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than OAE1a based on ammonites. This ambiguity in the age of the top of the
Cupido Formation would suggest that the biostratigraphic age controls are
not good enough to assign durations to the upward shallowing sequences.
Hinnov et al. (2013) sampled 143 m of the topmost part of the Cupido
Formation at two different localities separated by about 25 km. At Garcia
Canyon, inner-shelf facies were sampled and at Chico Canyon, middle-shelf
facies were collected. Unoriented samples were taken every 20-50 cm, each car-
bonate upward shallowing cycle was sampled at least four to five times. The
sampling was designed to be at a finer scale than the facies to assess if the cyclicity
observed by the rock magnetic measurements was independent of the upward
shallowing cycles. Two hundred and ninety-seven samples were collected at
Garcia and 283 samples at Chico. Samples were crushed into 2-4 mm size pieces
and placed in 2 × 2 × 2 cm plastic sample boxes for magnetic measurement. The
boxes were completely filled in order to immobilize the rock pieces during
measurement. All magnetic measurements were normalized by mass.
Rock magnetic tests in addition to the cyclostratigraphy rock magnetic
parameters were made in order to assess the magnetic mineralogy. Low tem-
perature and hysteresis measurements indicate that the magnetic carrier of
the cyclostratigraphy is pseudo single domain (PSD) to multidomain (MD)
(grain size 0.1-1 µm) magnetite. Based on these results, ARM was used for
the rock magnetic cyclostratigraphy. ARM was applied with a 100 mT peak
alternating field in the presence of a 97 μT bias field.
The log
10
ARM cyclostratigraphy shows strong correlation between the inner
shelf and mid-shelf localities (Figure 6.5). The log
10
of the ARM helps to stabi-
lize the variance making the cycles more obvious. The strongest feature in the
Garcia stratigraphic position (m)
60
140
143m
120
100
80
40
20
122m
88 m
4th order sequence boundaries
58 m
32m
-4.4
Inner shelf garcia
Middle shelf chico
-4.8
-5.2
-5.6
-6
-6.4
4th order sequence boundaries
-6.8
140m
11 8m
85 m
56m
33 m
120
100
80
Chico stratigraphic position (m)
60
40
20
Figure 6.5
log
10
ARM rock magnetic cyclostratigraphy for the Cretaceous Cupido Formation. Results from two
different localities (Garcia and Chico Canyons) show strong correlation of 30-35 m cycles with higher frequency
cycles superimposed. Source: Hinnov, Kodama, Anastasio, Elrick & Latta 2013. Copyright 2013 by the Geological
Society of London.
