Geology Reference
In-Depth Information
These rules give more weight to any reversed or even intermediate
direction samples than to normal polarity samples, because it is assumed
that all the samples suffer from varying amounts of normal polarity over-
printing. This approach was used by Kodama (1979) for the normally
overprinted Plio-Pleistocene Rio Dell Formation in northern California
and made sense for the young Rio Dell rocks since they were deposited
just before the present Brunhes normal polarity subchron (C1n). This
approach only works for the Triassic age rocks from the Dolomites because
the present-day geomagnetic field direction is nearly parallel to the Triassic
normal polarity direction. In general, there can be no one set of rules
for establishing a magnetostratigraphy. A careful paleomagnetic study
must be conducted to establish if there is significant secondary magnetic
overprinting and how it interacts, geometrically, with the primary magne-
tization in the rocks. Different situations will require a different set of
rules, but rules need to be established to make sure the polarity interpreta-
tion is consistently applied through the section.
A technique commonly used for establishing a magnetostratigraphy is to
plot the mean direction, or mean VGP latitude, determined for each strati-
graphic horizon. Sometimes only the declination or the inclination of the
mean paleomagnetic direction is plotted. This approach can work well if
there is no significant overprinting, hence reasonably small confidence limits,
and the stable demagnetized direction for a sample is easily determined from
the vector endpoint diagrams. When using declination or inclination instead
of VGP latitude to indicate a sample's polarity, it is important to realize that
declination is the best parameter to plot for low-latitude magnetizations that
have flat inclinations, and inclination is the best parameter to examine
for high-latitude magnetizations, where there can be a large variation in dec-
lination, even though the vertical magnetizations may change only a small
amount in direction. This caveat is particularly important for magneto-
stratigraphies of unoriented cores.
3.3
Tying to the GpTs
Once the local reversal stratigraphy has been established, the important final
step is to tie it to the GPTS. The GPTS is well established and dated as far
back as there is extant ocean floor and is based on the dating of the seafloor
magnetic anomalies, e.g., back to 170 Ma (
Chron
M44, (Gradstein et al.
2012)). Tying the reversal stratigraphy to the GPTS allows absolute ages to be
assigned to polarity interval boundaries and hence the establishment of the
sediment accumulation rate for the sedimentary sequence under study. This
in turn allows the determination of the duration of any cycles found in the
cyclostratigraphy.
Tying to the GPTS can be tricky, because sediment accumulation rates
can change throughout a section, unrecognized hiatuses can remove part of
