Geology Reference
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The goal of the cyclostratigraphy study was to determine the duration of
the Shuram carbon isotope excursion that has been found in the Rainstorm
Member (Corsetti & Kaufman 2003). The Shuram Excursion is a significant
negative excursion in δ 13 C isotope values, in which δ 13 C values plummet
from +5 to as low as −12‰ and recover slowly over millions to perhaps tens
of millions of years. It is observed globally, in South China, Oman, South
Australia, Namibia, India, Mexico, and Death Valley, and it is thought to
record the oxidation of organic carbon in an anoxic Neoproterozoic ocean
just before the explosion of multicellular life in the Cambrian (Kaufman
et  al. 2007; McFadden et  al. 2008; Grotzinger et  al. 2011). The Shuram
Excursion is observed stratigraphically for several hundred meters, starting
just above the Johnnie Oolite (Verdel et al. 2011), although the excursion is
not complete at the Nopah Range and Winters Pass Hills localities because
of a postdepositional incision event (Corsetti & Kaufman 2003). The depo-
sitional environment of the Rainstorm Member was a shallow marine
continental shelf and was apparently affected by strong storm events as
evidenced by thin siltstone units interbedded into carbonate (Summa 1993;
Pruss et al. 2008).
About 40 m of the Rainstorm Member was sampled in the Nopah Range,
and about 55 m was sampled in the Winters Pass Hills. Unoriented samples
were collected every 20-25 cm stratigraphically. Oriented samples were also
collected for paleomagnetic measurement to develop a magnetostratigraphy
to constrain the sediment accumulation rate for these two localities. Nine
horizons were collected every 4-6 m in the Nopah Range and 19 horizons
every 3-4 m in the Winters Pass Hills.
The Nopah Range locality gave better paleomagnetic results and a mean
paleomagnetic direction (D = 262.8°, I = 1.3°, α 95 = 16.4°) similar to that
observed by Van Alstine and Gillett (1979) for the Rainstorm Member in the
Desert Range of Nevada about 150 km to the northeast (D = 258°, I = −1°).
At the Nopah Range, an R-N-R-N magnetostratigraphy was observed over
the 40  m sampled. The paleomagnetic data from the Winters Pass Hills
locality suffered from extensive overprinting, probably a Cretaceous age
viscous thermal partial remagnetization in a north and down direction, on
the  east (reversed)-west (normal) and shallow directions from the
Neoproterozoic. The overprinting makes it difficult to determine polarity
intervals, particularly because the directions are not antipodal, but there is
clearly a long normal polarity interval of about 30 m in stratigraphic thick-
ness with a 7 m thick reversed polarity interval stratigraphically below and
mixed polarity layers above and below this sequence. The long normal
polarity interval at the Winters Pass Hills can be correlated with the ~20 m
thick lowermost normal polarity interval at  the Nopah Range. Thermal
demagnetization experiments show that  almost all of the remanence is
removed at temperatures near to 580°C, indicating magnetite as the paleo-
magnetic carrier. Magnetite is most likely a primary, depositional magnetic
mineral suggesting that the remanence is primary. The number of polarity
intervals in these sections can only provide an order of magnitude estimate
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