Magnetostratigraphy - concepts, definitions, and applications

The most characteristic feature of the Earth's magnetic field is that it reverses polarity at irregular intervals, producing a 'bar code' of alternating normal (north directed) and reverse (south directed) polarity chrons with characteristic durations. Magnetostratigraphy refers to the application of the well-known principles of stratiaraphy to the pattern of polarity reversals registered in a rock succession by means of natural magnetic acquisition processes. This requires that the rock faithfully recorded the ancient magnetic field at the time of its formation, a prerequisite that must be verified in the laboratory by means of palaeomagnetic and rock magnetic techniques. A sequence of intervals of alternatively normal or reverse polarity characterized by irregular (non-periodic) duration constitutes a distinctive pattern functional for correlations. Over the last 35 Myr, polarity intervals show a mean duration of 300,000 years, but large variations occur from 20,000 yr to several Myr and even up to tens of Myr. By correlating the polarity reversal pattern retrieved in a rock succession to a reference geomagnetic polarity time scale (GPTS), calibrated by radioisotopic methods and/or orbital tuning, the age of the rock succession can be derived. Magnetostratigraphy and correlation to the GPTS constitute a standard dating tool in Earth sciences, applicable to a wide variety of sedimentary (but also volcanic) rock types formed under different environmental conditions (continental, lacustrine, marine). It is therefore the stratigraphic tool of choice to perform correlations between continental and marine realms. Finally, we emphasise that magnetostratigraphy, as any other stratigraphic tool, works at best when integrated with other dating tools, as illustrated by the case studies discussed in this paper.