Geoscience Reference
In-Depth Information
•
Understand and exploit sedimentary building materials and
mineral deposits.
Sedimentary rocks form important
building materials. Sandstones and limestones are relatively
easy to extract and shape because the natural bedding
planes can be used to create blocks. Limestones are needed
for cement production and agricultural uses, while
mudstones are the raw component for bricks.
6
•
Refi ne the geological timescale.
The more continuous
nature of the sedimentary record compared with that of
igneous and metamorphic rocks, together with its fossil
content, makes it important for the construction of the
geological timescale. The sedimentary record is used for
biostratigraphy, constructing magnetic polarity patterns,
identifi cation of Milankovitch and other regular cycles,
radiometric dating of interbedded volcanic ashes, some
direct radiometric dating (e.g.
14
C, U series and Re-Os
isotope) and chemostratigraphy, all of which contribute a
major part of the composite geological timescale.
In addition to the basic geological equipment (Table 2.1, p. 4),
it is useful for the study of sedimentary rocks to also have
some of the optional equipment listed in Table 2.3 (p. 6). In
particular, ensure that you have a comparison chart for
identifying grain size, shape, rounding and sorting (see also
Appendix A6). A Munsell colour chart is useful if you are
recording rocks where there is a signifi cance to the variation in
colour, or, where colour is the main distinguishing feature. A
1 lb (
c
. 0.5 kg) geological hammer and a cold chisel are usually
suffi cient to collect samples.
6.2 Description, recognition and recording
of sedimentary deposits and sedimentary
structures
There are four aspects that need recording in the description of
sedimentary rocks: (1) the composition, which is relatively easy
compared with igneous and metamorphic rocks as there are not
many minerals that are common in sedimentary deposits
(Appendix A6, Table A6.2); (2) the texture of the rock; (3) the
sedimentary structures; and (4) the fossils within them.
Composition and texture are taken together as lithology
(Section 6.2.1), sedimentary structures are covered in Section
6.2.2 and the recording of fossils was covered in Chapter 5.
6.2.1 Recording sedimentary lithology
Assuming that the sedimentary section under study is divided
into units (Section 3.2.2), the next task is to describe and
classify the sedimentary rock type of the unit under
