Page 83 - Sedimentology and Stratigraphy

70 Field Sedimentology, Facies and Environments

data is essential and a strong hard-backed book Tucker (2003) and Stow (2005) provide a check-list
made with weather-resistant paper is strongly recom- of features to be noted. Once different formations have
mended. Also essential is a hand lens (10 magni- been recognised (19.3.3) it is normal for a graphic
fication), a compass–clinometer and a geological sedimentary log (5.2) to be measured and recorded
hammer. If a sedimentary log is going to be recor- from a suitable location within each formation.
ded (5.2), a measuring tape or metre stick is also Although it is sufficient to regard a rock unit as simply
essential and if proforma log sheets are to be used, a ‘red sandstone’ for the purposes of drawing a geologi-
clipboard is needed. For the collection of samples, cal map, any report accompanying the map should
small, strong, plastic bags and a marker pen are attempt to reconstruct the geological history of the
necessary. A small bottle containing dilute hydro- area. At this stage some knowledge of the detailed
chloric acid is very useful to test for the presence of character of the sandstone will be required, and suffi-
calcium carbonate in the field (3.1.1). It is good to cient information will have to be gathered to be able
have some form of grain-size comparator. Small cards to interpret the sandstone in terms of environment of
with a printed visual chart of grain sizes can be deposition (5.7).
bought, but some sedimentologists prefer to make a An in-depth study may involve recording a lot of
comparator by gluing sand of different grain sizes on data from sedimentary rocks, either to see how a
to areas of a small piece of card or Perspex. The particular unit may vary geographically, or to see
advantage of these comparators made with real how the sedimentary character of a unit varies verti-
grains is that they make it possible to compare by cally (i.e. through time) – or both. The data for these
touch as well as visually. palaeoenvironmental (5.7) or stratigraphic (Chapter
The most ‘hi-tech’ items taken in the field are likely 19) studies need to be collected in a systematic and
to be a camera and a GPS (Global Positioning Satel- efficient way, and for this purpose the graphic sedi-
lite) receiver. Photographs are very useful for provid- mentary log is the main method of recording data. A
ing a record of the features seen in the field, but only sedimentologist may spend a lot of time recording and
if a note is kept of where every photograph was drawing these logs, in conditions which vary from
taken, and it is also important that supplementary sunny beaches to wind-swept mountainsides (or
sketches are made. Global Positioning Satellite recei- even a warehouse in an industrial city – 22.3), but
vers have become standard equipment for field geol- the methodology is essentially the same in every
ogists, and can be a quick and effective way of instance. In conjunction with the data recorded on
determining locations. They are used alongside a logs, other information such as palaeocurrent data
compass–clinometer, and are not a replacement for will be collected, along with samples for petrographic
it: a GPS unit will not normally have a clinometer on and palaeontological analyses.
it, and a compass will work without batteries.

5.2 GRAPHIC SEDIMENTARY LOGS
5.1.2 Field studies: mapping and logging
A sedimentary log is a graphical method for rep-
The organisation of a field programme of sedimentary resenting a series of beds of sediments or sedimen-
studies will depend on the objectives of the project. tary rocks. There are many different schemes in use,
When an area with sedimentary rock units is mapped but they are all variants on a theme. The format
the character of the beds exposed in different places presented here (Fig. 5.1) closely follows that of Tucker
is described in terms used in this book. To describe (1996); other commonly used formats are illus-
the lithology the Dunham classification (3.1.6) can trated in Collinson et al. (2006). The objective of
be used for limestones, and the Pettijohn classifica- any graphic sedimentary log should be to present
tion for sandstones (2.3.3). Other features to be noted the data in a way which is easy to recognise and
are bed thicknesses, sedimentary structures, fossils interpret using simple symbols and abbreviations
(both body and trace fossils – 11.7), rock colour and that should be understandable without reference to
any other characteristics such as weathering, degree a key (although a key should always be included to
of consolidation and so on. Field guides such as avoid ambiguity).