Geology Reference
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the high
natural remanent magnetization (NRM)
intensities that Kent
et al. (2004) observed are certainly consistent with IRMs acquired naturally
due to the high currents caused by lightning. Kent et al.'s (2004) data showed
only one polarity, after samples were excluded because of their high NRM
intensities, for nearly all of the section. One polarity interval in the early
Triassic would suggest much less than one million years for the duration of
the entire Latemar sequence.
Spahn et al. (2013) studied the magnetostratigraphy at Rio Sacuz, a
section correlative to the Latemar deposited in the basin adjacent to the
Latemar platform. Rio Sacuz was selected because it was below the tree
line, less exposed, and less likely to be affected by lightning strikes. In
fact, the paleomagnetic data had none of the high NRM intensities
observed by Kent et al. (2004). A N-R-N-R polarity sequence was obtained
(Figure 3.4) that when correlated to the Triassic GPTS (Hounslow &
Muttoni 2010) indicated a one million year duration for the deposition of
the Rio Sacuz section, and by correlation, the entire 600 m Latemar
sequence. Spahn et al. (2013) also measured the MS throughout the 70 m
Rio Sacuz section on the paleomagnetic cores collected every meter of
section for the magnetostratigraphic study. MTM spectral analysis
revealed significant spectral peaks at 9.8 and 7.9 m which appeared to be
modulated by a longer ~25 m cycle which repeated a little more than
twice throughout the section (Figure 6.9). The less than significant 23 m
spectral peak is likely weak due to only two repetitions. The ~7-9 m
peaks are interpreted by Spahn et al. (2013) to be of short eccentricity
(125 and 95 kyr) and the 25 m peak is interpreted to be of long eccen-
tricity (405 kyr). Based on the MS cyclostratigraphy, the duration of the
Rio Sacuz section is more precisely determined to be ~800-900 kyr. These
results suggest that the meter scale shallowing upward cycles in the
Latermar are sub-Milankovitch in duration with a period of nominally
1700 years. The Triassic Daye Formation from China (see Section 6.6)
also observed sub-Milankovitch cycles of similar duration.
6.6 Daye Formation, Triassic Carbonates,
South China
Wu et al. (2012) studied the rock magnetic cyclostratigraphy of the 57 m
thick early Triassic Daye Formation. They collected an amazing 2440 speci-
mens, sampling the marine limestones, marls, and shales every 1-3 cm for
55.1 m of section. They crushed the samples and placed them in the stan-
dard 2 × 2 × 2 cm plastic boxes used in paleomagnetic studies and mass
normalized all their remanence measurements.
Rock magnetic measurements identified low coercivity magnetite
(~55 mT) with some evidence of a higher coercivity magnetic mineral,
probably hematite. This magnetic mineralogy suite is very similar to what
