Geoscience Reference
In-Depth Information
•
Sedimentary rocks show a range of porosities whereas
plutonic igneous and metamorphic rocks have a very low
porosity.
•
Metamorphic and igneous rocks have a higher density than
most sedimentary rocks.
3.3 Overview of possible data formats
3
The objectives of the fi eldwork will determine the most
appropriate format for the data collection. Chapters 5 to 9 cover
specifi c types of data collection relating to fossils, structures
and different rock types. It can also be useful to transfer
techniques between different rock types, so the range of fi eld
methods are briefl y reviewed here.
•
Sketches:
Sketches on a range of scales both in plan view
and cross-section to show the geometric relationships of
rock bodies are used in all aspects of geological fi eldwork.
They are covered in detail in Section 4.3.
•
Graphic logs:
These are graphic representations of a
stratigraphic succession of rocks, including information on
the facies, boundaries between different units, bed
thickness and lithostratigraphy. Graphic logs are commonly
used for recording changing facies in sedimentary rocks
(Chapter 6), but they are probably the best means of
recording any stratigraphic information whether it is
changing rock types along a cutting, the stratigraphic
succession of fossils (Chapter 5), or the features of
pyroclastic rocks (Section 7.2) or even a succession of lavas
(Chapter 7). The construction of a graphic log is covered in
detail in Section 6.3.
•
Cross-sections:
Field sketches of cliffs and exposures
provide one type of cross-section. Another type that is
worth sketching when in the fi eld is a generalized cross-
section of the inferred subsurface geology. This can be
particularly useful where the strata are folded and/or
faulted. A sketch cross-section helps to focus the task and
think about different possibilities. Cross-sections are
covered in Sections 4.3, 10.4.3 and 10.6.2.
•
Maps:
Sketch maps (Section 4.3.4) and base maps
(Section 10.5.2) provide an invaluable plan view of the
geological features showing the geometrical relationship
between the units and the location of large-scale geological
features.
•
Stereonets:
For structural studies and also palaeocurrent
measurements in sedimentary successions it is often
