Doing Rock Magnetic Cyclostratigraphy - Rock Magnetic Cyclostratigraphy - page 143

Long eccentricity was assumed to drive the fourth order sequence stratig-
raphy cycles in the Cretaceous Cupido Formation platform carbonates
(Figure 6.6, (Hinnov et al. 2013)). This assumption was supported by the
emergence of spectral peaks with 100, 44, and 20 kyr periods, once the rock
magnetic cyclostratigraphy data series was tied to time assuming the
sequence boundaries were spaced at 405 kyr intervals.
Of course, radiometric ages from volcanic ash fall beds throughout a sed-
imentary sequence could easily give enough time control to estimate the
sediment accumulation rate and hence the approximate duration of cycles
recognized in the time series analysis of the rock magnetic data series. The
distribution of the ash fall beds in the stratigraphic section and the errors on
the ages will limit the accuracy and resolution of the age model. For in-
stance, in the Latemar Triassic carbonate platform sequence of the Dolomites
in northern Italy, three volcanic ash fall beds are spaced over ~250 m in
>600 m thick section. The ages range between 241.2 and 242.6 Ma with over-
lapping error limits. A range of sediment accumulation rates calculated
from the ages varies by a factor of two from 14 to 28 cm/kyr (Mundil et al.
2003). Mundil et al. (2003) do present a better-constrained sediment
accumulation rate, but by bringing in additional geochronologic data by
correlation to a different sedimentary section.
When there is no good control on absolute time available, the ratio of the
periods for the cycles that emerge in the spectral analysis can be used as
evidence for astronomically forced climate cycles. Observations of 20:5:2:1
ratios, which are the approximate ratios for long eccentricity : short eccen-
tricity : obliquity : precession (400:100:40:20), or some portion of them, can
suggest Milankovitch forcing. This approach is the least desirable way to
detect astronomically forced cycles since misidentification can and will hap-
pen. The 5:1 bundling of the Latemar facies cyclicity, for instance, was the
primary evidence for short eccentricity : precession driving the sedimentary
cyclicity until geochronology and magnetostratigraphy suggested otherwise.
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