Geology Reference
In-Depth Information
Reconnaissance magnetostratigraphy had been published by Hogan
and Burbank (1996) before the rock magnetic cyclostratigraphic study was
conducted, so the researchers had an approximate idea of the sediment
accumulation rate to plan the sampling, and particularly to set the sam-
pling interval at about five times per predicted precessional cycle (every
4 kyr). Oriented samples for paleomagnetic measurement were collected
near reversal boundaries already located by Hogan and Burbank's (1996)
magnetostratigraphy, but at a 3 m stratigraphic spacing, to better resolve
the reversal boundary locations. More precisely located reversal bound-
aries would give more accurate absolute time control for calibrating the
cyclostratigraphy. Unoriented cyclostratigraphy samples were collected
every 20 cm for the bottom 100 m of section, where lithology and the
presence of glauconite suggested a condensed section and a slower than
average sediment accumulation rate, at 75 cm spacing for the middle 100-
500 m part of the section and at a 1.5 m stratigraphic spacing from 500 to
800 m at the top of the section where the magnetostratigraphy (Hogan &
Burbank 1996) and the coarser grained lithology suggested high-sediment
accumulation rates.
IRM acquisition measurements indicated that the ferromagnetic minerals
carrying the primary paleomagnetism of the rocks were magnetite, a primary
magnetic mineral, and Fe sulfides, which are secondary magnetic minerals,
formed by reductive diagenesis (Roberts & Weaver 2005). The magnetic
mineralogy of the Arguis Formation indicated that ARM would be the best
way to measure the rock magnetic cyclostratigraphy, with the caveat that
secondary Fe sulfides could be carrying at least some of the ARM.
The refined magnetostratigraphy identified eleven polarity intervals and
extended the Hogan and Burbank (1996) magnetostratigraphy 250 m up
section. Using biostratigraphy (planktonic foraminifera, benthic forami-
nifera, and calcareous nanoplankton), the reversal stratigraphy could be
correlated to Chrons C18-C16 in the Eocene (from about 40 to 36 Ma)
giving an average sediment accumulation rate of 23 cm/kyr to help identify
astronomically forced cycles in the cyclostratigraphy.
MTM spectral analysis of the ARM cyclostratigraphy revealed significant
spectral peaks with approximately 5 m wavelengths (5.6, 5.3, and 4.7 m)
which based on the average sediment accumulation rate would be ~22 kyr
in duration and likely to be precession. A significant peak was also observed
at 14.6 m (~63.5 kyr) that was not identified as astronomically forced.
Strong, significant spectral peaks were also observed at 30.7 m (~133.5 kyr),
53 m (~230.4 kyr), and 82 m (~356.5 m). The first peak was strongly sus-
pected to be short eccentricity and the 82 m peak could be long eccentricity
(Figure 6.4).
As explained in Chapter 5, the cyclostratigraphic age model for the Arguis
Formation was refined by multiple steps of tuning to account for changes in
sediment accumulation rate and unrecognized hiatuses throughout the section.
The ARM data series was tuned a total of three times to eccentricity. The series
was first filtered with a band-pass filter, centered at 405 kyr, to more accurately
