Geoscience Reference
In-Depth Information
vertical and horizontal sampling interval will depend upon the
purpose of the study. Secondary to this consider the following:
lithological variations, deposition rate, proximity to suspected
boundaries, suspected or known repetition, resources and fi eld
constraints. This section outlines the factors to be taken into
account depending on the intended purpose of the samples.
Care needs to be taken to avoid cross-contamination when
selecting samples for geochemical analysis, mineral analysis,
microfossils, molecular fossils and high-resolution studies.
13
13.1.1 Samples for thin-sections
A sample of about 10 cm thick × 5 cm × 5 cm is usually
suffi cient for producing one or more thin-sections provided the
rock is not too coarse-grained. If the rock is coarse-grained then
a sample size that is appropriate to the grain size should be
taken. The sample needs to be large enough to enable it to be
clamped in the rock saw and care needs to be taken with
sample position and shape if it needs to be orientated (see
below). In deformed rocks it is useful to cut two or even three
thin-sections at right angles to each other, so a slightly larger
sample may be required.
13.1.2 Orientated samples
Younging direction and approximately
orientated samples
For sedimentary rocks the younging direction is often required
and is usually recorded on the face perpendicular to bedding
with an arrow. If the bedding orientation is not clear this may
also need to be recorded so that the section can be cut
perpendicular to the bedding. For igneous rocks the younging
direction may need to be recorded for cumulates, pyroclastic
rocks, and igneous rocks containing fl ow features and vesicles.
For samples from metamorphic rocks it is also often necessary
to record the top surface because of the need to obtain thin-
sections in a particular orientation relative to the structural
fabric(s). A more precise method of orientating samples is
provided below.
Precisely orientated samples
Some samples need to be precisely orientated in the fi eld so
that this can be taken into account during later processing
(Figure 13.1, p. 252). This includes samples for
palaeomagnetism studies and some samples for more precise
metamorphic and structural analysis. To do this, fi nd a piece of
rock on the exposure that is going to be easily extractable, or if
the rocks fracture in a manner that is suitable, extract the rock
sample and then fi t it back into place exactly. Using a compass-
