Stratigraphy

The stratigraphy is a discipline of the Sciences of the Earth which studies the succession of the various geological layers or layer. It is about an integrated approach, in what results produced by the Géochimie, the Paléontologie, the Pétrographie, the Astronomie… are joined together and exploited through various methods: Biostratigraphie, Chimiostratigraphie, Lithostratigraphie, Magnétostratigraphie, Cyclostratigraphie… These methods, developed in a relatively independent way, are then reinvested in approaches plus general practitioners:

  • the Chronostratigraphie seeks to establish a geological time scale as precise and reliable as possible by organizing the data items which the “chronostratigraphic units constitute”;
  • the sequential Stratigraphy, exploiting the Seismic reflection, seeks to light the organization relative of the sedimentary bodies in connection with the cyclic variations of the marine level to the regional scales;
  • the genetic Stratigraphie seeks to release from the general sedimentological models to conduct forecasts in basement, in the absence of complete data (Forage S specific).

Great principles

The principles of stratigraphy are of number variable according to the authors. These principles are on the one hand Postulat S, which it is necessary to check by the observation, and on the other hand geometrical relations between the studied geological formations. The two principles which seem unanimously accepted are the principle of continuity and the principle of superposition.

General principles

  • the principle of continuity : the same layer has the same age on all its extent.

  • the principle of uniformitarianism : the last geological structures were formed by phenomena (tectonic, magmatic, sedimentary or different) acting as at our time. See also Charles Lyell.

  • the paleontological identity principle : two very layers having the Fossile S are regarded as having the same age. This principle is not related on the geometrical relationship between the layers, but to the Paléontologie; it bases on the existence of fossil stratigraphic. It makes it possible to correlate sedimentary sequences of distant areas.

The preceding principles suffer from many counterexamples ( cf will infra ) and must be validated by the observation of the studied situation. They are nevertheless starting points used by all the geologists in an unknown situation.

Geometrical relations between layers

  • the principle of superposition : in the absence of structural upheavals, a layer is more recent than that which it recovers and older than that which recovers it. It is the oldest principle in geology, it was formulated as of the XVIIe century by Nicolas Steno.

  • the principle of horizontality : the sedimentary layers settle horizontally; a sedimentary sequence which is not in horizontal position underwent posterior deformations with its deposit.

  • the principle of stepping : the sedimentary layers are older than the faults or the rocks which recut them.

  • the principle of inclusion : the pieces of rock included in another layer are older than their container.

Exceptions to the principles

For each one of these principles one can find exceptions. These exceptions depend on the mode of sedimentation and the scale to which one observes them:
  • the alluvial tablecloths most recent can be deposited after the cashing of the valley and be lower than the former alluvia (nevertheless, the recent alluvia are not covered by oldest).
  • riverwashes and deltaic do not settle horizontally, but in sedimentation obliques.
  • bio-built sedimentations are not obligatorily horizontal (a coral reef is not horizontal for example).
  • etc

Discordance

When there is interruption of the sedimentation, followed by a deformation (Faille S, swing or crumpling) and of a erosion, there is discordance between the layers (or layer S) the oldest deformations and those more recent, horizontal. There exist also sedimentary discordances. They are the result of a change of the medium of deposit. This change is caused by a variation of the marine level.

A angular unconformity exists between two superimposed layers from which the Pendage S are different on both sides from the unconformity plane. The lower series of layer S.A. then undergone of the deformations (swing in the case of a series Monoclinal E, crumpling in the case of a Synclinal or of a Anticline: in this case it is an angular unconformity on folded structure).

Certain concordant layers in a point, can gradually become unmatched : it is a progressive discordance .

Gap

When there is no chronological continuity between two layers, one speaks about gap . There are two types of gaps:
  • Gap of erosion: erosion removed layers, then sedimentation began again by leaving the gap.
  • Gap of sedimentation: for the period corresponding to the duration of the gap, sedimentation stopped. That can be due to a Marine regression.

Stratigraphy in archeology

One uses the same principles in the field of the Archéologie, the various layers allowing a dating of the excavated site are the result of the natural accumulation of sediments, human contributions, layer of destruction or fire. A stratigraphic cut makes it possible to highlight, by their difference of color and texture, the various layers present on the site and to proceed to the relative dating of the objects contained in these layers.

See too

stratigraphy|stratigraphy

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