Geology Reference
In-Depth Information
Fig. 6.1.
Cross sections through a circu-
lar cylinder.
a
Normal section.
b
Oblique section.
c
Offset
section.
d
Axial section
Fig. 6.2.
Structure contour map of an
anticline showing the section
lines.
A-A':
normal (trans-
verse) cross section perpen-
dicular to the trend of the
anticline.
B-B':
longitudinal
cross section.
C-C':
well-to-
well cross section
dated (Chap. 10 and 11) first. A grid of cross sections is needed for a complete three-
dimensional structural interpretation.
The reason that a structure section should be straight and perpendicular to the major
structural trend is that it gives the most representative view of the geometry. The sim-
plest example of this is a cross section through a circular cylinder (Fig. 6.1). If the
entire cylinder is visible, then it would readily be described as being a right circular
cylinder. The cross section that best illustrates this description is Fig. 6.1a, normal to
the axis of the cylinder, referred to as the normal section. Any other planar cross sec-
tion oblique to the axis is an ellipse (Fig. 6.1b). An elliptical cross section is also correct
but does not convey the appropriate impression of the three dimensional shape of the
cylinder. A section that is not straight (Fig. 6.1c) also fails to convey accurately the
three-dimensional geometry of the cylinder, although, again, the section is accurate.
Section c in Fig. 6.1 could be improved for structural interpretation by removing the
segment parallel to the axis, producing a section like Fig. 7.1a. A section parallel to the
fold (or fault) trend (Fig. 6.1d) is also necessary to completely describe the geometry.
Predictive cross sections are constructed using bed-thickness and fold-curvature
relationships that are appropriate for the structural style. In order to use these geomet-
ric relationships, or rules, to construct and validate cross sections, it is necessary to
choose the cross section to which the rules apply. Such a rule, in the case of the circular
cylinder in Fig. 6.1, is that the beds are portions of circular arcs having the same center
of curvature. In this simple and easily applicable form, the rule applies only to section a.
More complex rules could be developed for the other cross sections, but it is quicker
and less confusing to select the plane of the cross section that fits the simplest rule than
to change the rule to fit an arbitrary cross-section orientation.
