Geoscience Reference
In-Depth Information
fl uid according to what is appropriate and shows up clearly on
the rock (Section 13.2.1). An alternative method, provided that
there are no conservation issues, is to accurately mark up and
then to extract a large sample or set of samples and then to
subsample it back in the laboratory. Overlapping and/or
duplicate samples according to the aims of the project may be
required (Figure 5.8). Figure 13.2 shows part of a sedimentary
section marked ready for sampling. A corner section has been
selected for ease of extraction and a centimetre scale has been
marked using both a tile scribe and correction fl uid. The tile
scribe has been used as this creates a thin but permanent mark
that is retained after packaging and transportation, and the
correction fl uid has been used as it makes the scribe marks
easier to identify in photographs and a quick glance.
13
Contamination is an issue for high-resolution sample sets.
Depending on the nature of the exposure it might be best to
sample from the bottom up in order to avoid material falling
onto the part which is yet to be sampled. In addition the tools
used to collect the samples need to be cleaned carefully. An
effective means of sampling sedimentary or other softish rocks
where rock powders are required for chemical analysis is to
mark the rock and then to drill it with a masonry drill bit of
between 10 to 15 mm diameter, collecting the powder in a bag
held under the drill bit as the rock is drilled. Care needs to be
taken to clean the drill bit in between samples to ensure that
there is no cross-contamination. This method is not appropriate
for samples that will be analysed for metal isotopes because of
possible contamination from the close contact of the drill bit.
Very soft sedimentary deposits, particularly those with a high
water content, can be sampled with a knife by cutting out the
required samples. Drills designed for palaeomagnetism studies
that extract small cores of about 2 cm diameter can be used to
obtain small high-resolution samples of harder rocks.
Collecting contiguous
orientated samples of friable
sedimentary deposits such as
coals and mudstones can be
diffi cult as they can easily
break during transport. A
quick way of keeping them
intact is to wrap the samples
in stretchable self-adhering
plastic material such as 'cling
fi lm/ shrink wrap'.
In some cases samples of the whole bed or set of beds are
required. In this case it is best to extract large samples that
overlap stratigraphically. A suitable section should be found for
extraction and the rocks marked before extraction with either
hammer and chisels or a disc rock saw. Do not forget to obtain
permission if you are extracting large samples or if there are
conservation issues.
13.1.8 Labelling samples and their packaging
It is worth spending some time devising a suitable labelling
scheme for your samples. There are a variety of possibilities
depending on the purpose of the study. For regional studies the
locality number or an abbreviation of the name and then
sample number at that locality is often used, e.g. 24/3 or FB/3,
but an alternative way is to use the national grid square and
