Magnetostratigraphy

Palaeomagnetic results from rocks and sediments show that through geologic time the Earth's magnetic field direction has not been constant, but has periodically reversed in direction . In fact the field has reversed polarity many times. The changes in polarity, when correlated to a radiometric or biostratigraphic time scale form a magnetic polarity time scale (MPTS). 

When sediments are deposited, the direction of the magnetic field vector at that time can be recorded by the small amounts of magnetic minerals present in the sediments. By this process, the reversals of the Earth's magnetic field are recorded in sedimentary sequences.

How is the Magnetostratigraphy Evaluated? 

Sampling a sequence of rocks, and subsequent measurement of their palaeomagnetic record, enables chronostratigraphic dating of these rocks by comparison with the MPTS. Local correlation to adjacent cores can also be performed, if a clear match to the MPTS is not possible. Samples from boreholes provide an inclination measurement but not a declination measurement (unless the core is oriented), because of core section rotation during the coring process. 

Measurement

Sediments older than the Pleistocene rarely contain a perfectly preserved magnetic record of the Earth's magnetic field, so that palaeomagnetists apply 'cleaning techniques' (demagnetization) to the sediment magnetization. This serves two purposes:

• To remove magnetic components which have been acquired at a later stage in the sediment's history, perhaps through diagenesis or thermal events.
• To assess the integrity of the sample's magnetic record on the basis of the behaviour of the rock's magnetic components during demagnetization, integrated with the rock's likely magnetic and diagenetic history.

The application of the demagnetization techniques to the rock samples is a fundamental part of the dating procedure, since it is by this means that the fidelity of the rocks' magnetic record is examined and interpreted. Other information on the burial and diagenetic history also play a part in the interpretation process.

Sampling

The type of samples used in palaeomagnetic studies are 25mm diameter drill plugs cut from cores, with the up-core direction marked on the drill plug. The sampling intervals are located to take account of a) independent means of dating, and b) bedding dips for structural correction. Duplicate samples from selected core sections also provide a means of assessing the inter-sample variability and fidelity.

An example of magnetostratigraphy applied to commercial cores can be found in Hounslow et al., (1995).

Previous Clients

Dr. Mark Hounslow for further details ("m.hounslow" using dept adddress @lancs.ac.uk)