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.

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, 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, and southward to southeastern California,Ketner, Keith B. (1968). ''Origin of Ordovician quartzite in the Cordilleran miogeosyncline''. U.S. Geological Survey, Professional Paper 600-B. pp. 169–177.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. a north-south extent of about . An isolated exposure was identified in Sonora, Mexico south of its most southerly exposure in California, but that occurrence probably was tectonically displaced there from California. The Eureka and its correlatives are lenticular in cross-section: in Nevada and Utah the formation extends more than east-west, thinning out in both directions from maxima along its axis of more than . In Canada the east-west extent is much less but there also, it thins out to both east and west.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.

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. 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.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 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.

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. The surfaces of the quartz grains are almost universally "frosted" or abraded, indicating that they, at one time or another, occupied a subaerial environment. 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. 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;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. (2) the base of the formation decreases in age from north to south as determined by marine fossils in subjacent beds; (3) the formation becomes finer grained from north to south apparently due to progressive abrasion of the grains along the way; and (4) the radiometric age of constituent zircon grains points to a northern source.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.

See also

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

References

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Geologic formations of Alberta
Geologic formations of British Columbia
Geologic formations of California
Geologic formations of Idaho
Geologic formations of Nevada
Geologic formations of Utah
Ordovician System of North America
Ordovician Alberta
Ordovician British Columbia
Ordovician California
Ordovician Idaho
Ordovician geology of Nevada
Ordovician geology of Utah
Ordovician southern paleotropical deposits
Quartzite formations
Shallow marine deposits