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58-597: The bright orange-red slopes of the Hakatai Shale contrasts sharply against the grayish outcrops of the Bass Formation . The outcrop of the Hakatai Shale also contrasts greatly with the steep cliffs formed by Shinumo Quartzite as seen at the base of Isis Temple . In the central Grand Canyon north of Grand Canyon Village and viewed from the south at the South Rim, the bright orange-red unit can be seen below

116-671: 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

174-412: 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

232-436: 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

290-523: 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,

348-576: 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

406-516: 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

464-404: 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

522-405: 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

580-421: Is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record . The significance of angular unconformity (see below)

638-611: 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

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696-602: Is a prominent angular unconformity , which is part of the Great Unconformity . The differential erosion of the Unkar Group left resistant beds of the Cardenas Basalt and Shinumo Quartzite as topographic highs, ancient monadnocks , that are now buried by sandstones, shales, and conglomerates of the Tapeats Sandstone. These monadnocks served locally as sources of coarse-grained sediments during

754-560: 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

812-485: Is of probable marine deltaic origin. The Hatakai Shale is between 1,253 and 1,104 million years old. It is younger than a layer of volcanic ash in the underlying Bass Formation that has been dated at 1,253 million years. In addition, the Ar/Ar dating of detrital muscovite from the basal, Escalante Creek Member of the overlying Dox Formation indicates that it is younger than 1,140 million years. The overlying Dox Formation

870-608: Is older than the age of the Cardenas Basalt, which is 1,104 million years. Isis Temple-Cheops Pyramid is a small horst , located close to the geographic center of the Grand Canyon. It is bordered on the east by the Bright Angel Fault and on the south by Granite Gorge; the Bright Angel Fault continues to the south-southwest across Granite Gorge and up Garden Creek, up the Bright Angel Trail to

928-498: 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

986-515: Is the lowermost part, overlies deeply eroded granites , gneisses , pegmatites , and schists that comprise Vishnu Basement Rocks . Typically, the Hakatai Shale is subdivided into three informal members. In ascending order they are the Hance Rapids (lower), Cheops Pyramid (middle), and Stone Creek (upper) members. The total thickness of the Hakatai Shale varies from 135 m (443 ft) at Hance Rapids to nearly 300 m (980 ft) at

1044-610: The Bass Formation , Hakatai Shale, Shinumo Quartzite , Dox Formation , and Cardenas Basalt . In ascending order, the Unkar Group is overlain by the Nankoweap Formation , about 113 to 150 m (371 to 492 ft) thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation , about 60 m (200 ft) thick. The Grand Canyon Supergroup , of which the Unkar Group

1102-475: 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

1160-476: 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

1218-597: The marine transgression that deposited the Tapeats Sandstone and other members of the Tonto Group. The contact between the Hatakai Shale and the Tapeats Sandstone forms part of a relatively flat surface that lies between the monadnocks. Stromatolites have been found within spectacular exposures of the Hatakai Shale in Rodgers Canyon. At this outcrop, they occur in the transitional zone between it and

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1276-543: The type section in Hakatai Canyon in the Shinumo Creek area. First, the Hance Rapids (lower) member consists of purple to reddish-purple mudstone, interbedded sandy siltstone , and rare occurrences of thin-bedded sub arkose - to quartz arenite . Beds of the lower member of the Hakatai Shale grade upward into the fine-grained middle member, with a boundary drawn at or very near their color change. Second,

1334-440: 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

1392-585: The Bass Formation where stromatolitic carbonate beds are intimately interbedded with coarse deposits of the Hakatai Shale. The base of these deposits contains stromatolite onchaloids, small algal mounds, that are built upon coarse clasts of sandstone-conglomerate. In addition, the dark gray to black central cores of the reduction mottles found in the Cheops Pyramid (middle) member is possible of organic (stromatolitic?) origin. A number of spurious and discredited reports about other types of fossils found in

1450-462: 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

1508-484: The Bass Formation. 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;

1566-522: 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

1624-435: The Cheops Pyramid (middle) member consists of mudstone, siltstone, and subordinate sandy siltstone that exhibit a striking reddish-orange color. These distinctively red-colored strata are commonly mottled with small to large (up to 10 cm and more across), non-red, spherical to spheroidal reduction spots. These mottles have dark-gray to greenish-gray central nuclei that contain very dark gray to black central cores. Finally,

1682-667: 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

1740-400: The Hatakai Shale have been made. In one case, apparent trails of metazoans were reported from the Hatakai Shale. However, these reported trace fossils are now regarded to be nonbiogenic pseudofossils. The strata comprising the Hance Rapids (lower) member is interpreted to have accumulated beneath shallow, low-energy waters. The change from Bass Formation to the lower member likely represent

1798-745: The Isis Temple and Cheops Pyramid landforms at the intersection of Bright Angel Canyon and Granite Gorge; the Bright Angel Trail from the South Rim traverses through the geographic region to the north, the North Kaibab Trail in Bright Angel Canyon. The Hakatai Shale is part of a conformable sequence of sedimentary strata that comprise the Unkar Group . The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed, in ascending order, of

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1856-411: The Shinumo Creek area. The feeder dikes to these sills are not exposed. However, the feeder dikes for the Cardenas Basalt can be traced, discontinuously, to within a few meters of its base. Adjacent to the sills, the Hatakai Shale has been altered to knotted hornfels containing porphyroblasts of andalusite and cordierite that have been replaced by muscovite and green chlorite , respectively. In

1914-464: The South Rim. The north side of Isis Temple-Cheops Pyramid is bordered by the approximate east-west, Grandview Fault . Bass Formation 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

1972-561: The Stone Creek (upper) member of the Hakatai Shale consists of pale purple or lavender, fine- to coarse-grained, crossbedded coarse arkose . Basaltic sills and dikes intrude all of the Unkar Group below the Cardenas Basalt. Sills intruded only the Bass Formation and Hakatai Shale. Dikes intrude the Hakatai Shale, Shinumo Quartzite, and Dox Formation. The sills range in thickness from 23 m (75 ft) at Hance Rapids, eastern Grand Canyon, to 300 m (980 ft) in Hakatai Canyon in

2030-528: 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

2088-415: 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

2146-499: 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

2204-571: 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 Disconformity An unconformity

2262-432: 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

2320-406: 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

2378-501: 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

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2436-523: The contact is sharp, but conformable. The contact between the Hatakai Shale and the overlying Shinumo Quartzite is a distinct, significant disconformity . This disconformity is sharp and locally truncates cross-beds, and channel forms, within the sandstones of the underlying Hatakai Shale. Within the lower subarkose of the Shinumo Sandstone, a basal lag of conglomerate , which contains basement clasts up to 5 cm (2.0 in) across, lies on

2494-550: 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

2552-409: The eastern part of the Grand Canyon, the contact between the Hatakai Shale with underlying Bass Formation is typically gradational over an interval of a meter or so. For example, in Red Canyon, the contact consists of an interval in which stromatolitic limestone of the Bass Formation is intimately interbedded with coarse deposits of the overlying Hakatai Shale. In the eastern part of the Grand Canyon,

2610-410: The eroded surface that forms this disconformity. This basal conglomerate contains quartzite clasts that lack any known equivalents in the Grand Canyon region. As documented by the dating of detrital zircons , this disconformity is estimated to represent a period of about 75 million years. The contact between the Tapeats Sandstone and Hatakai Shale and the rest of the folded and faulted Unkar Group

2668-437: 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

2726-681: 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

2784-538: The next deposition. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus . It is a kind of relative dating . A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition. Disconformities are marked by features of subaerial erosion. This type of erosion can leave channels and paleosols in

2842-534: The overlying horizontal layers. The whole sequence may later be deformed and tilted by further orogenic activity. A typical case history is presented by the Briançonnais realm (Swiss and French Prealps) during the Jurassic. Angular unconformities can occur in ash fall layers of pyroclastic rock deposited by volcanoes during explosive eruptions . In these cases, the hiatus in deposition represented by

2900-419: 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

2958-543: The rock record. A nonconformity exists between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock. Namely, if the rock below the break is igneous or has lost its bedding due to metamorphism, then the plane of juncture is a nonconformity. An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with

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3016-617: 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 (below East Rim). In addition, thick, prominent, and dark-colored basaltic sills intrude across

3074-526: The unconformity may be geologically very short – hours, days or weeks. A paraconformity is a type of unconformity in which the sedimentary layers above and below the unconformity are parallel, but there is no obvious erosional break between them. A break in sedimentation is indicated, for example, by fossil evidence. It is also called nondepositional unconformity or pseudoconformity. Short paraconformities are called diastems . A buttress unconformity also known as onlap unconformity, occurs when younger bedding

3132-534: 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

3190-558: The waning stages of marine deposition during which the Bass Formation accumulated. The Cheops Pyramid (middle) member is regarded to have accumulated either in a shallow, near-shore marine environment or in coastal plain mudflats , or deltas . The greenish gray reduction mottles are similar in nature to reduction mottles observed in the lower middle and upper members of the Dox Formation, which are regarded as having accumulated in nonmarine environments. The Stone Creek (upper) member

3248-503: 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,

3306-423: 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

3364-517: Was shown by James Hutton , who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland. The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region or were subsequently eroded before

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