Geology Reference
In-Depth Information
the excursion of the geomagnetic field away from a given stable polarity
configuration for a brief (10
3
-10
4
years) period. These short excursions
appear to be in the continuum of geomagnetic field behavior and examples
have occurred in the recent past (e.g., the Mono Lake excursion (Liddicoat
& Coe 1979)). During an excursion, a full polarity reversal has not been
established (Laj & Channell 2007). Typically, once a horizon's polarity is
determined, a stratigraphic column is marked as either normal or reversed
polarity, and the pattern of reversed and normal polarity stratigraphic inter-
vals can aid the tie of the sedimentary sequence's reversal stratigraphy to the
GPTS (Gradstein et al. 2012).
The addition of significant magnetic overprinting complicates the pic-
ture and makes some set of rules necessary. Since the acquisition of
secondary magnetizations is typically spatially heterogeneous, it is not
uncommon for samples from one horizon to have acquired different
degrees of overprinting. Samples from one horizon can have two different
polarities. The establishment of “rules” for interpreting the polarity of a
horizon in this case lends consistency to the overall polarity stratigraphy
interpretation. If the case can be made for dominant normal polarity over-
printing, then more weight can be given to reversed polarity samples at a
horizon in the determination of the horizon's polarity. This scenario can
be the case for younger stratigraphic sections, particularly since the
geomagnetic field has been normal polarity for the past 790kyr and
viscous magnetization overprints will be normal polarity. Spahn et al.
(2013) were able to make the case that the magnetization of Triassic rocks
from the Dolomites were affected by present-day overprinting justifying
the heavier weighting of reversed polarity samples in determining a
horizon's polarity. Overprinting can cause intermediate directions, in
which all the samples at a horizon are partially overprinted and neither
reversed or normal polarity, even if the bounds around the normal and
reversed polarity VGP positions are expanded to 45° (Kent et al. 2004;
Spahn et al. 2013).
Short polarity episodes are observed in the GPTS; therefore, some-
times the finite sampling interval used in any magnetostratigraphic
study or the deterioration of the record due to overprinting causes a
paleomagnetic study to entirely miss short polarity periods. If only one
horizon marks a different polarity from the horizons directly above
and below it stratigraphically, it could be due to heterogeneous over-
printing affecting just that horizon, or it could be the accurate record of
a short polarity interval. Many times the rules established for deter-
mining the reversal stratigraphy choose to mark that horizon with less
weight, just a “half-bar” in the stratigraphic column showing the polarity
interpretation.
An example of the application of rules to establish a magnetostratig-
raphy can be shown in Spahn et al.'s (2013) study of the stratigraphic
section at Rio Sacuz in the Dolomites (Figure 3.4). The rules applied in
