Geoscience Reference
In-Depth Information
vertical sampling interval will depend upon lithological
variations, sedimentation rate, proximities to suspected
boundaries, suspected or known cyclicity, resolution
necessary for the purpose of the study, resources and fi eld
constraints (Chapter 13).
Continuous sampling , sometimes known as channel sampling,
has advantages over spot sampling for high resolution
studies if resources allow. Typically an exposure is fi rst
trenched to reveal fresh rock surfaces and then overlapping
short lengths of rock are taken throughout the section
under study (Figure 5.8c). The advantage of channel
sampling is that there is a continuous record for later
laboratory studies and the resolution of the study can be
modifi ed. Conservation issues and sampling restrictions
must, however, be taken into account. The disadvantage of
continuous sampling is the large amount of material that
will be collected. This needs to be assessed against spot
sampling where instead targeted samples can be collected,
for instance along megafossil shell beds, rather than
collecting material with no useful samples for the aims
of the study.
5
Figure 5.9 Example of a trench
through diatomite in China.
Successive samples were obtained
as the trench was dug. (Robert A.
Spicer, The Open University, UK.)
Some planar fossil material such as leaves and fi sh can be
viewed only by exposing bedding surfaces. On a wave-cut
platform this is usually not too much of a problem, but in a
cliff-face trenching is often the only option (Figure 5.9).
Throughout the section spot or continuous sampling can be
employed, but this needs to be done in such a way as to expose
large areas of bedding surfaces. In this case trenches several
metres square in horizontal dimensions might be required so
that successive bedding surfaces from top to bottom of the
section can be revealed.
If the aim of fi eldwork is to correlate between different sections
fossil occurrences should be recorded in stratigraphic context
by means of a graphic log (Chapter 6 and Figure 5.10, p. 92).
Record the fossil appearances, disappearances and changes in
abundance (frequency) as well as sedimentological information.
Near a boundary such as a break in sedimentation or a
suspected extinction event, sampling at close intervals is often
required to pinpoint the boundary precisely. In the case of
extinctions the last occurrence of a taxon is unlikely to be
recorded because the fossil record is always incomplete.
Similarly the origin of a taxon is diffi cult to pinpoint. The
problem becomes more acute the rarer the taxon. The net effect
is that even a sudden extinction will appear gradual. To
minimize this effect (the Signor-Lipps effect) more frequent
and larger samples are required near any boundary. What
constitutes 'near' will depend on the size and abundance of
taxa being studied.
Mark sample sites with
fl uorescent tape as you
collect (fi xed with rock or a
nail) so that at the end of
sampling you can log and
photograph the exact sample
locations. Remove the tape
and nails before you leave
the site.
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