magnetostratigraphy

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magnetostratigraphy Magnetostratigraphy is the application of the chronology of reversals in polarity of the geomagnetic field to the study of the stratigraphy of layered materials (i.e. sedimentary and volcanic rocks). An essential criterion for accurate magnetostratigraphic work is a reliable timescale for the reversal chronology of the Earth's magnetic field: hence the development of the geomagnetic polarity timescale (GPTS) (Fig. 1), in itself a remarkable story. Early work in palaeomagnetism demonstrated that some rocks were magnetized in a direction opposite to that of the present field. For example, a north-seeking compass needle, as we think of it today, would have pointed to the south pole. One explanation required that the geomagnetic field reversed itself at least once, if not several times, in the geologic past. Alternatively, it was argued that at least some rock types are capable of self-reversal, where the magnetization acquired is opposite to the field applied. The possibility that the geomagnetic field itself does reverse has important consequences for processes in the core and lower mantle and, as noted in the 1920s, is testable because, worldwide, rocks of the same age should have the same polarity. Research provided convincing evidence that the geomagnetic field has reversed its polarity several times in the past, and a crude GPTS was born.

The discovery of the relationship between alternating stripes of ocean floor that were normally or reversely magnetized and reversals in the geomagnetic timescale lead to the platetectonics paradigm and provided the opportunity for substantial refinements in the GPTS. Although first-order estimates of age durations of polarity magnetozones could be provided by date on marine magnetic anomalies, the absolute ages of magnetozones are based on independent geochronologic information (e.g. dating of lava flows just younger than and just older than a specific reversal) from the terrestrial record, and are constantly being refined. More recently, the combination of high-precision ⁴⁰Ar/³⁹Ar isotopic age spectra and magnetic polarity data for igneous rocks has led to improved estimates of ages of parts of the polarity timescale, particularly for the Cenozoic. For example, the age of the most recent complete reversal, from the reversed polarity Matuyama magnetozone to the present normal polarity Brunhes magnetozone, has been revised from 730 000 years to about 780 000 years. Further research will significantly improve the chronological framework provided by the geomagnetic polarity timescale and thus estimates of ages of sequences of rocks by magnetostratigraphy and geological processes based on the polarity timescale. In addition, detailed palaeomagnetic records from some sequences of lava flows and recent sediments have revealed the morphology of the transitional part of a reversal. Rather than an actual ‘flipping’ of the main dipole, the short transitional period (estimated to be about a few thousand years in duration) is characterized by a decay of the main dipole to an almost non-existent state, and domination by smaller, non-dipole components.

Magnetostratigraphic studies have several goals, among which are:(1) defining the polarity history of the Earth, in particular prior to about 160 million years, the age of the oldest oceanic lithosphere yielding interpretable marine magnetic anomaly data;(2) time correlation of different sections of strata; and(3) determination of absolute ages of overall sections of strata where independent information (e.g. isotopic age data) is available.

For any magnetostratigraphic study, the approach should be no different from that of conventional palaeomagnetism, with the caveat that the sampling strategy is to obtain a polarity record of a rock section that is as complete as possible. Several factors, including the amount of time missing in the rock sequence, the spacing of individual measurements, and the fidelity of the process by which magnetization is acquired, make magnetostratigraphic studies challenging and complicated. At each stratigraphic level, the quality of the magnetization signal and hence the reliability of the polarity determination must be documented. Furthermore, it must be demonstrated that the rocks possess a primary magnetization. This is relatively straightforward for sequences of lava flows but is more difficult in sedimentary rocks. As an example, a generally high remanence intensity and overall abundance in the geologic record have made haematite-cemented detrital sedimentary rocks (‘red beds’) a focus of much magnetostratigraphic as well as palaeomagnetic research. Perhaps by default, these rocks have also been the focus of considerable controversy that bears on the interpretation of magnetostratigraphic data. At the centre of this debate is the carrier and age of the magnetization characteristic of the rocks. Rock-magnetic data indicate that the magnetization is typically carried by haematite, but the haematite could occur as detrital grains, whose magnetic moments aligned themselves with the ambient field during deposition, or as the interstitial cement, which could have been precipitated at any time after deposition of the sediment. If a consensus is emerging, it is that the cement in many red beds is acquired relatively early after sedimentation; many red beds are thus considered to yield magnetostratigraphies of reasonably high fidelity.

John W M Geissman

Bibliography

Butler, R. F. (1992) Paleomagnetism: magnetic domains to geologic terranes. Blackwell Scientific Publications, London.
Tarling, D. H. (1983) Palaeomagnetism. Chapman and Hall, London.

magnetostratigraphic time-scale (polarity time-scale, geomagnetic reversal time-scale, reversal time-scale) A time-scale based on the periodic polarity reversals in the Earth's geomagnetic field. Magnetic minerals within a rock retain an orientation induced by the field at the time the rock was formed (see NATURAL REMANENT MAGNETISM). Provided they include suitable minerals, strata from all over the world thus contain a record of the normal (as at present) or reversed state of the geomagnetic field at the time of their formation. This reversal pattern has been correlated between different successions of rocks to produce a sequence that, when combined with a dating method such as potassium—argon dating, has given a time-scale measured in units of normal or reversed polarity. The scale was first established in detail for the last 4.5 Ma using data from terrestrial, mainly extrusive, rocks; it has now been extended back to the Upper Jurassic by means of the magnetic-anomaly patterns in oceanic crust. The terms proposed by the ISSC for geochronologic units of the magnetostratigraphic time-scale are polarity superchron, polarity chron (replacing the earlier term ‘epoch’), and polarity subchron (replacing the earlier term ‘event’). The corresponding terms proposed for rocks deposited during those polarity intervals are polarity superchronozone, polarity chronozone, and polarity subchronozone.

magnetostratigraphic time-scale(polarity time-scale, geomagnetic reversal time-scale, reversal time-scale) A time-scale based on the periodic polarity reversals in the Earth's geomagnetic field. Magnetic minerals within a rock retain an orientation induced by the field at the time the rock was formed (see natural remanent magnetism). Provided they include suitable minerals, strata from all over the world thus contain a record of the normal (as at present) or reversed state of the geomagnetic field at the time of their formation. This reversal pattern has been correlated between different successions of rocks to produce a sequence which, when combined with an appropriate dating method, has given a time-scale measured in units of normal or reversed polarity. The scale was first established in detail for the last 4.5 Ma using data from terrestrial, mainly extrusive, rocks; it has now been extended back to the Upper Jurassic by means of the magnetic-anomaly patterns in oceanic crust.

magnetostratigraphy Branch of stratigraphy based on geomagnetic polarity reversals.