Geology Reference
In-Depth Information
3.2
Measuring Magnetostratigraphy
3.2.1
Sampling Strategy
The first step is to design a field sampling strategy. If the ultimate goal is to tie
the reversal stratigraphy measured for the rocks to the GPTS, it is helpful to
know roughly how old the rocks are (i.e., whether Eocene, Cretaceous, or
Jurassic), so that the geomagnetic field reversal rate can be approximated.
The sediment accumulation rate should be estimated, too, since the objective
is to sample a polarity interval at a minimum of three separate horizons.
Magnetostratigraphic samples are usually collected from separate strati-
graphic horizons, or geologic “instants” of time. The most common technique
is to collect about three independently oriented cores, drilled with a gasoline-
powered sampling drill, from a separate horizon. The horizons should be
spaced as evenly as possible through the stratigraphic section at an interval
that would ensure about three horizons in a polarity interval (see Butler 1992).
Continental fluvial rocks typically have sediment accumulation rates of
~10 cm/kyr (Sadler 1981). With a reversal rate of 4-5/Myr in the Neogene, this
sediment accumulation rate would require sampling intervals between 6 and
8 m. By the same reasoning, the slower sediment accumulation rates of pelagic
marine sediments (<1 cm/kyr, Sadler (1981)) would indicate a sampling
interval of <0.6 m for the Neogene.
Multiple samples are usually taken at each horizon in order to check for
the effects of magnetic overprinting after deposition. Rocks can pick up
secondary magnetizations due to chemical changes in the rock growing new
magnetic minerals or the acquisition of secondary magnetizations by the
primary, depositional magnetic minerals from so-called viscous magnetiza-
tions (see Butler (1992) or Tauxe (2010)). The acquisition of secondary mag-
netizations is usually not homogeneously distributed throughout the rocks,
so collecting multiple samples from a horizon often yields at least one sample
per horizon that is not completely overprinted or not overprinted at all. If all
the samples from a horizon have similar paleomagnetic directions, then it is
meaningful to calculate a horizon mean direction, and N = 3 is the minimum
number of samples needed to conduct the Fisher (1953) statistics typically
used to analyze paleomagnetic directional data.
3.2.2
Sample Collection
Paleomagnetic samples are collected typically by drilling 25 mm diameter
cores with a gasoline-powered drill and diamond tipped drill bits
(Figure 3.1). The cores are oriented using a specially designed orienting
tool and a magnetic compass. If drilling is not allowed in the sampling area
or it is not practical to haul the drill, gasoline, and cooling water needed
