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acme zone

Overview page. Subjects: Earth Sciences and Geography — Ecology and Conservation.

Informally, a body of strata that contains the maximum abundance of a particular fossil taxon occurring within the stratigraphic range of that taxon, and after which the zone is named.

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addition rule

Overview page. Subjects: Earth Sciences and Geography.

With reference to crystallographic notation, the rule stating that the indices (see miller indexes) of two crystal faces in the same zone always add up to the indices of a face bevelling...

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biochron

Overview page. Subjects: Earth Sciences and Geography.

The length of time represented by a biostratigraphic zone.

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biostratigraphy

Overview page. Subjects: Biological Sciences.

The branch of stratigraphy that involves the use of fossil plants and animals in the dating and correlation of the stratigraphic sequences of rock in which they are discovered. A zone is...

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biozone

Overview page. Subjects: Biological Sciences — Earth Sciences and Geography.

1 The total, global, time-stratigraphic range of a given taxon, i.e. all the rocks laid down in the time interval during which that taxon existed.

2 A...

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b-zone

Overview page. Subjects: Ecology and Conservation — Earth Sciences and Geography.

A biostratigraphic zone that is distinguished by benthonic (see benthos) fossils (e.g. brachiopods and trilobites). The term was proposed in 1965 by T. G. Miller. Compare p-zone.

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geochronometry

Overview page. Subjects: Earth Sciences and Geography — Ecology and Conservation.

The determination of the length of time intervals. Geochronometric resolutions for zonations based on different organisms may be calculated by dividing the time-span of a series by the...

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index fossil

Overview page. Subjects: Biological Sciences — Earth Sciences and Geography.

A fossil whose presence is characteristic of a particular unit of rock (the zone) in which it occurs and after which that zone is named.

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informal

Overview page. Subjects: Earth Sciences and Geography.

The informal naming of a stratigraphic unit occurs when (a) the unit-term is referred to as an ordinary noun, and not in the context of a proper name, e.g. ‘the geologic periods’; and (b) w...

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parallel twins

Overview page. Subjects: Earth Sciences and Geography.

A twinned crystal (see crystal twinning) where the twin axis lies in the composition plane parallel to a zone axis, e.g. a Carlsbad twin in orthoclase feldspar.

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p-zone

Overview page. Subjects: Ecology and Conservation — Earth Sciences and Geography.

A biostratigraphic zone distinguished by pelagic fossils, e.g. ammonites (Ammonoidea) and graptolites (Graptolithina). The term was proposed in 1965 by T. G. Miller. Compare b-zone.

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Rhaetian

Overview page. Subjects: Earth Sciences and Geography.

1 A Late Triassic age, preceded by the Norian, followed by the Hettangian (Lias) and dated at 203.6–199.6 Ma (Int. Commission on Stratigraphy, 2004).

2 Th...

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Scythian

Overview page. Subjects: Earth Sciences and Geography.

Originally defined as a stage at the base of the Triassic in the Alps. Now it is generally regarded as a series divided, according to ammonite zones, into four stages. The corresponding...

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subzone

Overview page. Subjects: Earth Sciences and Geography.

Division of the fundamental unit (zone) used in biostratigraphy. The demarcation of a subzone is based on the fossil subspecies or assemblage contained within the rock sequence studied.

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typological method

Overview page. Subjects: Earth Sciences and Geography.

In stratigraphy, the describing of stratigraphic units in terms of the kind of rock or fossil they contain. In practice it is the lower-ranking units, e.g. zones, that are defined...

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zone axis

Overview page. Subjects: Earth Sciences and Geography.

An axis of a crystal, which normally passes through the centre of the crystal and may or may not be parallel to the crystallographic axes, but which is parallel to the edges of crystal...

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zone symbol

Overview page. Subjects: Earth Sciences and Geography.

The symbol which defines the position of the zone axis of a crystal with respect to the crystallographic reference axes of the mineral. It is enclosed in square brackets to indicate that it...

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zonule

Overview page. Subjects: Earth Sciences and Geography — Medicine and Health.

(zonula) n. (in anatomy) a small band or zone; for example the zonule of Zinn (zonula ciliaris) is the suspensory ligament of the eye. —zonular adj.

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