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and the 1.7 m peak is 16 kyr in duration. The less significant peaks at 2.6 and
2.1 m would indicate 24 and 19 kyr, respectively.
Laskar et al. (2011) indicate that eccentricity has not changed significantly
in duration throughout geologic time. Based on the magnetostratigraphy, the
13.3 and 10.3 m cycle are very close to the expected duration of the short
eccentricity peaks (125 and 95 kyr, see Chapter 5). The 1.7 m (16 kyr), 2.6 m
(24 kyr), and 2.1 m (19 kyr) peaks are reasonably close to Berger and Loutre's
(1994) estimate for precession band peaks at 298 Ma (20.7 and 17.4 kyr). From
the simple reconnaissance rock magnetic cyclostratigraphy study of suscepti-
bility, it appears that astronomically forced cycles can be recorded in a fluvial
environment, despite the potential for shredding caused by depositional and
erosional processes. The results from the hand-held susceptibility meter show,
too, that reconnaissance studies can be easily done to check if a sedimentary
sequence is suitable for rock magnetic cyclostratigraphy and what the appro-
priate sampling intervals should be for an intensive cyclostratigraphy study
with remanence measurements.
In a separate rock magnetic study of the Pottsville, Pennsylvania, outcrop
of the Mauch Chunk Formation, both isothermal remanent magnetization
(IRM) and susceptibility measurements were made in the laboratory on sam-
ples collected every 50 cm for 40 m of stratigraphic section. These results
showed that the laboratory remanence and susceptibility measurements both
detected the eccentricity cycles, suggesting ferromagnetic minerals dominate
the susceptibility measurements. If this is true, it indicates that the portable
susceptibility measurements detect variations in hematite concentration that
appear to record astronomically forced climate change.
6.8 Rainstorm Member of the Neoproterozoic
Johnnie Formation, Death Valley, California
The cyclostratigraphy and magnetostratigraphy of the Rainstorm
Member of the Neoproterozoic Johnnie Formation was studied by
Minguez et al. (2014) and Kodama and Hillhouse (2011). The Rainstorm
Member of the Johnnie Formation is comprised of gray, green, pink,
and purple marine siltstones and carbonates exposed near Death Valley,
California. It is made up of three units: a basal siltstone, a middle
carbonate unit, and an upper unit of interbedded siltstone and finely
bedded quartzite. Near the middle of the basal greenish gray to purplish
siltstone unit is a distinctive ~2 m thick oolitic layer, the Johnnie Oolite,
which serves as an important marker bed used for regional correlation.
The thickness of the Rainstorm Member varies significantly in the Death
Valley region. The cyclostratigraphy and magnetostratigraphy studies
were conducted at two localities, the Nopah Range and Winters Pass
Hills, and at these two localities, the Rainstorm Member is 120 and 60 m
thick, respectively (Verdel et al. 2011).
