Geoscience Reference
In-Depth Information
6.4 Rocks in space: Reconstructing
sedimentary environments and
their diagnostic features
A set of graphic logs in proximal and distal locations, together
with sketches and other information, can be used to record and
interpret the depositional environment(s) of the strata. The
graphic logs should provide a summary of features to allow
division of the succession into sedimentary facies (i.e. rock
bodies with similar composition, texture, fossils and
sedimentary structures that represent a particular set of
processes and depositional conditions). Using Walther's Law,
and knowledge of the different parts of depositional
environments and the processes within them, the facies can be
grouped together into architectural elements or facies
associations. The construction of facies associations is
particularly appropriate for depositional environments where
there are large lateral changes, such as within a fl uvial system.
The elements of completing such a study are illustrated in
Worked Example 6.2 (pp. 130-133).
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For pilot projects, broad-scale regional studies or simply if time
is short, generalized graphic logs recording the main
sedimentary features to an approximate scale can be useful (e.g.
Figure 6.11, p. 128). The advantage of this is that it is fast,
allowing time to visit more locations and gain a better overview.
The benefi t of carrying out facies analysis is that the facies
patterns are then predictable and can be used to assess
reservoir and source rock potential. In addition facies models
underpin the interpretation of changes in relative sea-level, the
establishment of stratigraphic gaps and can add to
interpretation of long timescale changes in the climate,
palaeolatitude and altitude.
Fine-grained hemipelagic and pelagic carbonate and
siliciclastic successions tend not to vary for hundreds of metres
or kilometres. Usually with this type of succession, because the
variation in facies is so small, there is no need to defi ne facies
associations. The visual differences between facies in the fi eld
are often very subtle, but may well refl ect large changes in
parameters such as run-off, temperature and productivity.
In summary, facies analysis needs to consider the following.
•
Is the area chosen for the graphic log and facies analysis
representative?
•
Are sections along a proximal to distal profi le available
(basin centre and basin margin)?
•
Do the facies obey Walther's Law or not?
