Magnetostratigraphy is a geophysical correlation 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 ChRMthat is, the polarity of Earth's magnetic field at the time a stratum was deposited.
This is possible because volcanic flows acquire a thermoremanent magnetization and sediments acquire a depositional remanent magnetizationboth of which reflect the direction of the Earth's field at the time of formation. This technique is typically used to date sequences that generally lack fossils or interbedded igneous rock.
When measurable magnetic properties of rocks vary stratigraphically they Magnetostratigraphic dating sim be the basis for related but different kinds of stratigraphic units known collectively as magnetostratigraphic units magnetozones. 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" or chrons. If the ancient magnetic field was oriented similar to today's field North Magnetic Pole near the Geographic North Pole the strata retain a normal polarity.
If the data indicate that the North Magnetic Pole was near the Geographic South Polethe strata exhibit Magnetostratigraphic dating sim polarity.
Oriented paleomagnetic samples are collected in the field using a rock core drill, or as hand samples chunks broken off the rock "Magnetostratigraphic dating sim." To average out sampling errors, a minimum of three samples is taken from each sample site.
In sedimentary layers, the preferred lithologies are mudstonesclaystonesand very fine-grained siltstones because the magnetic grains are finer and more likely to orient with the ambient Magnetostratigraphic dating sim during deposition.
Samples are first analyzed in their natural state to obtain their natural remanent magnetization NRM.
The NRM is then stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component. Magnetic orientations of all samples from a site are then compared and their average magnetic polarity is determined with directional statisticsmost commonly Fisher statistics or bootstrapping. The latitudes of the Virtual Geomagnetic Poles from those sites determined "Magnetostratigraphic dating sim" 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. Because the polarity of a stratum can only be normal or Magnetostratigraphic dating sim, 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 correlate zones of like polarities between different stratigraphic sections. To avoid confusion at least one isotopic age needs to be collected from each section.
In sediments, this is often obtained from layers of volcanic ash. Failing that, one can tie a polarity to a biostratigraphic event that has been correlated elsewhere with isotopic ages. 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 fossilschanges in sedimentary rock composition, changes in depositional environment, etc.
They also constrain the ages of cross-cutting features such as faultsdikesand unconformities. 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. This provides the rate in meters per million years which is usually rewritten in terms of millimeters per year Magnetostratigraphic dating sim 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 "Magnetostratigraphic dating sim" of the age at which the source rock passed through the generation window and hydrocarbon migration began.
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. Changes in sedimentation rate revealed by magnetostratigraphy are often related to either climatic factors Magnetostratigraphic dating sim 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. From Wikipedia, the free encyclopedia. Magnetic Domains to Geologic Terranes. Originally published by Blackwell Scientific Publications. Archived from Magnetostratigraphic dating sim original on Check date values in: Paleomagnetic principles and practice.
Search and Discovery Article American Association of Petroleum Geologists.