Magnetostratigraphy

Magnetostratigraphy is based on the polarity of the earth’s magnetic field as it is preserved in the sediments at the time when they were deposited. The magnetic minerals in the sediments orient themself in the direction of earth’s magnetic field and preserved.

• The current magnetic north pole is located in the eastern Canada at 77^o18′ N latitude and 101^o48′ W longitude.
• The current magnetic south pole is located in the eastern Antarctica at 65o48′ S latitude and 139o E longitude.

The earth’s magnetic poles have switched their position periodically in the geological past known as reversal of magnetic poles. The reversal of the magnetic poles takes place in two stages as following –

1. At first stage, the intensity of the earth’s magnetism slows down and the flow of electric currents of the earth’s core is gradually reduced to a near zero level.
2. At second stage, the reversal of the poles takes place and the earth’s magnetic intensity is gradually briught back to its original value.

The positive direction of magnetization of a rock is, by definition, its “north-seeking magnetization” (it points toward the Earth’s present magnetic North Pole), and the rock is said to have “normal magnetization”, or “normal polarity”. Conversely, if it points to the present magnetic South Pole, the rock is said to have “reversed magnetization”, or “reversed polarity”.

A problem arises because the north paleomagnetic pole is believed to have crossed the geographic equator in Paleozoic time, so that for some lower Paleozoic and older rocks it is unclear which is the direction of the North Pole and which the South Pole. Polarity must in these cases be defined with respect to the apparent polar wander path (APWP) for the crustal plate where it is found.

The basic formal unit in magnetostratigraphic polarity classification is the magnetostratigraphic polarity zone, or simply polarity zone. Polarity zones may be subdivided into polarity subzones and grouped into polarity superzones.

Magnetostratigraphic polarity zones may consist of bodies of strata unified by-

1. A single polarity of magnetization;
2. An intricate alternation of normal and reversed polarity of magnetization;
3. Having dominantly either normal or reversed polarity, but with minor intervals of the opposite polarity.