#zone fossil

[earthstory]

Zone Fossils (Index Fossils) The recognition and use of zone fossils is fundamental to biostratigraphic correlation. Fossil groups that were (i) rapidly evolving, (ii) widespread across different facies and biogeographic provinces (facies are the rocks that represent a particular life environment), (iii) relatively common, and (iv) easy to identify make the ideal zone fossils. In the Early Paleozoic macrofauna, graptolites are the closest to being ideal zone fossils, whereas during the Mesozoic, the ammonites(http://on.fb.me/1LYL0Lm) are most useful.

The use of efficient zone fossils ensures that relatively short intervals of geological time can be correlated, often with a precision of a few hundred thousand years, over long distances through different facies belts around the world. In practice there are no ideal zone fossils. Most long-range correlations involve use of intermediate faunas with mixed facies. For example, in Ordovician and Silurian rocks, deep-water facies are correlated by means of the rapidly-evolving and widespread graptolites; these fossils are rare in shallow-water shelf deposits where trilobites and brachiopods are much more common. Nevertheless, facies with both graptolite and shelly faunas may interdigitate in deep-shelf and slope sequences, allowing correlation through these mixed facies from deep to shallow water. Parallels can be drawn with a neritic ammonites and benthic bivalves and gastropods of the Mesozoic seas. Microfossils are widely used for correlation in hydrocarbon exploration; the amount of rock available in drill cores or cuttings is usually limited and a range of fossil microorganisms including foraminiferans (members of a phylum or class of amoeboid protists), and radiolarians (protozoa) together with dinoflagellates (flagellate protists), spores and pollen form the basis of the correlation schemes used by petroleum companies. ~ JM Image Credit: http://bit.ly/1grt7ZC Labelled for re-use. More Info: Index Fossils: https://pubs.usgs.gov/gip/geotime/fossils.html http://abt.cm/1dPEOY3

[earthstory]

Zone Fossils (Index Fossils)

The recognition and use of zone fossils is fundamental to biostratigraphic correlation. Fossil groups that were (i) rapidly evolving, (ii) widespread across different facies and biogeographic provinces (facies are the rocks that represent a particular life environment), (iii) relatively common, and (iv) easy to identify make the ideal zone fossils. In the Early Paleozoic macrofauna, graptolites are the closest to being ideal zone fossils, whereas during the Mesozoic, the ammonites(http://on.fb.me/1LYL0Lm) are most useful.

The use of efficient zone fossils ensures that relatively short intervals of geological time can be correlated, often with a precision of a few hundred thousand years, over long distances through different facies belts around the world. In practice there are no ideal zone fossils. Most long-range correlations involve use of intermediate faunas with mixed facies. For example, in Ordovician and Silurian rocks, deep-water facies are correlated by means of the rapidly-evolving and widespread graptolites; these fossils are rare in shallow-water shelf deposits where trilobites and brachiopods are much more common. Nevertheless, facies with both graptolite and shelly faunas may interdigitate in deep-shelf and slope sequences, allowing correlation through these mixed facies from deep to shallow water. Parallels can be drawn with a neritic ammonites and benthic bivalves and gastropods of the Mesozoic seas.

Microfossils are widely used for correlation in hydrocarbon exploration; the amount of rock available in drill cores or cuttings is usually limited and a range of fossil microorganisms including foraminiferans (members of a phylum or class of amoeboid protists), and radiolarians (protozoa) together with dinoflagellates (flagellate protists), spores and pollen form the basis of the correlation schemes used by petroleum companies.

~ JM

Image Credit: http://bit.ly/1grt7ZC Labelled for re-use. Sourced on 09/07/15.

More Info: Index Fossils: http://on.doi.gov/1grsu2b http://abt.cm/1dPEOY3 http://1.usa.gov/1TppNMW