Geology Reference
In-Depth Information
Depth Rank Series
4pi MTM spectrum, Forcellone
4
3.5
3
2.5
2
1.5
1
0.5
0
2
118 cm
1
80 cm
60 cm 40 cm
500 cm
0
-1
0
500
1000
1500
2000
2500
3000
3500
4000
0
0.01
0.02
0.03
0.04
0.05
cycles/cm
ARM Series
4pi MTM spectrum, Forcellone ARM
15
1600
500 cm
12
1200
9
800
110 cm
85 cm
66 cm
6
400
3
0
0
0
0.01
0.02
0.03
0.04
0.05
0
500
1000
1500
2000
2500
3000
3500
4000
cycles/cm
stratigraphic position
Fig. 8.20 Depth rank series and ARM cyclostratigraphy for the Forcellone section of the Latemar dolomites. The power
spectra of these series show periodicities at 100 cm (118 and 110 cm) bundled at 500 cm, giving the 5 : 1 ratio expected for the
Milankovitch frequencies of short eccentricity and precession. Figure from Kodama & Hinnov (2007). (See Colour Plate 26)
However, the rock magnetic and lithologic cyclos-
tratigraphy is clearly at odds with the U-Pb and
magnetostratigraphic interpretations. The magneto-
stratigraphy collected from the main Latemar sequence
was compromised by remagnetization caused by light-
ning strikes, so another magnetostratigraphic section
was sampled at Rio Sacuz (Spahn 2011; Spahn et al.
2011). The Rio Sacuz rocks are equivalent in age to the
Cimon Latemar carbonate platform rocks, but Rio
Sacuz was off the carbonate platform. Stratigraphic
equivalence is shown by the identifi cation of the ash
layers at Rio Sacuz that can be tied to the ash layers in
the main carbonate platform sequence at Latemar and
Forcellone (Preto et al. 2007). The Rio Sacuz section is
however in a protected stream valley below the tree line
and was not affected by lightning strikes. The magne-
tostratigraphy for this section identifi ed more than one
polarity for the Latemar, a N-R-N-R sequence that,
when tied to an integrated geomagnetic polarity times-
cale for the Triassic (Hounslow and Muttoni, 2010),
indicates that the 670 m of the Latemar platform car-
bonate sequence was deposited over only about 1-2
million years. This is in good agreement with the U-Pb
geochronology and the previously reported magneto-
stratigraphy (Kent et al. 2004 ).
This result has important implications for rock mag-
netic cyclostratigraphic interpretations. It suggests
that precession, if recorded by the rock magnetics,
occurs at a 5 m period rather than at a 1 m period. What
should then be made of the 5 : 1 m bundling touted
as evidence for eccentricity: precession periodicities
and an astronomically forced interpretation? If the
1 m upward shallowing sequences are the results of
autocycles, why are they bundled at 5 m? Is it just a
fortuitous juxtaposition of autocycles and precession?
While this last point may be what happened in reality
at the Latemar, it is hardly a satisfying result. What
it does show is the importance of having independ-
ent absolute time control to bolster a cyclostratgraphic
interpretation. Absolute time control can come from
sequence stratigraphy, magnetostratigraphy, bios-
tratigraphy, geochronology or some other well-accepted
technique. It is apparent from the Latemar results that
5 : 1 bundling can fool workers into an astronomically
forced interpretation; it may be that 5 : 1 bundling has
a deeper meaning than simply astronomical forcing.
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