Geology Reference
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that fit the theoretically predicted peaks quite well; however, the non-
Milankovitch 178 kyr peak is not eliminated at this level of tuning. Tuning,
therefore, should be approached with caution, and the power spectrum
should be checked at each step to assess the effects of the tuning.
It is clear from the Arguis Formation study that despite the presence of
secondary Fe sulfides in the marine sediments, a successful cyclostratigra-
phy and a successful magnetostratigraphy can be extracted. These results
would suggest that the primary depositional magnetite dominates both the
paleomagnetism and the rock magnetic cyclostratigraphy and that the Fe
sulfides do not make significant contributions to the rock magnetic
cyclostratigraphy.
Kodama et al. (2010) get at the encoding mechanism in a unique way.
They conduct a coherency analysis that shows the eccentricity-tuned ARM
data series has precession that is in phase with October-November insola-
tion for the Arguis's paleolatitude of 35°N. It is important to note that the
Arguis Formation ARM data were not tuned at the precessional scale. ARM
precession in phase with October to November insolation would coincide
with a Mediterranean-climate's fall rainy season, hence the encoding mech-
anism is thought to be driven by runoff delivering continentally derived
sediment into a relatively constant production of marine carbonate. ARM
peaks would occur during greater runoff from the continent.
6.4 Cupido Formation Platform
Carbonates, Northeastern Mexico
The rock magnetic cyclostratigraphic study of the Cretaceous (Barremian-
Albian) Cupido Formation platform carbonates is an important study
because it demonstrates that rock magnetism can detect astronomically
forced climate cycles while a repeating sequence of upward shallowing, peri-
tidal cycles does not (Hinnov et al. 2013). The Cupido Formation is located
in the Sierra Madre Oriental fold belt of northeastern Mexico. It is 940 m
thick and is characterized by meter scale (1-8 m, average thickness: 3.4 m),
upward shallowing cycles including subtidal, intertidal, and supratidal
facies. The uppermost 150 m of the formation is characterized by facies
patterns that indicate a gradual deepening of the depositional environment.
The upward shallowing cycles in the Cupido Formation indicate sea level
fluctuations that could be due either to processes internal to the deposi-
tional basin or to a driving force external to the basin, possibly astronomi-
cally forced climate change. Goldhammer et al. (1991) interpreted the
upward shallowing cycles to represent obliquity based on biostratigraphic
age control. Unfortunately, the biostratigraphic age controls are not robust.
At two different localities, the base of the overlying La Pena Formation could
either be older than the Oceanic Anoxic Event 1a (OAE1a or “Selli” event,
(Schlanger & Jenkyns 1976)) based on planktonic foraminifera or younger
