Relative dating

The relative dating gathers the whole of the methods of Datation making it possible to order chronologically geological events or biological, the ones compared to the others.

Geometrical principles

The succession of the geological phenomena involves modifications of the Roche S and Paysage S which can escape the direct observation. Nevertheless, the rock whole out of preserve of the traces which it is possible to interpret, on the basis of the principle that the laws physico - chemical did not change (principle of Actualisme). This reconstitution rests on three principles.

The principle of superposition

The principle of Superposition can be stated as follows: when Sédiment S settle, or that there is a succession of volcanic castings, the Strate located low will be older than that which is above.

A Strate is a unit sedimentary or volcanic delimited by two more or less parallel Surface S which corresponds to discontinuity S or changes of composition.

Before applying this principle, the Géologue S must seek if the stacking of the layers were modified by an accident Tectonique (in particular thanks to the criteria of polarity).

The principle of stepping

An event (magmatic intrusion , Faille, Plissement, discordance, erosion) which causes a change in the Géométrie of the rocks is posterior with the last layer that it affects and former to the first nonaffected layer.

Any geological event which recuts another him is posterior.

When horizontal layers rest on folded layers, there is an abnormal zone of contact between these two units: one speaks about angular unconformity. That indicates that there was crumpling then erosion.

The principle of continuity

Although the base of a layer is older than its top, it is considered that it has the same age on all its extent even if its composition changes.

This principle cannot apply in the zones of transgression or Marine regression.

The principle of inclusion

The pieces of rock included in another layer are older than their container.

The paleontological identity principle

According to the Fossil S contained in a layer, and by referring to a sedimentary Sequence chosen and dated, one can determine the relative age of a layer or a rock. One thus defines a Biozone (an time interval during which several Espèce S lived together).

The species of Ammonite S being renewed very quickly, they had one very short lifespan. On the other hand, they occupy vast a geographical Territoire. Thanks to these characteristics, the ammonites allow a dating relative of sedimentary grounds even to very large Distance S. a stratigraphic Fossile is a fossil having one relatively short lifespan but a vast geographical distribution. It thus allows a relative dating of very distant grounds and not having inevitably the same composition thanks to the following principle: two grounds presenting the same stratigraphic fossils are same age.

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