Page 84 - Sedimentology and Stratigraphy

Graphic Sedimentary Logs 71

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Fig. 5.1 An example of a graphic sedimentary log: this form of presentation is widely used to summarise features in
successions of sediments and sedimentary rocks.

5.2.1 Drawing a graphic sedimentary log succession under description, for example, by the
superimposition of the letter ‘G’ to indicate a glau-
The vertical scale used is determined by the amount of conitic sandstone, by adding dots to the brickwork
detail required. If information on beds a centimetre to represent a sandy limestone, and so on. In many
thick is needed then a scale of 1:10 is appropriate. A schemes the lithology is shown in a single column.
log drawn through tens or hundreds of metres may be Alongside the lithology column (to the right) there is
drawn at 1:100 if beds less than 10 cm thick need not space for additional information about the sedi-
be recorded individually. Intermediate scales are also ment type and for the recording of sedimentary
used, with 1:20 and 1:50 usually preferred in order to structures (see below). A horizontal scale is used to
make scale conversion easy. Summary logs that pro- indicate the grain size in clastic sediments. The
vide only an outline of a succession of strata may be Dunham classification for limestones can also be
drawn at a scale of 1:500 or 1:1000. represented using this type of scale. This scheme
Most of the symbols for lithologies in common use gives a quick visual impression of any trends
are more-or-less standardised: dots are used for sands in grain size in normal or reverse graded beds, and
and sandstone, bricks for limestone, and so on in fining-upwards or coarsening-upwards successions
(Fig. 5.2). The scheme can be modified to suit the of beds.