Magnetostratigraphy

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

    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 core samples are collected in the field using a Pomeroy Drill. 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 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. The stable component is usually interpreted to be the DRM.

    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 (Fig. 1).

 


 
Figure 1: Magnetostratigraphic sampling of the stratigraphic column on the left (measured in meters) produced the results in the center column which is a plot of stratigraphic level vs. Virtual Geomagnetic Pole Latitude. Black circles are statistically significant sites. White circles lack statistical significance but have the same polarity as an adjacent black circle. Sites plotting to the right of the centerline are normally polarized while those to the left are reversely polarized. The data are abstracted to the standard black and white column on the right in which black is Normal and white is Reversed. An isotopic age was obtained from a volcanic airfall deposit in the section to calibrate the magnetostratigraphic column. 

 


Correlation & 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 paleontologic 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 (Fig. 2) with the Global Magnetic Polarity Time Scale (GMPTS). We currently use the GMPTS produced by Cande and Kent (1995). 

    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.
 
 

Figure 2: Correlation of the local paleomagnetic column from Figure 1 with the GMPTS. The correlation suggests that the rocks in the column were deposited between 8.2 and 1.7 million years ago.


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.

   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 (Fig. 3). 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. In the section used in Fig.3, the oldest growth strata suggest the initiation of anticlinal growth. The data suggest that growth of trapping structures predated the initiation of hydrocarbon maturation and migration. 

   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. I often use changes in sandstone composition for this type of interpretation. 

Figure 3: Although there is slight variation in the sediment accumulation rate at the bottom of the section, the 4 km-thick section produced a roughly linear rate of â0.6 mm/year or 600 m/million years. Linear rates are typical in distal foreland sections such as the one from which these data were obtained. Using outcrop thicknesses, the Source Rock passed through the generation depth (assumed here to be 4 km; red arrows) during the 4.8-4.1 Ma interval (orange arrows). Growth strata deposition commenced about 5.2 Ma (purple arrows). The data suggest that local structural traps were available during initial maturation and migration.


Existing Sections

    Numerous Neogene magnetostratigraphic sections have been completed in the Andean foreland of northwestern Argentina. Fig. 4 locates all sections (published and unpublished) completed by Dr. Reynolds with Magstrat and as an academic researcher. It also locates Neogene sections published by other workers.
                                                    

Figure 4: Location Map of existing Neogene magnetostratigraphy sections in northwestern Argentina. Gold X's indicate my sections and purple X's are the published sections of other workers. Province names and some cities are also shown.


Sandstone Petrography

    An attempt is made during paleomagnetic sample collection to collect medium-grained sandstone samples at about 100 m stratigraphic intervals. Thin sections made from these samples are stained to help determine feldspar compositions. Each slide is then subjected to a 300-sample point-count using the modified Gazzi-Dickinson technique. Abundances and compositions of quartz, feldspars, heavy minerals, and lithic fragments are recorded for each slide.

    It is often possible to determine exactly which lithostratigraphic formation provided the lithic fragments. Changes in lithic fragment composition are particularly important. For instance, the sudden appearance of metamorphic fragments in the sandstones indicates that metamorphic rocks became uplifted and exposed somewhere in the sediment source area. Because the ages of the strata are relatively well known from the magnetostratigraphy, the age of the uplift can be determined.
 


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