Geoscience Reference
In-Depth Information
For the less common rock types, e.g. ironstones and
evaporites, the ornament used may vary slightly. Some of
the commonly used symbols for graphic logs are shown in
Appendix A6, Figure A6.16.
•
Sedimentary structures.
The sedimentary structures are
represented either by very idealized symbols or by a sketch
(to scale) of the structures as seen. The latter has the
advantage of not having to identify the structure
immediately and/or showing the particular geometry of the
sedimentary structure in that succession. For instance, the
typical thickness of the cross-stratifi cation sets or the
variation in angle can be shown.
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Other information:
Other information includes the
lithostratigraphic nomenclature, fossils, biostratigraphic
zones, sample and photograph information, bed numbers,
palaeocurrent data and sequence stratigraphy. These are
usually put into columns adjacent to the main log with the
information aligned horizontally with the particular level
that it pertains to. These columns are usually ranked in a
logical order, e.g. for lithostratigraphy: group name, formation
name, member name and bed number. If the data you have
collected need to be compared with those of previous studies
this is often also shown in a summary format. It is a good
idea to work out the correlation in the fi eld where a variety
of features are more obvious and different correlations can
be tested. This is essential if you are relying on previous
data sets for biostratigraphic or geochemical information.
•
Stratigraphic order:
The rock units should, if possible, be
examined and recorded in the notebook in stratigraphic
order, i.e. with the notes on the overlying unit above the unit
it supersedes. This enables the contacts between the units to
be depicted graphically. Sometimes in the fi eld, because of
particular conditions, it becomes necessary to log the youngest
units fi rst and work down through the stratigraphy. In this
case the units should still be recorded in the correct
stratigraphic order in the notebook; this is easily achieved by
working down from the top of the notebook pages.
Variations on the conventions for graphic logs
The common variations on the main conventions are:
•
using the same or separate columns for the lithology and
sedimentary structures - exactly the same information
from Figure 6.4 is represented in a split column format in
Figure 6.6;
•
where the succession is mainly or entirely composed of
carbonates, omitting the brick symbol used for carbonates
(e.g. Figure 6.10, p. 126);
