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Magnetostratigraphy

Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their characteristic remanent magnetization (ChRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. This is possible in sedimentary rocks because when very fine-grained magnetic minerals (< 17 micrometres) fall through the water column, they orient themselves with Earth's magnetic field. In volcanic flows the magnetic minerals cystalize in orientation with the Earth's field. Upon burial, the orientation of the magnetism is preserved. The minerals, in effect, behave like tiny compasses.

Technique

When measurable magnetic properties of rocks vary stratigraphically they may be the basis for related but different kinds of stratigraphic units known collectively as "magnetostratigraphic units" ("magnetozones"). The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. Such reversals of the polarity have taken place many times during geologic history. They are recorded in the rocks because the rocks may record the direction of the Earth's magnetic field at or near the time of rock formation (see paleomagnetism). The direction of the remnant magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called "magnetostratigraphic polarity units"[1] or chrons.

If the ancient magnetic field was oriented similar to today's field (North Magnetic Pole near the North Rotational Pole) the strata retain a Normal Polarity. If the data indicate that the North Magnetic Pole was near the South Rotational Pole, the strata exhibit Reversed Polarity.

Sampling procedures

Oriented paleomagnetic samples are collected in the field using a Pomeroy Drill, or as hand samples. A minimum of three samples is taken from each sample site for statistical purposes. Spacing of the sample sites within a stratigraphic section depends on: 1) the type of depositional environment: The farther away from the orogenic front, the closer the sample spacing due to generally lower rates of deposition; and 2) the suitability of the rocks for paleomagnetic analysis. Mudstones, siltstones, and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. It is more likely that these samples will deliver a reliable paleomagnetic signal.

Analytical procedures

Samples are first analyzed in their natural state to obtain their natural remnant magnetism (NRM). The NRM is then stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component. The stable component is usually interpreted to be the DRM or detrital remanent magnetization.

DRM orientations of all samples from a site are then compared and their magnetic polarity is determined with Fisher statistics. Using Watson's criteria, the statistical significance of each sample site is evaluated. The latitudes of the Virtual Geomagnetic Poles from those sites determined to be statistically significant are plotted against the stratigraphic level at which they were collected. These data are then abstracted to the standard black and white magnetostratigraphic columns in which black indicates normal polarity and white is reversed polarity.

Correlation and ages

Because the polarity of a stratum can only be Normal or Reversed, variations in the rate at which the sediment accumulated can cause the thickness of a given polarity zone to vary from one area to another. This presents the problem of how to differentiate different zones of like polarities between different stratigraphic sections. To overcome the possibility of confusion at least one isotopic age (or at least an age based on paleontological data) needs to be collected from each section. These are usually obtained from intercalated airfall volcanic material. With the aid of the independent isotopic age or ages, the local magnetostratigraphic column is correlated with the Global Magnetic Polarity Time Scale (GMPTS).

Because the age of each reversal shown on the GMPTS is relatively well known, the correlation establishes numerous time lines through the stratigraphic section. These ages provide relatively precise dates for features in the rocks such as fossils, changes in sedimentary rock composition, changes in depositional environment, etc. They also constrain the ages of cross-cutting features such as faults, dikes, and unconformities.

Sediment accumulation rates

Perhaps the most powerful application of these data is to determine the rate at which the sediment accumulated. This is accomplished by plotting the age of each reversal (in millions of years ago) vs. the stratigraphic level at which the reversal is found (in meters). This provides the rate in meters per million years which is usually rewritten in terms of millimeters per year (which is the same as kilometers per million years).

These data are also used to model basin subsidence rates. Knowing the depth of a hydrocarbon source rock beneath the basin-filling strata allows calculation of the age at which the source rock passed through the generation window and hydrocarbon migration began[2]. Because the ages of cross-cutting trapping structures can usually be determined from magnetostratigraphic data, a comparison of these ages will assist reservoir geologists in their determination of whether or not a play is likely in a given trap.

Another application of these results derives from the fact that they illustrate when sediment accumulation rates changed. Such changes require explanation. The answer is often related to either climatic factors or to tectonic developments in nearby or distant mountain ranges. Evidence to strengthen this interpretation can often be found by looking for subtle changes in the composition of the rocks in the section. Changes in sandstone composition are often used for this type of interpretation.

References

1. ^ Chapter 9 of Butler,R.F. 1992. Paleomagnetism: Magnetic Domains to Geologic Terranes
2. ^ Reynolds,J. 2002. Magnetostratigraphy Adds a Temporal Dimension to BasinAnalysis


See also

* Biostratigraphy
* Lithostratigraphy
* Tectonostratigraphy

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