Geology Reference
In-Depth Information
Understanding the encoding of the obliquity and precession scale orbitally
forced signals in the Stirone Formation by susceptibility is problematic
simply because susceptibility can be carried by ferromagnetic, paramagnetic,
and diamagnetic minerals. Furthermore, many of the ferromagnetic min-
erals are secondary iron sulfides, as shown by Gunderson et al.'s (2012) work
and earlier work by Channell et al. (1994) and Mary et al. (1993). Measurement
of the susceptibility of representative samples at liquid N temperatures (77 K)
indicated that the increase in susceptibility over room temperature values
was not great enough to indicate that all the susceptibility was carried by
paramagnetic minerals, but paramagnetic minerals did make significant
contributions to the susceptibility.
Gunderson et al. (2012) concluded that the susceptibility was carried by a
combination of paramagnetic and ferromagnetic minerals. Since precession
tends to affect insolation at low and middle latitudes, and would thus affect
monsoon strength and runoff from the nearby continent, it was easy to see
how precession could control the delivery of magnetite and detrital clays to
the depositional basin. However, obliquity does not strongly affect insolation
at the low-to-mid latitudes of the Stirone Formation in the Plio-Pleistocene.
Obliquity is a strong control on sea level, due to obliquity's heavy influence
on high latitude insolation and hence to changes in global ice volume. Using
the magnetostratigraphic tie points (reversal boundaries), Gunderson et al.
(2012) showed that susceptibility highs are coincident with obliquity minima
(and therefore sea level low stands). Gunderson et al. (2012) speculate that
lower sea levels lead to restricted Mediterranean circulation, more anoxia,
and perhaps enhanced production of iron sulfides.
Therefore, the encoding envisioned for the Stirone Formation comprises
two parts: a simple erosional model for the encoding of precession and a
more complicated low sea level, more anoxia, more iron sulfides model for
the obliquity signal.
6.3
Arguis Formation, Spanish Pyrenees
This important rock magnetic cyclostratigraphy study (Kodama et al. 2010)
was used as an example for astronomical tuning discussed in Chapter 5.
Details of the identification of the astronomically forced cycles and the
tuning procedure can be found in Chapter 5.
The Arguis Formation is a sequence of Eocene marine marls exposed in
the External Sierra of the Jaca Basin of the Spanish Pyrenees. The Arguis
Formation pro-deltaic sediments are folded, via salt tectonics, into the Pico
de Aguilla anticline. Samples were collected from 800 m of growth strata on
the flanks of the anticline to provide a high-resolution age model for timing
the growth of the fold. The Arguis Formation coarsens up section in grain
size from mudstones at the base- to medium-coarse sandstones at the top of
the section.
