Geology Reference
In-Depth Information
cyclostratigraphy is a long wavelength cycle with a period of about 30-35 m
that is similar in length to the spacing of fourth order sequence stratigraphy
boundaries recognized in the Cupido rocks (Goldhammer 1999). These long-
period cycles can be easily matched between Chico and Garcia and are seen to
diminish in amplitude up section. The decrease in amplitude may be due to
increasing water depth suggested by the facies patterns. Higher frequency
variability is superimposed on the dominant 30-35 m cycle. No magneto-
stratigraphy was measured for the Cupido cyclostratigraphic study. In lieu of
direct measurement of absolute time, identification of orbitally forced cycles
may be accomplished by tying one period to a given Milankovitch cycle to see
if other astronomically forced cycles are enhanced in the power spectrum. This
approach was used for the log
10
ARM data series in the Cupido Formation. The
fourth order sequence boundaries were assumed to be the 405 kyr long eccen-
tricity period. This assumption is supported by tying the two oldest and
youngest fourth order sequence boundaries to the Barr6, AP1, and AP2 global
sequences suggesting the section sampled is about 1.62 myr in duration. When
this tie to time is made, the MTM spectral analysis shows that spectral peaks
emerge for the log
10
ARM data series with periods of 100, 44, and 20 kyr that
can be easily tied to short eccentricity, obliquity, and precession (Figure 6.6).
This support for astronomically forced rock magnetic cycles is bolstered by the
(a)
(b)
-4
20
405-kyr
-4.5
-5
-5.5
15
0.035, 0.045, 0.055
precession
passband
10
-6
-6.5
-7
-7.5
-500
100 kyr
5
44 kyr
20 kyr
0
0
0
500
Kiloyears upsection
1000
1500
2000
0.02
0.04
Cycles/kyr
0.06
0.08
0.1
(c)
(d)
1
0.8
0.4
0.2
0
-0.2
400 kyr
0.6
0.4
0.2
0
0
95 kyr
-0.4
-500
0
500
Kiloyears upsection
1000
1500
2000
0.01
0.02
Cycles/kyr
0.03
0.04
0.05
Figure 6.6
Figure from Hinnov et al. (2013) showing log
10
ARM data series for Garcia (a) and its power spectrum
(b) after the fourth order sequence boundaries have been tied to 405 kyr long eccentricity. Astronomically forced cycles
emerge at 100 kyr (eccentricity), 44 kyr (obliquity), and 20 kyr (precession). Further testing of the astronomically forced
nature of these cycles shows that the amplitude envelope (c) of the log
10
ARM series filtered at the precessional scale shows
(d) spectral peaks of the 400 kyr period of long eccentricity (expected) and the ~100 kyr period of short eccentricity.
Source: Hinnov, Kodama, Anastasio, Elrick & Latta 2013. Copyright 2013 by the Geological Society of London.
