Geology Reference
In-Depth Information
time period (Merrill et al. 1996; Constable 2000). For the past 170 million
years, the maximum reversal rate was about 4-5 reversals per million years,
so for a 10 cm/kyr sediment accumulation rate, a magnetostratigraphic sam-
pling interval of 6-7 m would be necessary to acquire three horizons per
polarity interval. At each horizon, at least three oriented cores should be
collected. If drilling cores in the field isn't a possibility, then oriented blocks
can be collected and cores can be drilled from them back at the laboratory.
All the techniques needed to drill and collect oriented cores are covered
in standard paleomagnetism text books (Butler 1992; Tauxe 2010).
Finally, it is important to determine whether the magnetic mineralogy
in the rocks that will likely carry the cyclostratigraphic signal are primary
depositional minerals. This is difficult to know without conducting a detailed
paleomagnetic study. At a minimum, evidence of the chemical changes asso-
ciated with present-day weathering, i.e., rusty stains on the rock or yellow
brown weathering crusts, should be avoided in the sampling and should be
trimmed away during sample preparation in the laboratory.
7.2
Field Sampling
Once the cyclostratigraphy study has been designed, field sampling is fairly
straightforward. Compared to a standard paleomagnetic study, many more
samples are collected, but the samples do not need to be oriented, so the
field procedures are simpler. Usually small hand samples are collected at the
stratigraphic interval chosen during the design stage of the study. The hand
samples are typically ~5 × 5 × 3 cm blocks that are carefully labeled and usu-
ally transported to the laboratory in sample bags, either plastic or cloth. The
most important part of the sampling procedure is to keep track of each
sample, since the correct sequence of measurements is critical for detecting
cycles in the data. Good field notes and prelabeled sample bags can help the
field worker avoid missed samples or samples with identical labels collected
from different stratigraphic horizons.
Accurately measuring stratigraphic thickness or “measuring section” is a
critical part of a rock magnetic cyclostratigraphy study. A commercially
available or home-made Jacob's staff is the standard way of measuring sec-
tion. Once the bedding attitude has been determined with a Brunton
compass, the inclinometer on the Jacob's staff is used to orient the staff per-
pendicular to bedding and the small telescope or sight on the staff is used to
determine the location of the next sampling horizon (Figure 7.1). Of course,
apparent dip must be calculated if the sampling transect is not perpendic-
ular to the strike of the bedding. Figure 7.2 shows samples collected for a
rock magnetic cyclostratigraphy study of the Rainstorm Member of the
Neoproterozoic Johnnie Formation. Sampling sites were determined with a
Jacob's staff and marked with surveying tape. Samples were broken off with
a geologic pick and placed in prelabeled plastic bags. The samples were left
