Geology Reference
In-Depth Information
eccentricity was identified and the sequence of Newark basin lake
deposits could be scaled to that time assignment, precession and short
eccentricity cycles were recognized as the shorter cycles in the record.
Biostratigraphic records can also provide time control for a sedimentary
sequence, but magnetostratigraphy allows more precise and accurate age
control because geomagnetic polarity reversals are relatively quick
geologically (~10 3 -10 4 years) and are not time transgressive, like many
biostratigraphic boundaries. Magnetostratigraphy can provide accurate
time calibration for cyclostratigraphic studies, but at a lower resolution
than the astronomically forced cycles potentially measured with
cyclostratigraphy. Without some kind of independent, absolute time
control, a serious pitfall of cyclostratigraphic interpretations is misiden-
tification of astronomically forced cycles, the best example being the
Latemar controversy (Chapter 6). Magnetostratigraphy and cyclostratig-
raphy can and should be used together to achieve precession-scale
high-resolution chronostratigraphy.
Magnetostratigraphy is a technique in which paleomagnetic mea-
surements of samples, typically from a sedimentary sequence, are used
to determine the polarity intervals that record the polarity of the
geomagnetic field at the time of the sediment's deposition. Once the
sequence or pattern of polarity intervals is identified, a correlation to
the geomagnetic polarity time scale (GPTS) (Gradstein et  al. 2004,
2012) assigns absolute time. The polarity interval boundaries provide
absolute time tie points throughout the sequence and allow average sed-
iment  accumulation rates to be calculated. Magnetostratigraphy has,
at best, a 10 4 years time resolution, depending on the reversal rate of the
geomagnetic field at the time the rocks were deposited. Over the
past ~160 Ma, the shortest polarity intervals occurred in the last ~20 Ma
and about 150 Ma when the geomagnetic field reversed on average up
to  5  times/Myr. Further back in time, the reversal rate declines to 0
by  the  Late Cretaceous and doesn't reverse polarity at all from about
84 Ma to about 126 Ma (Gradstein et al. 2012). The same variability in
geomagnetic field reversal rate is also observed in the Proterozoic
(Pavlov & Gallet 2010). The GPTS is calibrated mainly by dating of
seafloor magnetic anomalies recorded in the world's oceans. Since the
oldest extant seafloor is Jurassic in age, the well-determined GPTS only
goes back to ~170 Ma.
The basic techniques needed to collect and measure the paleomag-
netic samples necessary to determine a magnetostratigraphy for a sedi-
mentary sequence are covered in great depth in excellent paleomagnetic
textbooks (Butler 1992; Tauxe 2010). Opdyke and Channell's (1996)
book gives comprehensive coverage of past magnetostratigraphic studies.
We will provide only a basic outline of the steps involved for establishing
a magnetostratigraphy and urge the reader to consult the textbooks cited
above for more detail.
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