Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 120

Fig. 8.18 Time series analysis of Arguis Formation ARM cyclostratigraphy without any tuning, simply tied to absolute time
with magnetostratigraphy, shows spectral peaks at expected Milankovitch frequencies. Figure from Kodama et al. (2009) .
(See Colour Plate 24)

carbonate sequence (670 m) in the Triassic rocks in the
Dolomites of northern Italy. U-Pb radiometric dating
of zircons from ash layers in the Latemar sequence give
ages very close to each other, suggesting the whole
sequence was deposited in 2 million years (Mundil
et al. 2003). The sequence contains c. 1 m thick upward
shallowing sequences often observed in platform car-
bonates. A cyclostratigraphic analysis of these layers
suggests that they are bundled in 5 : 1 packages (Gold-
hammer et al. 1990). The bundling is highly suggestive
of short eccentricity to precession bundling and a
Milankovitch forcing for the stratigraphy. However,
this interpretation means that the Latemar had to have
been deposited over nearly 12 million years, an order
of magnitude difference with U-Pb dating results.
Resolution of the controversy clearly has important
implications for the validity of orbitally forced interpre-
tations of stratigraphic cycles, particularly the 5 : 1
bundling often used to justify an astronomically
forced interpretation. One additional piece of evidence

bearing on the depositional duration of the Latemar is
a magnetostratigraphic analysis of the main Latemar
sequence (Kent et al. 2004) observing only one polarity
in the whole 670 m sequence, again suggesting deposi-
tion over only 1 million years or less. However, the
magnetostratigraphy of these exposed rocks at high
elevations in the Dolomites was plagued by lightning
strikes that remagnetized a good portion of the dataset
by application of natural IRMs, thus calling into ques-
tion the validity of the magnetostratigraphy.
A rock magnetic cyclostratigraphy for a short
sequence (40 m with c. 25 cm spacing) of Latemar
equivalent rocks at Cimon Forcellone (Kodama &
Hinnov 2007) was measured to see if rock magnetics
would identify the same cyclicities observed in the
lithologic cyclostratigraphy. This study was followed up
by a larger rock magnetic study of the main Latemar
sequence (100 m with c. 10 - 20 cm spacing; Marangon
2011). Both studies used ARM as a measure of mag-
netite concentration variations. Rock magnetic meas-