Faunal stage
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A stage is in stratigraphy the succession of rock strata laid down in an age of the geologic timescale. It consists of the depositions matching and paired with the geologic age of the same name. Stages can be divided into hierarchically smaller stratigraphic units called chronozones and are themselves subdivisions of series.

In the geological timescale, ages are units of time in the Geologic time scale, delineating a span of years, each much shorter than the successively larger geological epochs, periods, eras and eons, all of which are defined in the hierarchy in terms of consisting of, and each containing multiple geological ages (or other smaller units of lesser hierarchical rank) as their building blocks. All the units of the hierarchy share (or encompass) the same boundary criteria.

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The use of the term stage is therefore referring to chronostratigraphy ("rock history"), the sequence of and characteristics of the layers of rock laid down during a span of time, and connote an interdisciplinary criteria for comparatively evaluating previously unstudied formations. For such evaluations, the question of whether a specific outcrop or section of such depends on whether when studied it satisfies (belongs within) a given stages' definition criteria, or whether it possesses features or characteristics that place it elsewhere in the time line of Earths history.

Dual hierarchys

The time span in earth history during which the stage accrued is the matching age (geochronological time unit). The Earth Scientists in international fora have decided that geological stages, during which the rocks comprising a stage were formed, each also form the building blocks of the larger geological hierarchy of deposits, the successively larger geological divisions—the series, systems, erathems, and eonothems— each having certain characteristics which set them apart from the others, and each corresponding to a matching time unit covering larger spans of years (listed above in the same hierarchical order).

Stages, or more completely, Faunal stages (see Concept of stage below) are the base subdivisions of the various distinct rock layers hierarchically established within a geological series laid down during the formation the rock. The beginning point in the series of each stage is defined by the International Commission on Stratigraphy of the International Union of Geological Sciences, based on unique and unambiguous identification factors and know sequences of fauna succession or faunal combinations and ratios over the age when the rock formed. The beginning point of the next older stage ends the younger, forming a continuum across the strata classified hierarchically into the successively larger geological series, systems, erathems and eonothems of stratigraphy. The span of years between the two boundary delineating reference points is called an age, and the stage (rock strata laid down over time, having various describable characteristics and features) and the age (a unit of time) share the same name. An age is unit of time, corresponding to and named the same as the stage it is paired with. Moreover, in the geologic time system, each higher type of time unit (epoch, period, era, eon) and their paired classification schema (series, system, erathem, eonothem) are organized with geological ages and stages as their subdivisions, sharing the same boundary points. A stage generally (excepting strata greater than 542-630 Mya, where the fossil record becomes spotty) needs microscopic analysis (petrological analysis) to be accurately determined, including biostratigraphic analysis, since it's macroscopic characteristics are insufficient for accurate classification. In short stages are primarily determined by the fossil record which each contains within the rock column in a particular locale and generally independent of gross physical characteristics and so therefore lithostratigraphical methods are of limited utility.

The geological ages are hierarchically the shortest unit of time used in the Earth sciences, but not the shortest unit used in the field—Stages are subdivisions of series and are themselves (or may be, according to the International Commission on Stratigraphy (ICS) a chronozone) each of which, may also be divided into other chronozones.[1]—the distinction being that a chronozone is any convenient grouping of geographically consistent well defined successions of layers, whether long or short.

International benchmarks

As in 2008, the International Commission on Stratigraphy (ICS) is nearly finished a task begun in 1974, subdividing the Phanerozoic eonothem into internationally accepted stages, and establishing a series of important basal "international benchmarks" (See the articles GSSP and GSSA, the two types of benchmark transition points the ICS' strata correlations in good outcrops on each continent are marking) at the boundaries of each based on a rigid set of criteria. In many regions around the world local subdvisions and classification criteria are still used along with the newer internationally coordinated uniform system, but once the research establishes a more complete international system, they should be abandoned eventually. The benchmarks will give a much greater certainty that results can be compared with confidence in the date determinations, and such results will have farther scope than any evaluation based solely on local knowledge and conditions.

Stage is a term defining a package of rocks formed in a certain interval in time; it is equivalent to the term age defining the interval of time itself, although the two words are often confused in informal literature.

Concept of stage

Further information: biostratigraphy#Concept of stage

A stage is a major subdivision of strata, each systematically following the other each bearing a unique assemblage of fossils. Therefore, stages can be defined as a group of strata containing the same major fossil assemblages. French palaeontologist Alcide d'Orbigny is credited for the invention of this concept. He named stages after geographic localities with particularly good sections of rock strata that bear the characteristic fossils on which the stages are based. Interdisciplinary co-ordination and cooperation has over the years given us the Geologic time scale system, wherein

Defining stages

Typically, a stage will be defined on fossil content (biostratigraphy) or paleomagnetic polarity in the rock. Usually one or more index fossils that are found worldwide, are common, easily recognized, and limited to a single, or at most a few, stages are used to define the stage's bottom. Thus, for example, in the (still used) local North American subdivision paleontologist finding fragments of the trilobite Olenellus would identify the beds as being from the Waucoban Stage whereas fragments of a later trilobite such as Elrathia would identify the stage as Albertan.

Stages were very important in the 19th and early 20th century as they were the major tool available for dating rock beds until the development of seismology and radioactive dating in the second half of the 20th Century.

Stages and lithostratigraphy

Stages can include many lithostratigraphic units (for example formations, beds, members, etc.) of differing rock types that were being laid down in different environments at the same time. In the same way, a lithostratigraphic unit can include a number of stages or parts of them.

See also

Multidiscipline comparison

e  h
Units in geochronology and stratigraphy
Chronostratigraphic
strata units of the
geologic record
Geochronological
units of the
geological time system
 Main article: geologic time scale

Notes
Eonothem
Eon
 4 total, largest spans of years
Erathem
Era
  12 total, roughly span multiples
of a hundred million years
System
Period
  Some familiar names, defined in
21 major divisions and 2 minor
Series
Epoch
 48 units & each spanning
tens of millions of years
Stage
Age
 100+ units & most all
spanning millions of years
Outside the heirarchy, units cross stage and age boundaries as needed
if the strata has good dating characteristics.
Chronozone Chron  only in more recent strata*
* Most and the most time specific time divisions[2]


Related other topics

External links

Notes and references

  1. ^ chronozone, footnote on ICS usage
  2. ^ International Commission on Stratigraphy. "International Stratigraphic Chart". Retrieved on 2008-06-17.

References
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