Bass formation
The bass formation is a geological formation in the Grand Canyon . It forms the basis of the Grand Canyon Supergroup belonging Unkar Group and during Mesoproterozoikums in Rodinium / ectasian before about 1,250 million years BP deposited.
Type locality and first description
A type locality of the bass formation, also called bass limestone , has not yet been identified. The formation was named after Bass Canyon , a tributary of the Colorado River . The name of Bass Canyon is derived from prospector William Wallace Bass . The first scientific description of the formation and its name assignment took place in 1914 by LF Noble . In 1976 the bass lineup was revised by Elston and Scott and made part of the Unkar Group.
Occurrence
The bass formation is limited in its occurrence to the central part and eastern section of the Grand Canyon in Coconino County in the US state of Arizona . She is particularly open-minded in the Granite Gorge on the Colorado River, at the Isis Temple and in the southwestern Bright Angel Canyon .
lithograph
The low lying or weakly dipping 10 to 15 ° to the northeast bass formation follows transgressively with a pronounced angular discordance on the metamorphic basement rocks of the Vishnu Basement Rocks . In turn, it is conformally overlaid by the Hakatai Shale .
Lithologically, the formation is predominantly dolomites and sandy dolomites. The thin to medium-layered dolomites are fine to coarse-grained. In between, however, there are also thin- layer sandstones of fine to coarse grain size, arkoses , silty sandstones, prominent conglomerate layers , intraformational breccia horizons , argillites and limestone . The usually gray colored dolomites and limestones can also take on reddish-gray hues and occasionally contain biscuit-like or bio-thermal stromatolite layers . The conglomerates, breccias, sandstones and argillites change their color from purple-brown to dark red to red-brown. Dolomites and limestone dominate the formation in the central section of the Grand Canyon, while sandstones, conglomerates and argillites dominate in the eastern section. Within the formation, a decrease in grain size towards the hanging wall can generally be observed.
The thickness of the formation varies between 37 and 104 meters, with a general increase in thickness towards the northwest. Their average thickness is 80 meters. Low thicknesses such as at Crystal Creek with 57 meters indicate an undulation of the transgression surface (topographical high in the basement). Maximum thicknesses of 100 meters prevail at Phantom Creek north of the Utah Flats .
The bass formation forms steep walls or a stair step morphology in the terrain, whereby resistant dolomite layers are responsible for the steps, the steps were carved out of easily erodible argillites.
The bass formation has various sedimentary structures, including ripple marks , dry cracks , cone structures ( cone-in-cone structure ), intraformational breccias or conglomerate layers , small-scale graded stratification (normal and inverse) and locally filled flow channels.
The carbonate games within the formation documents according to their sedimentation Alterationsprozessen as dolomitisation , recrystallization , Stylolithbildung and silicification .
Hotauta member
The lowest lying Hotauta Conglomerate Member (formerly also just Hotauta Conglomerate ) is the discontinuous base conglomerate of the marine transgression from the west over the eroded basement stump. The pebbles contained were washed up by the surging waves of the slowly penetrating sea. The pebbles consist of chert , granite , quartzite , quartz , plagioclase crystals and micropegmatites , the matrix consists of sand . About 80% of the pebbles consist of granite and quartzite. The quartzite pebbles have no counterpart inside the Grand Canyon and therefore come from outside. The Hotauta member is very open to the Hance Rapids (river mile 77) as well as the South Kaibab Trail and the North Kaibab Trail .
Deposit conditions
Lithology and sedimentary structures suggest that the bass formation, with the exception of the Hotauta member, is of marine origin. The transgression had come from the west.
The basal Hotauta Member consists of river sands and rubble that had been deposited on the Vishnu Basement Rocks in depressions of the former erosion surface. Its alien quartzite pebbles show that the catchment areas of the transporting rivers extended far beyond the area of the Grand Canyon. Marine transgression sediments then covered the Hotauta Member with a smooth surface with a maximum height difference of no more than 46 meters.
The dolomites of the Bass Formation were originally felled as limestone and were dolomitized only through later diagenesis . The limes were formed in clear, fairly warm and shallow sea water through both organic and inorganic processes. During the peak of transgression, limestone and deep water silt accumulated in the western part of the Grand Canyon, whereas stromatolite-bearing shallow water silt was deposited in the eastern part. After that, the sea slowly retreated in the hanging wall of the bass formation, recognizable by the facies now near the beach, which are marked by ripples, dry cracks, oxidized shale and other indicators for occasional drying out. During this regressive phase, the conditions for the formation of evaporites may also have been reached. In the end, a shallow water environment near the coast consisting of mud flats and deltas had established itself in the Grand Canyon , which led to the sediments of the Hakatai Shale being deposited.
Fossil content
The bass formation has fossil stromatolites , although diagnostic columnar forms are relatively rare. Finds of Collenia undosa (Walcott), Collenia symmetrica (Fenton and Fenton) and Collenia frequens (Walcott) have been made. As the associated sediments show, these stromatolites grew in the calm, shallow marine area. The frequent occurrence of flow ripples and dry cracks indicates temporary dryness. In addition, thin breccia layers reveal brief turbulent events. However, there are no direct signs of an intertidal environment near the beach.
Other fossils are also said to be present in the bass formation, for example jellyfish , sponges , worms and mussels (bivalvia). Critical investigations have shown, however, that the alleged sponges are only inorganic silica concretions, the alleged jellyfish represent gas escape structures or algae colonies , and the worm traces also only represent inorganic sediment structures. The suspected bivalvia are now interpreted as rounded clay patties or pellets, which are probably due to algae oncolites.
Volcanism
Volcanic ash layers occur within the bass formation . They consist of white, very fine-grained tephra , which alternate with dolomites and clay stones in the horizontal part of the formation. Their basic contacts are usually sharp and clear and they can be graded internally. They differ from the surrounding layers, which are characterized by carbonates and calcium silicate, by a diversified silicate mineralogy. The layers of ash could be dated radiometrically using the zirconia.
Like the other formations of the Unkar Group below the Cardenas Lava , the bass formation is also criss-crossed by basalt ducts . Basaltic storage corridors along dolomitic areas triggered a metasomatosis due to the heating of the silica-containing sediment , which, with recrystallization, produced asbestos containing chrysotile in the contact area of the intrusions . Chrysotile needles up to 10 centimeters long can now be found up to 3 meters from the contact.
The asbestos deposits in the Bass Formation have been known since the Powell Expedition and were then mined by William Wallace Bass and John Hance around 1900. Because of the difficult access, only several tons of chrysotile could be extracted at that time. In the 1960s, GH Billingsley found veins of asbestos up to 7.6 centimeters thick associated with chlorite , garnet and talc at the confluence of Tapeats Creek in the Colorado River . But these were never dismantled.
Age
The age of the bass formation can be narrowed down by means of uranium-lead dating of zirconia in an ash layer and by means of argon-argon dating of overlying basalts. Zircons in volcanic ash layers from the lower portion of the formation had a uranium-lead age of 1254.8 ± 1.6 million years BP. In the younger section of the overlying Cardenas Basalt, 1104 million years BP have been determined. In addition, a study of the Vishnu Basement Rocks (also using the uranium-lead method) found that these had reached the surface no later than 1250 million years BP. The formation is likely to have been deposited between 1255 and 1240 million years BP.
swell
- Hendricks, JD and Stevenson, GM: Grand Canyon Supergroup: Unkar Group . In: Beus, SS and Morales, M. (Eds.): Grand Canyon Geology . Oxford University Press / Museum of Northern Arizona, 1990.
Individual evidence
- ^ Noble, LF: The Shinumo quadrangle, Grand Canyon district, Arizona . In: Bulletin no.549 . US Geological Survey, Reston, Virginia 1914.
- ^ Elston, DP and Scott, GR: Unconformity at the Cardenas-Nankoweap contact (Precambrian), Grand Canyon Supergroup, northern Arizona . In: Geological Society of America Bulletin . 87, no.12, 1976, p. 1763-1772 .
- ^ Dalton, Russell O., Jr .: Stratigraphy of the Bass Formation (Late Precambrian, Grand Canyon, Arizona). Unpublished thesis . Northern Arizona University, Flagstaff, Arizona 1972, p. 140 .
- ↑ Nitechi, MH: pseudo-organic structures from the Precambrian Bass limestone in Arizona . In: Fieldiana Geology . tape 23 (1) , 1971, pp. 1-9 .
- ↑ Bloch, JD, Timmons, JM, Gehrels, GE, Crossey, LJ and Karlstrom, KE: Mudstone petrology of the Mesoproterozoic Unkar Group, Grand Canyon, USA: provenance, weathering and sediment transport on intracratonic Rodinia . In: Journal of Petrology . tape 76 (9) , 2006, pp. 1106-1119 .
- ↑ Billingsley, GH, Spamer, EE and Menkes, D .: Quest for the Pillar of Gold: the Mines & Miners of the Grand Canyon . Grand Canyon Association, Grand Canyon, Arizona, 1997, p. 112 .
- ↑ Timmons, J. M, Karlstrom, KE, Heizler, MT, Bowring, SA, Gehrels, GE and Crossey, LJ: Tectonic inferences from the ca.1254-1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis . In: Geological Society of America Bulletin . tape 117 (11/12) , 2005, pp. 1573-1595 .