Geoscience Reference
In-Depth Information
6 Recording features of
sedimentary rocks
and constructing
graphic logs
6
Angela L. Coe
6.1 Introduction
Sedimentary rocks, particularly coarse-grained siliciclastic
rocks, are rewarding to study in the fi eld. This is because you
can gain a lot of information about their mode of formation
directly from fi eld observations and start to make an
interpretation (Figure 6.1) without the need to wait for thin-
sections or geochemical analyses, which are required for many
igneous and metamorphic rocks. Some carbonate successions
and fi ne-grained mudstones can, however, be tricky to interpret
in the fi eld and their study can benefi t greatly from follow-up
microscope work and/or geochemical analyses.
Sedimentary rocks and the fossils they contain should always
be considered together because, as 'smart particles' (Section
5.1), fossils provide vital clues on the processes and
environment of deposition of the sedimentary deposits. Fossils,
for instance, can provide immediate clues on whether the rocks
are marine or non-marine, were deposited over a long or short
period of time, what conditions were like on the sea or lake
fl oor, as well as, in many instances, providing an immediate
relative dating method.
There are a variety of specifi c reasons for collecting data from
sedimentary deposits aside from the general ones of geological
mapping or constructing a geological history for an area. These
are to:
Understand sedimentary processes and depositional
environments. This leads to a better understanding of
natural processes on the Earth's surface.
Understand the potential of a sedimentary basin or unit for
hydrocarbon recovery or for water resources. An
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