Eureka Quartzite

The Eureka Quartzite is an extensive Paleozoic marine sandstone deposit in western North America that is notable for its great extent, extreme purity, consistently fine grain size of Quartzite, and its tendency to form conspicuous white cliffs visible from afar.

Eureka Quartzite Stratigraphic range: Ordovician
Type	Geologic formation
Underlies	Hanson Creek Formation
Overlies	Copenhagen Formation
Thickness	150 ft (46 m) (in southern Nevada)
Lithology
Primary	Quartzite
Location
Region	Nevada California Idaho British Columbia Alberta
Country	United States, Canada
Extent	2,200 km (1,400 mi)
Type section
Named for	Eureka, Nevada

The Eureka is commonly underlain and overlain by contrasting slope-forming limestone and dolomite strata, all of Ordovician age. It was named in 1883 for the Eureka mineral district in Nevada,[1] and that name is used almost exclusively in Nevada, but, in ensuing years, as extensions of the deposit were discovered in other areas, the same formation was given many other local names.

Description

Extent

By whatever name, the Eureka can be traced, with gaps, from Nevada northward through Idaho into western Canada along the British Columbia-Alberta boundary,[2][3] and southward to southeastern California,[2][4] a north-south extent of about 2,200 kilometres (1,400 mi). An isolated exposure was identified in Sonora, Mexico 950 kilometres (590 mi) south of its most southerly exposure in California, but that occurrence probably was tectonically displaced there from California.[5] The Eureka and its correlatives are lenticular in cross-section: in Nevada and Utah the formation extends more than 300 kilometres (190 mi) east-west, thinning out in both directions from maxima along its axis of more than 150 metres (490 ft). In Canada the east-west extent is much less but there also, it thins out to both east and west.[3]

Composition

Cliff-forming quartzite, the principle part of the Eureka, is composed of more than 99 percent quartz, which includes both the sand grains and the cement that binds them.[6] The quartz cement accounts for its outstanding hardness and resistance to erosion. Minor constituents are grains of zircon and tourmaline and a trace of feldspar.[6] All of the constituents in Nevada and Utah are less than 1 mm (0.039 in) in diameter; those in Canada are slightly larger. Bioturbation is believed to account for the scarcity of internal bedding.[6]

Origin

Almost all of the constituent grains of the formation were deposited in a near-shore environment, mainly in shallow water, and to a much lesser extent on the beach as determined by the nature of bedding from place to place.[6] The surfaces of the quartz grains are almost universally "frosted" or abraded, indicating that they, at one time or another, occupied a subaerial environment.[6] Almost all of the constituent grains were determined to have originated in Canada, and were carried southward by currents along the eastern shore of the Paleozoic sea.[2] This concept is supported by several lines of evidence: (1) the only plausible source of such a large volume of sand is in Canada at about 56° north latitude where Cambrian sandstone was exposed extensively in Ordovician time;[7] (2) the base of the formation decreases in age from north to south as determined by marine fossils in subjacent beds;[2][4] (3) the formation becomes finer grained from north to south apparently due to progressive abrasion of the grains along the way;[2] and (4) the radiometric age of constituent zircon grains points to a northern source.[8]

See also

• List of fossiliferous stratigraphic units in Nevada
• List of fossiliferous stratigraphic units in California

References

1. Hague, Arnold (1883). Geology of the Eureka Mining District, Nevada. U.S. Geological Survey, Third Annual Report. pp. 237–290.
2. Ketner, Keith B. (1968). Origin of Ordovician quartzite in the Cordilleran miogeosyncline. U.S. Geological Survey, Professional Paper 600-B. pp. 169–177.
3. Norford, B.S. (1966). Ordovician-Silurian of the Cordillera in Geological History of Western Canada. Alberta Society of Petroleum Geologists, Canadian Sedimentary Basins Symposium, Chapter 4, part 2. pp. 42–48.
4. Ross, R.J., Jr. (1964a). Middle and Lower Ordovician formations in southernmost Nevada and adjacent California. U.S. Geological Survey, Bulletin 1180-C. pp. C1–C101.
5. Ketner, Keith B (1986). Eureka Quartzite in Mexico?--Tectonic implications. Geology 14: 1027–1030
6. McBride, E.F. (2012). Petrology of the Eureka Quartzite (Middle and Late Ordovician) Utah and Nevada U.S.A]]. Rocky Mountain Geology 47: 81–111
7. deMille, George (1958). Pre-Mississippian History of the Peace River Arch, in Scott, J.C., ed., Symposium on the Peace River Arch. Alberta Society of Petroleum Geologists Journal 6: 61–68.
8. Gehrels, G.E., Dickinson, W.R., Riley, B.C.D., Finney, S.C., and Smith, M.T. (2000). Detrital zircon geochronology of the Roberts Mountains allochthon, Nevada, in Soreghan, M.J. and Gehrels, G.E., eds., Paleozoic and Triassic paleogeography and tectonics of western Nevada and northern California, Special Paper 347. Boulder, Colorado: Geological Society of America. pp. 19–42.