Geology Reference
In-Depth Information
for the duration of their deposition. Assuming a maximum geomagnetic
field, reversal rate similar to that of the last 160 million years (Merrill et al.
1996) occurred during the Proterozoic (Pavlov & Gallet 2010); a minimum
duration for deposition of the sediment at these localities would be some-
what less than one million years.
Magnetic susceptibility, ARM, and IRM were measured on the unori-
ented samples to determine the magnetic mineral concentration varia-
tions throughout the two sections and construct cyclostratigraphy data
series for time series analysis. ARM was applied with a 100 mT peak
alternating field in the presence of a 97 μT bias field. The IRM was acquired
in a 5 T field. Since magnetite appears to be carrying the paleomagnetism
and the successful observation of a magnetostratigraphy that can be corre-
lated between the two localities suggests it is a primary magnetization, this
bodes well for the cyclostratigraphy, particularly the ARM data series
which targets the magnetite in the rocks. Kodama and Hillhouse (2011)
also constructed a magnetic parameter ratio to measure the goethite to
hematite ratio in the rocks. The goethite to hematite ratio of a sedimentary
sequence is used to detect changes in precipitation or moisture availability
in the source area (e.g., Harris & Mix 2002), a magnetic goethite to hema-
tite ratio was measured as a possible way to detect climate variability.
Goethite was measured by the intensity loss from thermal demagnetiza-
tion of a 4T IRM at 130°C. Hematite was determined from the 4T IRM,
after goethite had been removed by thermal demagnetization and magne-
tite by alternating field demagnetization (or subtraction of the ARM
applied in a 100 mT alternating field). The goethite/hematite (G/H) ratio
can be written as:
IRMIRM
−
G
H
4
T
4
T
130
°
C
=
(6.1)
IRM
4
T
130
°
C AF
100
mT
where IRM
4T
is an IRM applied in a 4 T field, IRM
4T 130°C
is the 4 T IRM ther-
mally demagnetized at 130°C, and IRM
4T 130°C AF100mT
is the 4 T IRM that has
been thermally demagnetized and alternating field demagnetized in a
100 mT peak alternating field. Significant spectral peaks in the MTM power
spectra of the MS, ARM, and IRM data series with periods of 16-18 m, 5-6
m, 1-2 m, and 0.5-0.6 m were observed at both the Nopah Range and
Winters Pass Hills localities. Based on the approximately one million year
duration for the deposition of the sections, this would indicate that the
16-18 m spectral peak probably represents the 405 kyr long eccentricity
which according to Laskar et al. (2011) should not vary significantly through
geologic time. This time assignment is supported by the observation that if
18 m represents 405 kyr of time, then 5 m is ~110 kyr and is probably short
eccentricity, 1-2 m is ~34 kyr which is most likely obliquity, and 0.6 m is
13.5 kyr which is probably precession. Berger and Loutre (1994) estimate the
periods for the Milankovitch cycles at different times in the geologic past,
