Geology Reference
In-Depth Information
For the Devonian and earlier, the GPTS is not well determined or contin-
uous, and a tie to the GPTS for calibrating cyclostratigraphy is not possible.
Only estimates of the duration of a sedimentary sequence are possible
assuming the range in reversal rates observed over the well-constrained
0-170 Ma period (from ~5 to 0 reversals/Myr; Merrill et al. 1996) and the
number of reversals observed in the section. For example, if for a sequence
of sedimentary rocks older than 350 Ma five polarity intervals are observed,
the minimum duration for the deposition of the sequence would be esti-
mated to be 1 million years. This is the approach that had to be taken for a
Neoproterozoic magnetostratigraphy and rock magnetic cyclostratigraphy
conducted by Minguez et al. (2014) and discussed in Chapter 6.
3.4 providing the Best Time Resolution
from Magnetostratigraphy
The main objective of establishing a magnetostratigraphy for the sedimen-
tary sequence targeted for rock magnetic cyclostratigraphy is to have abso-
lute time tie points in the section to get the best estimate of the sediment
accumulation rate. An accurate sediment accumulation rate allows the most
accurate determination of the average duration for any cycles observed by
the cyclostratigraphic measurements. The best estimate of the sediment
accumulation rate depends on pinning down as accurately as possible the
stratigraphic position of polarity interval boundaries. One way to do this is
to use an iterative process. A reconnaissance magnetostratigraphy allows a
first-order tie to the GPTS. Once the approximate positions of the polarity
boundaries are known, the stratigraphic section can be resampled near to
those positions at very close stratigraphic intervals in order to pin down the
polarity boundary locations with the highest possible stratigraphic resolu-
tion. This approach was used for the development of a rock magnetic
cyclostratigraphy for the Eocene Arguis Formation from the Spanish
Pyrenees (Kodama et al. 2010) and is detailed in Chapter 6.
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