Chert

A hard, dense sedimentary rock composed of fine-grained silica (SiO2). Chert is characterized by a semivitreous to dull luster and a splintery to conchoidal fracture, and is most commonly gray, black, reddish brown, or green. Chert is also used as a field term to describe silica-rich rocks which may be impure; common impurities include carbonates, iron and manganese oxides, and clay minerals. When impurities change the texture of the rock to the extent that it is less dense and hard than chert, and has the appearance of unglazed porcelain, the rock is then called porcellanite or siliceous shale. The term flint is synonymous with chert, but its use has become restricted to archeological artifacts and to nodular chert that occurs in chalk. The term chert, however, is preferred for the nodular deposits. Jasper refers to red or yellow quartz chert associated with iron ore or containing iron oxide. Novaculite is a white chert of great purity and uniform grain size, and is composed chiefly of quartz; the term is mostly restricted to descriptions of Paleozoic cherts in Oklahoma and Arkansas. Chert synonyms that have become obsolete include silexite, petrosilex, phthanite, and hornstone. See also: Jasper

Composition

The silica that composes chert occurs in four forms: quartz, chalcedony, opal-CT, and opal-A. Chalcedony is a microscopically fibrous form of the mineral quartz with water trapped in void spaces parallel to the fibers. The poorly understood opal-CT phase is probably an interstratified mixture of the silica minerals cristobalite and tridymite, although it has been suggested to be solely disordered tridymite. In chert, the chalcedony, quartz, and opal-CT are secondary forms of silica derived from the dissolution and later precipitation of a primary silica phase, which for most chert is biogenic silica (opal-A). The main rock-forming types of biogenic silica are the tests and frustules of the siliceous plankton diatoms, radiolarians, silicoflagellates, and ebriidians, and the siliceous spicules of sponges. The composition of chert changes with time and temperature, beginning with opal-A, which transforms to opal-CT and finally to quartz or chalcedony. Under special conditions, opal-A can transform directly to quartz. See also: Chalcedony; Opal; Quartz

Diagenesis

Opal-A and opal-CT are metastable phases within the Earth's crust. The diagenetic transformation of opal-A to opal-CT and then to quartz is primarily controlled by temperature and time, but also by surface area, pore-water chemistry, sediment permeability, and host-sediment composition. The mineralogical transformations are commonly, but not invariably, accompanied by textural transformations. For example, diatom or radiolarian ooze (unconsolidated marine sediment composed mostly of diatom frustules or radiolarian tests) or diatomaceous or radiolarian earth (weakly consolidated oozes that have been incorporated into on-land deposits), which are composed of opal-A, transform into porcellanite (most commonly composed of opal-CT), which in turn transforms into chert (most commonly composed of quartz). See also: Diagenesis; Marine sediments

Occurrence

Chert occurs mainly in three forms: bedded sequences (see illus.), nodular, and massive. Bedded chert (called ribbon chert if beds show pinch-and-swell structure) consists of rhythmically interlayered beds of chert and shale; chert and carbonates; or in some pre-Phanerozoic formations, alternations of chert and siderite or hematite. Bedded sequences can be hundreds of feet thick stratigraphically and cover areas of hundreds of square miles. Individual beds are commonly ½–8 in. (1–20 cm) thick. Intercalated shale layers are commonly, but not invariably, thinner than the associated chert layers. In many places individual chert beds are laminated. Chert nodules and lenses occur primarily in chalk, limestone, and dolomite, and are the only types of chert recovered by the Deep Sea Drilling Project from open-ocean sediment. Chert nodules are well known from many places in western Europe, where their dark color contrasts strikingly with the white host chalk. Nodules and lenses vary in size from ½ in. to 30 ft (1 cm to 9 m). Fossils and sedimentary structures characteristic of the host rock are preserved within the nodules. Massive cherts occur in the interstices between basalt pillows, and as the basal member of bedded chert that overlies pillow basalts in ophiolites. See also: Basalt; Chalk; Dolomite rock; Limestone

 

Rhythmically bedded radiolarian chert from the Franciscan Complex, Diablo Range, California; of Late Jurassic age.

Origin

When a supply of silica is available, chert forms in four ways: (1) by replacement of mainly carbonate rock; (2) by deposition from turbidity currents composed primarily of biogenic silica; (3) by increasing the deposition of silica relative to terrigenous input, commonly by increased productivity of biogenic silica; and (4) by precipitation of silica from water under either hydrothermal or low-temperature hypersaline conditions. The replacement of chalk and limestone by silica has been well documented in Deep Sea Drilling Project cores and in some deposits on land. The silica is derived from the dissolution of biogenic silica deposited with the biogenic carbonate. The resultant chert most commonly forms nodules, but the silica may also replace beds of limestone completely, thereby forming bedded chert. Most bedded chert, however, is probably turbidite sequences, although additional work is necessary to confirm this hypothesis. Transient, sediment-laden density currents (turbidity currents) that flowed down the sides of submarine basins carried mostly biogenic silica which settled in layers. The layers later transformed into cherts through diagenetic processes. Pelagic or hemipelagic clays were deposited in the basin between times of turbidite deposition, thus creating the rhythmically bedded chert-shale sequences found in many mountain belts. In some places, the shales may be the turbidites and the cherts the pelagic deposits. The rhythmic bedding can also be produced by productivity cycles of biogenic silica. Of relatively minor volumetric importance in post-Archean rocks is the primary precipitation of silica. Inorganic precipitation of silica takes place in hypersaline lakes and lagoons. Chert also forms by deposition of silica from hydrothermal solutions into the interstices between basalt pillows, and in other localized areas within ophiolite sequences. See also: Turbidite; Turbidity current

Distribution

The distribution of chert is tied to the distribution of siliceous plankton, which in turn depends on oceanographic conditions. After the plankton die, their shells accumulate in great quantities on the sea floor, especially around the polar regions (diatoms) and the equatorial belt of high biological productivity (radiolarians and diatoms). Shells also accumulate in other areas where cold, nutrient-rich subsurface water upwells, such as off the west coasts of continents and in marginal seas such as the Philippine and Japan seas. Marine siliceous deposits, along with the underlying oceanic crust, are incorporated into mountain belts by uplift of oceanic crust or by obduction and thrust faulting at subduction plate margins. Such on-land cherts of every age are found at many locations around the world. See also: Bacillariophyceae

Radiolarians and sponge spicules occur in Phanerozoic chert. Diatoms first evolved during the Mesozoic Era and came into prominence in the Tertiary, when they became the dominant precursor of chert. The source of silica for pre-Phanerozoic cherts, which are commonly associated with banded iron formations, is equivocal. Direct evidence of a biological source is lacking, and inorganic precipitation of silica is generally postulated. Most Archean (before 2.5 billion years ago) cherts likely formed from hydrothermal or other abiotic processes. Future studies may yield information that would indicate additional sources of silica for pre-Phanerozoic cherts. See also: Sedimentary rocks

Bibliography

 

  • S. R. Aston (ed.), Silicon Geochemistry and Biogeochemistry, 1983
  • P. J. Heaney, C. T. Prewitt, and G. V. Gibbs (eds.), Silica: Physical Behavior, Geochemistry and Materials Applications, Mineralogical Soc. Amer. Rev. Mineralogy, vol. 29, Washington, DC, 1994
  • J. R. Hein, Bibliography of Fine-Grained Siliceous Deposits, USGS Open-File Rep., no. 80–391, 1980
  • J. R. Hein (ed.), Siliceous Sedimentary Rock-Hosted Ores and Petroleum, Van Nostrand Reinhold, New York, 1987
  • J. R. Hein and J. Obradović (eds.), Siliceous Deposits of the Tethys and Pacific Regions, Springer-Verlag, New York, 1989
  • Ali Fazeli = egeology.blogfa.com
  • Ali Fazeli = Springerlink.blogfa.com
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  • A. Iijima, J. R. Hein, and R. Siever (eds.), Siliceous Deposits in the Pacific Region, 1983
  • Ali Fazeli = egeology.blogfa.com
  • Ali Fazeli = Springerlink.blogfa.com