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42-784: In 1875, G. K. Gilbert recognized that strata similar to those, named the Tonto Group , found in the Tonto Basin are exposed near the bottom of parts of the Grand Canyon. Based upon this similarity, he also assigned these strata to the Tonto Group . From youngest to oldest, Gilbert further subdivided the Tonto Group into three subunits, the Marbled limestone , the Tonto shale , and the Tonto sandstone . However, he did not specify

84-489: A deepening shallow-marine shelf. These shallow marine deposits overlie intertidal deposits reworked from braided fluvial and deltaic sediments deposited in front of an eastward advancing shoreline. Together, the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone form a typical transgressive sedimentary sequence. In 2018, Karlstrom and others studied the age of detrital zircons from the Tapeats Sandstone in

126-720: A hilly and weathered paleosurface underlain by the Grand Canyon Group and Vishnu Basement Rocks. In one basin, the Cambrian Sixtymile Formation is preserved in a synclinal paleotopographic low in the Great Unconformity and separates the Tapeats Sandstone from it and the Grand Canyon Supergroup. The highest paleomonadnocks on this paleosurface are composed of Shinumo Quartzite and other erosion resistant strata of

168-614: A large river system. The age of the Shinumo Quartzite is approximately known. Based upon recent dates of volcanic ash layers, detrital zircon grains, and detrital mica flakes, geologists agree that the Unkar Group was deposited between about 1254 and 1100 Ma. The Hakatai Shale, Shinumo Quartzite, and Dox Formation samples yield clusters of zircon as young as 1170 Ma. Thus, these strata must be younger than 1170 Ma. The Dox Formation, which overlies

210-489: A large variety of horizontal tracks and trails. Many of the vertical burrows were most likely created by annelid worms and other soft bodied animals. The horizontal tracks and trails were likely created by gastropods and as well as arthropods (trilobites and crustaceans ). The Tapeats Sandstone consists of a mixture shallow marine, intertidal, and coastal plain deposits associated with an eastward advancing shoreline. The upper transitional beds represent sediment deposited on

252-592: A specific type locality for any of these stratigraphic units. In 1914, L F. Noble officially redefined and renamed Gilbert's subdivisions of the Tonto Group. He renamed the Tonto sandstone as the Tapeats Sandstone and the Tonto shale as the Bright Angel Shale . He named the Tapeats Sandstone after Tapeats Creek in which this sandstone is exposed in the creek's bed. Later in 1922, Noble renamed

294-604: A variety of environments ranging from fluvial to shallow- marine . Overall, the strata within it are conformable with the exception of a significant disconformity between the Hakatai Shale and Shinumo Quartzite. The Unkar Group is overlain in ascending order 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

336-593: 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

378-482: Is a thinner layer, known as the transitional beds , that consists of interbedded fine- to medium-grained sandstone and mudstone. The bedding in this upper layer is thinner than below and they exhibit largely trough and ripple cross stratification and horizontal lamination. The transitional beds grade upward into and interfinger with the overlying Bright Angel Shale. They form the surface of the Tonto Platform. In

420-481: Is called a stratotype , whereas the standard of reference for unlayered rocks is the type locality. The concept is similar to type site in archaeology . Shinumo Quartzite The Shinumo Quartzite also known as the Shinumo Sandstone , is a Mesoproterozoic rock formation , which outcrops in the eastern Grand Canyon , Coconino County, Arizona , ( Northern Arizona ). It is the 3rd member of

462-411: Is composed of "rusty red" quartzite and overlies the lower-middle member. The "rusty-red" quartzite of the upper-middle member grades downward into the lower-middle member through an interval of interbedded purple and red-brown quartzite. Their colors and cementation appear to be the result of ground or connate water alteration after their deposition. Finally, the upper member of the Shinumo Quartzite

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504-508: Is composed of reddish brown and locally purple sandstone and an overlying well-cemented gray quartzite. The Shinumo Quartzite is characterized as a quartzite, but the gray quartzite within the upper member of the Shinumo Quartzite contains mudstone -rich intervals. In addition, the gray quartzite was subjected to cementation by silica and bleaching that removed its original reddish brown and purple hematite pigments, after its deposition. Basaltic sills and dikes intrude all formations of

546-570: Is either an angular unconformity truncating tilted strata of the Grand Canyon Supergroup or a nonconformity with the Vishnu Basement Rocks and other Precambrian metamorphic or plutonic rocks. Within the eastern Grand Canyon, the Tapeats Sandstone is separated from the underlying Precambrian rocks by the Cambrian Sixtymile Formation. In the eastern Grand Canyon, the Tapeats Sandstone unconformably overlies

588-415: Is inferred to have accumulated in the delta plain , delta front, and mouth-bar of an ancient delta . The deformed strata of the upper member are interpreted as evidence of reoccurring earthshocks along regional fault and fold systems that were active during its deposition. The contact between the Shinumo Quartzite and Dox Formation indicates a shift from deposition in coastal deltas to fluvial deposition by

630-770: The Sixtymile Formation , about 60 m (200 ft) thick. The Unkar Group, as the base section of the Grand Canyon Supergroup , overlies deeply eroded granites , gneisses , pegmatites , and schists that comprise Vishnu Basement Rocks . In sharp contrast to argillaceous strata above and below it, the Shinumo Quartzite consists predominately of beds of red, brown or purple sedimentary quartzites that are firmly, to extremely well cemented by silica , and characterized by poorly developed bedding. It also contains beds of massive white, red or purple sandstone and conglomeratic sandstone. One conglomeratic sandstone layer that lies about 21 m (69 ft) above

672-485: The facies change, from quartz arenite, to mudstone and fine-grained arkose – is gradational. Between the three-member Tonto Group (above) and the Shinumo Quartzite, and the rest of the folded and faulted Unkar Group, is a prominent angular unconformity , which is part of the Great Unconformity . Though this surface is typically a plane, differential erosion (also areal erosion) of the tilted strata of

714-521: The marine transgression to form the Tonto Group. No fossils have been reported from the Shinumo Quartzite. The sedimentary strata comprising the Shinumo Quartzite are inferred to have accumulated in a variety of environments. The lower member is regarded to have been deposited by ancient river systems and later, in coastal tidal flats . The lower-middle, and upper-middle members are argued to have accumulated in nearshore marine environments and coastal tidal and supratidal flats . The upper member

756-541: 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 (orange, red, yellow, etc) strata of the underlying slope-forming Hakatai Shale . Overlying the Shinumo, dark green to black, fissile, slope-forming shales of the Dox Formation create

798-483: The Dox Formation directly overlying Shinumo Quartzite consists of predominantly dark green to black, fissile, slope-forming shale that contains thin sandstone beds. This shale makes a distinct notch between the resistant cliff-forming quartzites of the Shinumo Quartzite underlying them and resistant cliff-forming arkosic sandstones of the Dox Formation overlying them. The change in topographic expression, color, and

840-487: The Grand Canyon Group form paleocuestas of lesser and intermediate height. In the western Grand Canyon, the Tapeats Sandstone unconformably overlies a similar hilly and weathered paleosurface underlain by the Vishnu Basement Rocks. This paleosurface is quite irregular and was before burial a rolling paleotopography of resistant crystalline basement hills rising above the surrounding lowlands. The relief on

882-610: The Grand Canyon and southern Nevada using U-Pb dating . They interpreted the results of this study to indicate that the maximum depositional age of the Tapeats Sandstone to be from 505.4 ± 8 Ma to 501.4 ± 3.8 Ma and that it accumulated as an ancient shoreline gradually migrated eastward during the Cambrian Epoch 3. The flat lying Tonto Platform has hiking trails that cross it from the South Rim to North Rim, Grand Canyon for instance. The extensive Tonto Trail lies on parts of

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924-623: The Grand Canyon, the Tapeats Sandstone is a medium- to coarse-grained, thin-bedded, cliff-forming and locally conglomeratic sandstone that weathers to a tan or reddish-brown. In its outcrops within the eastern part of the Grand Canyon, the Tapeats Sandstone is commonly divided into two layers. The lower portion of it consists of a lower layer of sandstone that outcrops as a cliff-forming cliff consisting of beds that are each typically less than 0.9 m (3.0 ft)thick. The beds exhibit sedimentary structures that include planar and trough cross-bedding and crudely developed horizontal bedding. Typically,

966-475: The Great Unconformity in the Grand Canyon represents ancient valleys and hills that influenced sedimentation patterns within the Tapeats Sandstone along with the local basement lithology. For example, there are numerous places where the Tapeats Sandstone thins across or terminates against ancient crystalline basement highs. Where the Tapeats Sandstone is absent, the Bright Angel Shale directly overlies

1008-542: The Great Unconformity. Throughout the Grand Canyon area, the Precambrian bedrock underlying the Tapeats Sandstone and Great Unconformity was extensively weathered and eroded during prolonged periods of subaerial exposure. Last studied by Sharp in 1940, a highly altered ancient weathering horizon is locally preserved within the Precambrian rocks immediately underlying the Tapeats Sandstone. He concluded that extensive chemical weathering, which occurred prior to deposition of

1050-456: The Hakatai Shale. The relief at this unconformity is small, being about 10 m (33 ft). The Shinumo Quartzite overlying this unconformity consists of arkosic conglomeratic sandstone that contains quartzite and granite pebbles. Within the Unkar Group, the upper contact of the Shinumo Quartzite with the Dox Formation appears to be gradational and is marked by a change in topographic expression and color. The basal 12 m (39 ft) of

1092-775: The Marbled limestone as the Muav Limestone . According to Noble's definition, the Muav Limestone consisted of an upper set of dolomite beds and a lower set of limestone beds. The Muav Limestone is now applied only to Noble's lower set of limestone beds and the upper set of dolomite beds now constitutes the Frenchman Mountain Dolostone. E. D. McKee and C. E. Resser and subsequent researchers retained Noble's usage for this formation despite infrequent misgivings about their stratigraphic complexity. In

1134-444: The Tapeats Sandstone include five species of the lingulate genus Lingula and two species of paterinate brachiopods, An unidentified species of obolellid brachiopod has been reported from the Tapeats Sandstone but remains unverified. The Tapeats Sandstone, especially its transitional beds, are often bioturbated and contains a wide variety of trace fossils. These trace fossils commonly consist of ubiquitous vertical burrows and

1176-407: The Tapeats Sandstone, and the platform on the south side of Granite Gorge. Examples of the Great Unconformity . The Tapeats approximately 200 ft thick. Type locality (geology) Type locality , also called type area , is the locality where a particular rock type , stratigraphic unit or mineral species is first identified. If the stratigraphic unit in a locality is layered, it

1218-424: The Tapeats Sandstone, of exposed Precambrian rocks created a highly weathered bedrock, or regolith , as much as 15 m (49 ft) thick. Generally, this regolith is less than 30 m (98 ft) thick having been likely partially eroded and redeposited as Tapeats Sandstone. Where the Tapeats Sandstone rests on unaltered Precambrian basement, the regolith probably was removed by erosion either prior to or during

1260-434: The Tapeats Sandstone. However, because of the gradational and interfingering natural of Tapeats Sandstone with the Bright Angel Shale and uncertainties in locations of collecting sites, at least some of these taxa are probably collected from the Bright Angel Shale instead. The arthropods include the trilobites, Olenellus and Spencella and the marine arthropods, Indianites and Walcottella . The brachiopods recorded from

1302-712: The Unkar Group below the Cardenas Lava. The Hakatai Shale, Shinumo Quartzite, and Dox Formation are intruded by these dikes. They can be traced, discontinuously, to within a few meters of the base of the Cardenas Lava. The Shinumo Quartzite exhibits a variety of sedimentary structures. The sandstones of the lower and lower-middle members exhibit centimeter- to meter-scale planar tabular cross-stratification and trough cross-beds. These cross beds record north-directed paleocurrents along with subordinate bidirectional paleocurrents. The upper-middle member exhibits abundant cross beds, clay galls, and mudcracks . The sandstones of

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1344-547: The Unkar Group left resistant beds of the Cardenas Basalt and Shinumo Quartzite as ancient hills, called monadnocks , that are up to 240 m (790 ft) tall. Thin drapes of Tapeats Sandstone of the Tonto Group now cover most of these ancient monadnocks. However, a few of these monadnocks protrude further up into the Bright Angel Shale ( Isis Temple prominence example). These monadnocks served locally as sources of coarse-grained sediments that accumulated during

1386-477: The base of the Shinumo Quartzite near the South Kaibab trail contains jasper pebbles . Typically, the Shinumo Quartzite has been subdivided into four poorly defined, unnamed members. First, the basal lower member consisting of purplish arkosic conglomeratic sandstone. Unlike the rest of the Shinumo Quartzite, it contains quartzite and granite gravels up to 5 cm (2.0 in) in diameter. As in

1428-486: The case of the Hotauta Conglomerate, the quartzite gravel of the lower member lacks any known equivalents in the Grand Canyon region. Second, the lower-middle member is composed of purple cross-bedded quartzite, sometimes is subdivided into two units, and overlies the lower member. The purple quartzites of the lower-middle member exhibit crisp, well-defined reduction spots. Third, the upper-middle member

1470-557: The deposition of the Shinumo Quartzite. These deformed beds can be seen at river level in the vicinity of Nevilles Rapid, ( List of Colorado River rapids ), at river mile 75. The Shinumo Quartzite ranges in thickness from 330 m (1,080 ft) in the Bright Angel Canyon area to 345 m (1,132 ft) at Papago Creek and 405 m (1,329 ft) in the Shinumo Creek area. In the Bright Angel Canyon area,

1512-410: The initial deposition of the Tapeats Sandstone. Despite the abundance of invertebrate burrows and trails ( trace fossils ) found in the Tapeats Sandstone, the body fossils of invertebrates, such as brachiopods and trilobites are absent, except where it interfingers with the overlying Bright Angel Shale. As summarized by Lassiter and others 13 invertebrate taxa are reported as having been found in

1554-476: The main Unkar Group outcrop area (below East Rim). The Shinumo Quartzite is a middle member 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 consists of a sequence of sedimentary rocks that accumulated in

1596-401: The thickness of the bedding and cross-bedding decreases upwards in an outcrop. In paleotopographic lows along the Great Unconformity, the lower layer often has at its base a few beds composed of conglomeratic quartz and feldspar . The composition of these conglomeratic beds typically reflects the lithology of the underlying, local Precambrian basement rocks. Overlying the cliff-forming layer,

1638-492: The upper 12 to 15 m (39 to 49 ft) belong to the transition beds underlying the Bright Angel Shale. Throughout the majority of its extent, the Tapeats Sandstone lies unconformably on the surface of underlying Precambrian rocks forming the Great Unconformity . Where the Sixtymile Formation underlies the Tapeats Sandstone, the Great Unconformity lies at the base of the Sixtymile Formation. This unconformity

1680-459: The upper member exhibit trough cross-beds, suggesting a more northerly transport direction. The sandstone and quartzite beds of the upper member exhibit an abundance of complexly contorted, gnarly bedded, fluid expulsion structures and dramatic convolute bedding . The beds that exhibit these soft-sediment deformation structures are meters- to tens-of-meters thick. They are cited as evidence for frequent earthquake activity and fluid migration during

1722-465: The upper member is about 18 m (59 ft) thick, the upper middle member is about 80 m (260 ft) thick, and the lower middle member is about 130 m (430 ft) thick. The contact of the Shinumo Quartzite with the Hakatai Shale is the only documented unconformity that has been found within the Unkar Group . This unconformity truncates across beds and channel deposits of

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1764-402: The western part of the Grand Canyon, a mudstone-rich layer often occurs sandwiched between these sandstone layers. The thickness of the Tapeats Sandstone exposed within the Grand Canyon varies from very thin to absent where it accumulated over prominent paleotopographic highs of the Great Unconformity to as thick as 90 m (300 ft) thick within paleotopographic lows. Where it is thickest,

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