flats), and 57 m (187 ft) thick at Crystal Creek. The thin section of Bass Formation at Crystal Creek likely reflects the presence of a Vishnu Basement Rocks topographic high in the paleosurface on which it accumulated. The base of the Bass Formation

1125-819: Is a hiking trail in the North Rim side of the Grand Canyon , in Grand Canyon National Park , located in the U.S. state of Arizona . Access to this part of the park by car is seasonal, open from mid-May to mid-October or depending on snow cover from the previous winter. It is possible to reach the North Kaibab Trailhead by crossing the canyon on foot from the South Rim or by snowshoe or cross-country ski beginning at Jacob Lake, Arizona . The North Kaibab Trail begins at

1200-549: Is a major unconformity that also forms the base of the Grand Canyon Supergroup and the Unkar Group. This unconformity separates the underlying and deeply eroded crystalline basement , which consists of granites, gneisses, pegmatites, and schists of the Vishnu Basement Rocks, from stratified Proterozoic rocks of the Grand Canyon Supergroup. This contact is a remarkably smooth surface that has

1275-656: Is a sequence of strata of Proterozoic age that are subdivided into five geologic formations and exposed within the Grand Canyon , Arizona , Southwestern United States . The Unkar Group is the basal formation of the Grand Canyon Supergroup . The Unkar is about 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 . The Cardenas Basalt and Dox Formation are found mostly in

1350-619: Is available seasonally at the Supai Tunnel, Roaring Springs, the Caretaker's Dwelling, and Cottonwood Campground; and year-round at Bright Angel Campground and Phantom Ranch . Features along the trail include Roaring Springs, Ribbon Falls (140 ft), The Box (a slot canyon), and Phantom Ranch . The North Kaibab Trail is also part of the Arizona Trail system, crossing the state of Arizona from Mexico to Utah . The trail

1425-400: Is estimated to represent a period of about 75 million years. The age and exhumation history of the underlying Vishnu Basement Rocks constrain the age of the Unkar Group. Radiometric dating of these basement rocks demonstrate that these basement rocks underwent metamorphism and deformation at mid-crustal depths of about 20–25 km (12–16 mi) between about 1840 and 1660 Ma ago. Prior to

1500-635: Is joined by South Kaibab Trail which is located to the south of the trail. Grand Canyon National Park categorizes the North Kaibab Trail as a corridor trail . With this designation it receives regular maintenance and patrols by park rangers. Hikers may only camp at the Bright Angel or Cottonwood Campgrounds, where they can stay overnight with a permit issued by the Grand Canyon National Park Backcountry Information Center. Use of

1575-560: Is marked by the Hotauta Member of the Bass Formation. It is a prominent discontinuous, conglomerate, initially named the Hotauta Conglomerate and later redesignated the Hotauta Member. This conglomerate consists of rounded, pebble to cobble -sized clasts of chert , granite, quartz , plagioclase crystals , and micro pegmatites in a quartz sand matrix. About 80% of the gravel-size clasts consist of granite and quartzite . The quartzite gravel lacks any local equivalents in

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1650-634: 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, (today seen as ancient & weathered 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

1725-574: The Shinumo Quartzite consists characteristically of beds that are red, brown, or purple sedimentary quartzites and lesser massive white, red, or purple sandstone; also conglomeratic sandstone. Within these cliff-forming sandstones, mudstone-rich intervals occur. Some of these sandstone beds exhibit well-developed soft-sediment deformation structures. No fossils have been found in the Shinumo Quartzite. The lower and middle parts of

1800-542: 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 Bass Formation is the lowermost formation, overlies deeply eroded granites , gneisses , pegmatites , and schists that comprise Vishnu Basement Rocks . There has been some discussion about the nomenclature of the Bass Formation. It

1875-473: The Hotauta Conglomerate in 1914 as a separate unit of formation status for Hotauta Canyon. It was later included in the Bass Formation as a member where it has remained. The Bass Formation consists of interbedded sandstone ( arkose ), and silty sandstone, prominent interbeds of conglomerate and dolomite, and subordinate interbeds of argillite and limestone. Dolomite and sandy dolomite are

1950-490: The Shinumo Creek area. Basaltic sills form very prominent, dark gray cliffs in the area below Desert View and west of Palisades of the Desert. Exposures of the Shinumo Quartzite, and Dox Formation expose several basaltic dikes. The feeder dikes to the basaltic sills are not exposed. However, the feeder dikes for the Cardenas Basalt can be traced, discontinuously, to within a few meters of their bases. The base of

2025-475: 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 Grand Canyon Supergroup. In general, the strata comprising the Unkar Group dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of

2100-558: The marine transgression that deposited the Tapeats Sandstone (Tapeats Sea), and other members of the Tonto Group. Within the Unkar Group, the contact between the Hakatai Shale and overlying Shinumo Sandstone is a distinct disconformity. This contact is the only significant unconformity that occurs within the Unkar Group. This disconformity is sharp and locally truncates cross-bedding and channels exhibited by sandstones in

2175-437: The Bass Formation have yielded dates of about 1200 Ma. These dates suggest that the 1254 Ma age may be a bit too old. However, these zircons likely underwent some lead loss, and, as a result these apparent dates are younger than their actual ages. As a result, the 1254 Ma dates remain the best estimate for the age of initial deposition of the Unkar Group. Geologists have attempted to date the Cardenas Basalt for many years. On

2250-440: The Bass Formation so far. The environment, in which these stromatolites grew, as judged by the associated sediments, was one of quiet, shallow marine waters. The common presence of ripple-marks and mud-cracks suggest intermittent desiccation. Thin layers of flake-breccia associated with them indicate occasional periods of turbulence of brief duration. However, direct evidence specifically indicating an intertidal environment close to

2325-461: The Bass Formation, metasomatism and recrystallization have produced chrysotile asbestos , both above and below the sills. Veins of asbestos with fibers up to 10 cm (3.9 in) in length, often commonly occur within 3 m (9.8 ft) of the upper and lower contacts of the sills. The Bass Limestone shows generally, an increase in thickness to the northwest ranging from 37 (121 ft) to 104 m (341 ft) thick. Its average thickness

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2400-519: The Bass formation have been affected by post-depositional alteration including: dolomitization , recrystallization, stylolitization , and silicification . Basaltic sills and dikes intrude all strata of the Unkar Group that underlie the Cardenas Lava. Only sills, whose feeder dikes are not exposed, can be observed intruding the Bass Formation. Where the sills have intruded siliceous dolomites in

2475-418: The Cardenas Basalt consists of cliff-forming basaltic and andesitic lava flows that are interbedded with beds of breccia, sandstone, and lapillite. No fossils have been found in the Cardenas Basalt. The Cardenas Basalt was formed by the subaerial eruption of basaltic and andesitic magma in wet coastal environments such as river deltas or tidal flats. Angular unconformities of vastly differing magnitudes separate

2550-425: The Cardenas Basalt from the overlying Nankoweap Formation and Tonto Group . Mafic sills and dikes ( basalt resp. diabase ) intrude all strata of the Unkar Group below the Cardenas Basalt. They consist of black, medium- to coarse-grained, olivine-rich basalt that contains plagioclase , olivine , clinopyroxene , magnetite - ilmenite , and biotite . Their chemical composition indicates that they share

2625-573: The Comanche Point Member. The Dox Formation locally interfingers with, and is baked by, basalt lava flows of the overlying Cardenas Basalt. Within the central Grand Canyon, pre-Tapeats Sandstone erosion has removed parts of the Unkar Group above the level of the middle part of the Dox Formation. The missing part of the Dox Formation and overlying Cardenas Basalt and Chuar Group can be found in a prominent syncline and fault block in

2700-664: The Grand Canyon, indicating a distant source. Excellent exposures of the Hotauta Member occur at the Colorado River level near Hance Rapids (river mile 77) and along the South Kaibab and North Kaibab Trails . A variety of sedimentary structures have been reported from the Bass Formation. They include ripple marks ; mudcrack covered surfaces; cone-in-cone structures ; interformational breccias/conglomerates; both normal and reversed small-scale, graded bedding ; and local channel fills. The dolomite and limestone within

2775-640: The Grand Canyon. The grabens and half grabens filled with strata of the Unkar group are associated with northwest–southeast trending Mesoproterozoic fault systems that have curving, southwest-dipping traces. These fault systems were later reactivated during the Neoproterozoic, to form basins in which the following Chuar Group accumulated, and during the Cenozoic, to form geologic structures, i.e., faults, anticlines, synclines, and monoclines, that are exposed at

2850-463: The Hakatai Shale contrasts sharply against the grayish outcrops of the Bass Formation. The sloping exposures of the Hakatai Shale also contrast greatly with the steep cliffs formed by the overlying Shinumo Quartzite. Stromatolites occur in the transitional zone between the Hakatai Shale and Bass Formation. The Hakatai Shale accumulated in low-energy, shallow, near-shore, marine environments. In sharp contrast to argillaceous strata above and below it,

2925-424: The Hotauta Conglomerate, Bass Limestone, Hakatai Shale, Shinumo Quartzite, and Dox Sandstone. The still unnamed Cardenas Basalt is only briefly noted as exposures of it are absent in the Shinumo 15-minute quadrangle. Although a recognized part of the Unkar Group, the basalt lava flows that overlie the Dox Formation were generally ignored and simply described as "basalt and diabase." In 1938, Charles R. Keyes applied

3000-526: The Hotauta Member, fills paleovalleys cut into the underlying Vishu basement complex at its base. The Bass Formation also contains stromatolite beds and thin volcanic ash layers. The Hotauta Member is regarded to be fluvial in origin. The remainder of the Bass Formation accumulated in relatively warm shallow marine waters. The Hakatai Shale consists of purple, reddish-purple, reddish-orange, and pale purple or lavender mudstone, sandy siltstone, siltstone, and arkosic sandstone. The brightly colored slopes of

3075-500: The Shinumo Quartzite accumulated in coastal tidal flats and the upper part of it represents the deposits of river deltas. The gradational contact between the Shinumo Quartzite and Dox Formation above indicates a shift from deposition in coastal deltas – to fluvial deposition by a large river system. Of note, the 'soft-sediment deformation' seen in this Shinumo Quartzite formation indicates significant earthquake and tectonic activity during its deposition. The Dox Formation consists of

Bass Formation - Misplaced Pages Continue

3150-526: The Tapeats Sandstone and other members of the Tonto Group . The contact between the Bass Formation and the Tapeats Sandstone forms part of a relative flat surface that lies between the monadnocks. Fossil stromatolites occur within the Bass Formation. Diagnostic “columnar forms” of stromatolites are uncommon. Of these forms, only Collenia undosa Walcott, Collenia symmetrica Fenton & Fenton, and Collenia frequens Walcott, have been reported from

3225-461: The Tonto Group either cover or drape onto most of these ancient monadnocks. However, the summits of the highest monadnocks protrude up through the base layer Tapeats Sandstone and are blanketed by overlying Bright Angel Shale as can be seen at Isis Temple . Lava Butte is a partially exhumed prehistoric monadnock associated with this unconformity that consists of Cardenas Basalt. These monadnocks served locally as sources of coarse-grained sediments during

3300-554: The Unkar Group and the remainder of the overlying Grand Canyon Supergroup are preserved in a prominent syncline and fault block that is exposed in the eastern Grand Canyon. Examples of these fault blocks can be seen at the Isis Temple prominence, "Cheops Pyramid," and the intersection of Phantom Creek with the Bright Angel Canyon, ( North Kaibab Trail ). The Unkar Group also contains thick basaltic sills and

3375-403: The Unkar Group is a major unconformity that also forms the base of the Grand Canyon Supergroup. This unconformity is a nonconformity that separates the underlying and deeply eroded crystalline basement, which consists of granites, gneisses, pegmatites, and schists of the Vishnu Basement Rocks, from stratified Proterozoic rocks of the Unkar Group. This contact is a remarkably smooth surface that has

3450-415: The Vishnu Basement Rocks. The quartzite gravel indicates that the conglomerates of the Hotauta Member were deposited by river systems that extended some unknown distance outside of the Grand Canyon region. Marine sediments buried the fluvial deposits of the Hotauta Member as a smooth surface, with a local relief of probably no more than 150 feet (46 m) of the Vishnu Basement Rocks, being submerged by

3525-499: The asbestos deposits of the Sierra Ancha - Salt River Canyon region of Gila County, Arizona . As in the case of the Bass Formation, these chrysotile asbestos deposits are of the contact metamorphic type that occur in magnesium-bearing Mesoproterozoic dolomites and limestones altered by basaltic dikes and sills. As in the case of the Grand Canyon asbestos deposits, dolomite and limestone reacted with silica-bearing fluids, heated by

3600-568: The basalt intrusions, forming the serpentine mineral chrysotile. Much like the Grand Canyon asbestos deposits, these basaltic sills and dikes range in age from 1,050 to 1,140 Ma. Quite unlike the Grand Canyon asbestos deposits, the asbestos deposits of the Sierra Ancha-Salt River Canyon region have been extremely productive. 36°06′29″N 112°07′15″W  /  36.1080°N 112.1209°W  / 36.1080; -112.1209 Unkar Group The Unkar Group

3675-430: The base of the formation. These layers are characterized by sharp basal contacts, some graded-bedding , and a diverse silicate mineral assemblage that differs from the carbonate or calcsilicate dominated mineral assemblages of adjacent strata. Zircons from one of these volcanic ash beds has been dated using Uranium-lead dating techniques. The base of both the Bass Formation and Unkar Group in eastern Grand Canyon

3750-472: The basis of other geologic criteria, geologists have found that the dates, which range from 1,000 to 700 million years ago, obtained for the age of the Cardenas Basalt and upper age of the Unkar Group, were too young, and something was clearly perturbing the dating systematics. The current interpretation is that the deposition of the overlying Chuar Group, in a marine setting, disrupted the potassium-argon (K-Ar) radiometric system. Apparently, fluids associated with

3825-475: The campground overnight is regulated by the National Park Service, and they call for a maximum number of groups (7 to 11 people) and parties (1 to 6 people), as well as a maximum total number of persons. Use permits are available on a first-come, first-served basis from the park's Backcountry Information Center. Requests are taken beginning on the first day of the month, up to four months before

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3900-402: The central part of the Grand Canyon, while the sandstone, conglomerate, and argillite dominate the Bass Formation in the eastern part of the Grand Canyon. The Bass Formation typically becomes generally finer grained toward the top. Volcanic ash layers occur in the Bass Formation. They consist of layers of white, very fine-grained tephra , that are interbedded with dolomite and argillite toward

3975-499: The contact consists of an interval in which stromatolitic limestone of the Bass Formation is intimately interbedded with coarse clastic sedimentary rock of the overlying Hakatai Shale. In the eastern part of the Grand Canyon, the contact is sharp, but conformable. The contact between the Tapeats Sandstone and Bass Formation and the rest of the folded and faulted Unkar Group is a prominent angular unconformity , which

4050-523: The deposition of the Bass Formation, these rocks were uplifted to the Earth's surface from mid-crustal depths and eroded to form the surface of the nonconformity on which the Unkar Group lies. As these rocks were uplifted from mid-crustal depths, the timing of their cooling was recorded in cooling ages of feldspars and other minerals. As determined from these cooling ages, these basement rocks were uplifted from depths of 25 to 10 km (15.5 to 6.2 mi) between 1750 and 1660 Ma. Then, they were uplifted from

4125-574: The deposition of the Chuar Group have altered the older Cardenas Basalt, partially degraded the minerals, and therefore producing a disruption in the K-Ar systematics. Using newer dating techniques and approaches not available to earlier geologists, the Cardenas Basalt and intrusive sills have been re-dated. New data acquired using newer dating techniques and approaches, indicate that the Cardenas Basalt erupted about 1,104 million years ago. This date marks

4200-547: The early 1900s, William Wallace Bass mined asbestos in the Shinumo Creek-Hakatai Canyon area and John Hance mined asbestos in Asbestos Canyon. Although these deposits had good quality asbestos, little asbestos, at most a few tons, was produced from them because these deposits were fairly small and difficult to access. These asbestos mines are now abandoned and protected as historical sites. In

4275-442: The eastern Grand Canyon. The Dox Formation accumulated in a variety of marine, coastal, estuarine, and fluvial environments. The Cardenas Basalt is composed mainly of thin discontinuous beds of pahoehoe lava flows of olivine-rich basalt. The lower part of this formation consists of complexly interbedded, thin, and discontinuous beds of basalt, hyaloclastite, and sandstone that form low, talus -covered slopes. The upper unit of

4350-429: The eastern region of Grand Canyon. The Shinumo Quartzite, Hakatai Shale, and Bass Formation are found in central Grand Canyon. The Unkar Group accumulated approximately between 1250 and 1104 Ma (1,104 million years ago, 1.1 billion). In ascending order, the Unkar Group is overlain by the Nankoweap Formation , about 113 to 150 m (371 to 492 ft) thick; the Chuar Group , about 1,900 m (6,200 ft) thick; and

4425-480: The end of the deposition of the Unkar Group. These radiometric dates are corroborated by the radiometric dating of detrital micas and zircons from the Hakatai Shale, Shinumo Quartzite, and Dox Formation. Based on all of these radiometric dates, researchers have concluded that the Unkar Group was deposited between about 1254 and 1100 Ma, with a hiatus of unknown duration between the Hakatai Shale and Shinumo Quartzite. North Kaibab Trail The North Kaibab Trail

4500-412: The folded and faulted Unkar Group. Though this surface is typically a plane, differential erosion of the tilted strata of the Unkar Group left resistant beds of the upper layer Cardenas Basalt and the middle layer Shinumo Quartzite as ancient hills, called monadnocks . These ancient hills, which are ridges formed by block faulting, are up to 240 m (790 ft) tall. Thin drapes of Tapeats Sandstone of

4575-437: The general level of a relatively flat surface. The Vishnu Basement Rocks underlying this surface are often deeply weathered to an average depth of 3 m (9.8 ft) below it. Where it has not been removed by erosion, prior to, and during the deposition of the overlying Bass Formation, a residual regolith developed by subaerial weathering of the underlying basement rocks – is present. Typically, this regolith consists of

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4650-459: The general level of a relatively flat surface. The Hotauta Member of the Bass Formation fills shallow paleovalleys that are part of this nonconformity. The Vishnu Basement Rocks underlying this surface are often deeply weathered to an average depth of 3 m (9.8 ft) below it. Where it has not been removed by erosion prior to and during the deposition of the overlying Bass Formation, a residual regolith – developed by subaerial weathering of

4725-482: The head of Roaring Springs canyon and ends at the Colorado River . The trailhead is at a parking area on Arizona State Route 67 , about 1-mile (1.6 km) north of the North Rim's Grand Canyon Lodge. The Ken Patrick Trail and Uncle Jim Trail are also accessible from this parking area. The trail is 14 miles (23 km) long, with camping available by permit at Cottonwood Camp at 7 miles (11 km) and Bright Angel Camp at 14 miles (23 km). Treated water

4800-473: The main Unkar Group outcrop area (below East Rim). Within the central Grand Canyon, Unkar strata occur in small, rotated, downfaulted blocks or slivers where they commonly are only partially exposed. Within this center part of the Grand Canyon, the Unkar Group is incomplete because pre- Tonto Group erosion has removed strata above the level of the middle part of the Dox Formation . The missing part of

4875-679: The mid-1960s, Gregory H. Billingsley found several veins of asbestos up to 7.6 cm (3.0 in) thick at the mouth of Tapeats Creek . These veins outcrop upstream along the Colorado River for nearly two miles up to Stone Creek. Green chlorite , green garnet , and talc are associated with the asbestos veins, which occur in a 0.6-to-0.9 m (2-to-3 ft) thick contact-metamorphism -zone associated with basaltic sills. For unknown reasons, claims were never filed on these veins and no attempts were made to mine them. The Grand Canyon asbestos deposits are very similar in origin and nature to

4950-735: The name "Cardenesan Series" to the basaltic volcanic rocks within the Unkar Group. In 1973, the current definition of the Unkar Group developed when the Nankoweap Formation, which had been earlier added to the Unkar Group was formally removed from it and the unconformity that separates the Nankoweap Formation from the Unkar Group – was recognized. The Bass Formation not only contains gray to red-gray dolomite and sandy dolomite but also interbedded purple-brown to dark red and reddish brown sandstone (arkose), and silty sandstone, prominent interbeds of conglomerate, and subordinate interbeds of argillite and limestone. A prominent conglomerate,

5025-419: The predominant lithologies. Beds of intraformational breccia are also found throughout the Bass Formation. The dolomites and limestones vary in color from gray to red-gray and sometimes contain biscuit-form and biohermal stromatolite beds. The conglomerates, breccias, sandstones, and argillites vary in color from purple-brown to dark red and reddish brown. Dolomite and limestone dominate the Bass Formation in

5100-418: The surface. Major unconformities separate the Unkar Group from the strata overlying and underlying it. First, the Unkar Group, as the bottom unit of the Grand Canyon Supergroup, lies directly upon deeply eroded granites , gneisses , pegmatites , and schists that comprise Vishnu Basement Rocks . Second, an angular unconformity, with a dip of less than 10°, separates the base of the Nankoweap Formation from

5175-416: The underlying Hakatai Shale. Within the Shinumo Quartzite, a basal lag composed of a layer 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 zircon , this disconformity

5250-482: The underlying Unkar Group. Finally, a well-defined angular unconformity at the base of the relatively flat-lying Tonto Group separates it from the underlying faulted and folded strata of the Unkar Group and the rest of the Grand Canyon Supergroup that are typically tilted at angles of 10°–30°. The western section of the Unkar group can be highlighted in a photo of three Unkar units below Isis Temple sitting on Vishnu Basement Rocks of Granite Gorge. The Unkar Group

5325-532: The underlying Vishnu Basement Rocks that indicate they were exhumed from depths of 25 to 10 km depths between 1,750 and 1,660 Ma and from a depth of 10 km to the surface on which the Bass Formation lies between 1,660 and 1,250 Ma. The presence of chrysotile asbestos veins in the Bass Formation was first noted in early exploration of the Grand Canyon by the Powell expedition . Shortly after 1890, mining claims were filed on these asbestos deposits. Later in

5400-494: The underlying basement rocks – is present. Typically, this regolith consists of dark-reddish brown, structureless, ferruginous sediment that is usually a few centimeters to 30 cm (0.98 ft) thick. This contact is regarded to be a classic example of an ancient peneplain . The contact between the Tonto Group and Unkar Group is a prominent angular unconformity, which is part of the Great Unconformity . The surface of this angular unconformity truncates dipping strata comprising

5475-503: The western Grand Canyon, while stromatolites and shallow-water mud accumulated in the eastern Grand Canyon. Following the maximum incursion of marine waters, the sea slowly regressed, and it accumulated in nearshore and coastal environments as indicated by ripple marks , mudcracks , oxidized shales , and other evidence of periodic subaerial exposure found in the upper part of the Bass Formation. Evaporite -forming conditions probably occurred, also during this regressive phase. Eventually,

5550-417: Was first recognized and named by Charles D. Walcott as the "Unkar terrane" in 1894. It and his “Chuar terrane” comprised what was then named the “Grand Canyon series” of "Proterozoic (Algonkian)" age. He regarded the Cardenas Basalt, unnamed at that time, as the uppermost unit in his "Unkar terrane." In 1910 and 1914, Levi F. Noble later divided what he called the "Unkar Group" into five subunits, which were

5625-486: Was originally named the Bass Limestone in 1914 for Bass Canyon, where it is typically exposed. The Bass Limestone has been reclassified as the Bass Formation by geologists because it consists of heterogeneous mixtures of clastic and carbonate sedimentary strata of which dolomite is the predominant rock type and limestone is only a minor rock type. In addition, its Hotauta Member was originally designated as

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