Geoscience Reference
In-Depth Information
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are a convenient way of testing and making correlations
between sections of similar age from different places.
There are many variations on the style in graphic logs, with
authors tending to develop a format according to the overall
objectives and personal style. There are, however, several set
conventions that are generally followed and these are discussed
in Section 6.3.1 before some of the common types of graphic
log and different conventions are illustrated. Section 6.3.2
explains the stages in constructing a graphic log.
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6.3.1 Conventions for graphic logs
The set conventions for recording graphic logs can be
summarized as follows.
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The vertical scale:
This represents cumulative thickness,
above a particular datum on the exposure (distance above
datum increases upwards) or in the case of a borehole core
the depth down from the top (distance below datum
increasing downwards). Graphic logs can represent
successions that range from centimetres to hundreds of
metres depending on the scale of observation. The scale
factor also varies. It is usual to choose a scale factor that is
easy to work with, e.g. 1 m of rock = 1 cm on paper (1 : 100)
or 10 m of rock = 2 cm on paper (1 : 500).
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The horizontal scale:
For siliciclastic deposits this always
represents the average size of the grains, and by implication
deposition from high- or low-velocity currents or
suspension (Figure 6.5a and b). Usually the grain size is
shown increasing to the right. In the case of carbonate
deposits the grain size is more complex because the size of
the clasts is often a function of factors other than the energy
(e.g. size of the biota producing the bioclasts or whether
ooids or peloids or pisoliths are forming). Nevertheless the
horizontal scale is based on decreasing amounts of
carbonate mud towards the right (Figure 6.5c).
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Lithology:
The rock type is represented by particular
ornaments, e.g. stipple for sandstone, bricks for limestone.
clay silt f m c
sand
granules
(a)
clay silt
vf
f mc
vc
sand
(b)
mic = micrite
g = grains
sp =sparite
gs =grain supported
ms =matrix supported
(c)
Figure 6.5
Variety of different grain-size scales. (a) Basic scale for
siliciclastic rocks. (b) A more technically correct scale with each of the
subdivisions representing a doubling in grain-size diameter for the sand
subdivision (but this can be harder to distinguish in the fi eld and does not
necessarily add that much more information). (c) Grain-size scale for
carbonate rocks. (d) Potential subdivisions based on composition for
mudstone successions. For mixed siliciclastic carbonate rocks both grain-
size scales are often added to the graphic log.
(d)
