Geology Reference
In-Depth Information
6.5 Latemar Massif, Triassic Carbonates,
Northern Italy
The Triassic Latemar massif platform carbonates have been studied exten-
sively as an example of an astronomically forced lithologic cyclostratigra-
phy. Goldhammer et al. (1987) recognized meter-scale upward shallowing
peritidal cycles that are bundled at a 5:1 ratio suggesting precession being
modulated by short eccentricity. A controversy erupted when the predicted
duration for the deposition of the ~600 m thick sequence (~12 myr),
assuming precession drove the meter scale upward shallowing cycles,
deviated significantly from the duration of deposition based on tightly
clustered U-Pb ages for zircons in ash beds distributed throughout the
sequence (~1-2 myr) (Mundil et al. 2003). Therefore, the Latemar rocks
were a perfect target for rock magnetic cyclostratigraphy given the results
from northeastern Mexico that suggested rock magnetic measurements
could give a cyclostratigraphy independent of lithologically determined
upward shallowing sequences for a platform carbonate.
Kodama and Hinnov (2007) conducted a preliminary rock magnetic
cyclostratigraphy study of 40 m of the Latemar rocks exposed at Forecellone.
Two hundred samples were collected at a 20 cm stratigraphic spacing. Depth
rank analysis of the shallowing upward facies shows a strong spectral peak
with a period of 118 cm (Figure 6.8), consistent with the observation of
strong lithologic cyclicity at the meter scale at Forcellone. The depth rank
analysis also observed a peak at 500 cm, showing the 5:1 bundling that char-
acterizes the Latemar's lithologic cycles.
IRM acquisition measurements showed that low coercivity magnetite
(~50 mT mean coercivity) appeared to be the main magnetic mineral
with small (<10%) amounts of a higher coercivity mineral (~200 mT)
that was identified as hematite (Kodama & Hinnov 2007). The MTM
spectral analysis of the MS, ARM, saturation IRM (SIRM), ARM/SIRM,
and
S-ratio
data series all support the depth rank power spectra and the
recognition of meter scale lithologic cycles bundled at 5:1. ARM, in
particular, shows spectral peaks at 110 and 500 cm, as does SIRM, show-
ing that magnetite concentration variations easily pick up the observed
lithologic cycles.
Alessandro Marangon completed a PhD dissertation at the University of
Padua studying, in part, the rock magnetic cyclostratigraphy of the
Latemar (Marangon 2011). He collected a longer section (100 m) of the
Latemar rocks at 20 cm spacing with 484 unoriented samples. His IRM
acquisition work supported what was observed by Kodama and Hinnov
(2007). Low-coercivity magnetite (~30 mT mean coercivity) appeared to
be the dominant magnetic mineral in the rocks, with small amounts
(~10%) of higher coercivity hematite. MTM spectral analysis of depth
rank, ARM, log
10
ARM/MS, SIRM data series all showed essentially the
same thing: strong spectral peaks at ~1 and 5 m.
