Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 116

study may be needed before a detailed rock magnetic
cyclostratigraphic sampling is completed.
Time series analysis used in rock magnetic cyclo-
stratigraphy uses mathematical techniques that are
essentially modifi cations of Fourier analysis. These
mathematical analyses break the data series down into
its component frequencies and assume that the repeat-
ing cyclicities are in the form of sinusoids. It's best to
provide a rough estimate of time to the data series
being studied, so that when the data series is converted
into the frequency domain it is a power spectrum that
is a function of time frequencies (e.g. Weedon 2003;
fi g. 1.3). If this isn't possible, a magnetic parameter
data series that is a function of stratigraphic position
can be converted to a spatial frequency power spec-
trum for initial evaluation. The multi-taper method
(MTM, Thomson 1982) is typically used for time series
analysis and the signifi cance of the spectral peaks is
determined from their emergence above robust red
noise calculated for the spectrum (Mann & Lees 1996).
The basic techniques will be illustrated in the following
case studies.

nate explanation is however possible: that autocycles
are responsible for the repetitive upward shallow-
ing sequences. For platform carbonates the autocycles
could be caused by slow subsidence of the platform,
perhaps due to cooling of the lithospheric plate, provid-
ing accommodation space for the biogenic production
of carbonate. When water depth hits some critical
depth, carbonate can be produced relatively quickly
until the space is fi lled and thus relative sea-level shal-
lows until supratidal environments are reached. This
sequence repeats itself as the platform slowly 'drowns'.
There is no need to appeal to orbitally forced climate
change to create the repeating facies and there is no
' climate signal ' to be found.
Rock magnetic cyclostratigraphy was used to study
the Cupido shallowing upward sequences in more
detail and from a different perspective. Time could not
be easily assigned to the sequence by magnetostratig-
raphy since its Aptian age puts it at the beginning of
the Cretaceous normal polarity superchron, when the
geomagnetic fi eld did not reverse for tens of millions
of years. A sequence stratigraphy analysis of the rocks
indicates its Aptian age ( c. 120 Ma) and suggests
that the formation was deposited over about 3.6 million
years, indicating that the shallowing upward cycles
have a non-Milankovitch duration of about 73 kyrs.
The ' fourth - order ' stratigraphic sequences in the
section would have a 730-900 kyr duration, again not
a Milankovitch periodicity. Furthermore, no 5 : 1 bun-
dling of the sequences is observed.
The Cupido Formation was sampled at high resolu-
tion using a 10 cm sample stratigraphic spacing for
ARM measurements. Many samples were taken within
each of the shallowing upward sequences; 140 m were
sampled. A beautiful rock magnetic cyclostratigraphy
resulted (Fig. 8.15) showing both long and short cycles
that could be correlated between the Chico and Garcia
localities. Furthermore, the longer wavelength maxima
seen in the ARM data are in phase with the sequence
boundaries determined from sequence stratigraphic
analysis. Another important observation is that the
ARM cyclicities are decoupled from cycles defi ned by
the lithologic facies (Fig. 8.16), indicating that they
arise independently. Rock magnetic analysis indicates
that the ARM is carried by magnetite and it has a grain
size, based on both the ARM/SIRM ratio and examina-
tion of a magnetic separate by SEM (Fig. 8.5), consist-
ent with eolian dust. The data suggest that the ARM
is a record of dust being blown into the sampling local-
ity during the Early Cretaceous, independent of the

CUPIDO FORMATION: CRETACEOUS
OF MEXICO

The fi rst example of a rock magnetic cyclostratigraphic
study is the work of Latta et al . (2006) on the lower
Cretaceous Cupido Formation from north-eastern
Mexico. The Cupido Formation is a 940 m thick
sequence of marine platform carbonates deformed
during the Laramide in the Coahuila marginal fold-
thrust province. Latta et al . (2006) sampled the Cupido
at two localities: Potrero Chico and Potrero Garcia. The
Garcia rocks were deposited in an inner shelf environ-
ment and the Chico rocks in a middle shelf envi-
ronment. At both localities, there are meter-scale
sequences of facies that indicate repetitive upward
shallowing of the depositional environment. This
behavior has been observed in other platform carbon-
ate stratigraphic sections, notably the Latemar cycles
in the Triassic of northern Italy that we will discuss in
the section on the Latemar controversy below.
One obvious interpretation of the upward shallow-
ing facies is that relative sea level is bouncing up and
down. The favored cyclostratigraphic interpretation
is that an oscillating outside forcing function, i.e.
orbitally forced climate change, is driving sea-level
fl uctuations at Milankovitch frequencies. An alter-