GEOL 4334
Lab 06: Geologic Cross Sections
v. 2015
LAB 06: Constructing Geological Cross Sections
Objectives:
Ø
Ø
Construct a viable geologic cross section based on map and subsurface data sets at appropriate
scale
Interpret existing geologic cross sections in terms of geometric accuracy and geologic validity
Materials:
tracing paper, grid paper, pencils, ruler, protractor, divider, calculator, protractor
Introduction
One of the goals of a structural geologist is to understand the three dimensional geometry of deformed
rocks. Unfortunately, all that can be directly observed are rocks at the Earth’s surface or, in special cases,
one dimensional information obtained from well logs. The shape of the structures below the surface and
the projection of those features into the air (before they were eroded) must be inferred. Geologic crossrd
sections represent interpretations of the subsurface structure. In this way they provide the 3 dimension
that compliments 2-dimensional geologic maps.
Constructing a Cross Section.
1. Determine the line along which you are going to draw the section. Several considerations must go
into selecting an appropriate section. Things to consider might include:
a. The section should be representative of the area being studied, and should cross major structural
features such as faults, folds, etc.
b. There should be appropriate data on the map or in a well log(s) to draw a complete section.
E) Completed cross section
Y
Y’
500’
500’
400’
400’
300’
300’
200’
200’
100’
c. One of the most common ways to draw
cross
sections
is
to
orient
them
perpendicular to major structural features.
Sections drawn perpendicular to map-scale
structural features avoid the problem of
determining apparent dips. In figure A, note
that the bedding symbols strike E-W and dip
to the north. Therefore, a cross section from
Y-Y’ would illustrate the north-dipping
structural grain of the map area. In contrast,
what would an EW-trending cross section
depict? (A lot of horizontal lines, parallel to
the strike of the beds! Try to visualize this.)
sandstone
shale
thrust fault
bedding
bedding form lines
100’the method of cross section construction to use. We will focus on three methods: 1) the Busk
d. Decide
V = H – sometimes referred to as “kink-band” method; and 3) the Busk method. Most of
method; 2) dip-domain
SL
the work SL
you
will
as a300’
professional
be using the free-hand or dip-domain methods. All
0’ 100’do 200’
400’ 500’ will
600’probably
700’ 800’
sections should be drawn with no vertical exaggeration.
GEOL 4334
Lab 06: Geologic Cross Sections
A
v. 2015
p1
Methods for drawing sections
D
p2
Busk method (Figure 2A)
A
B
E
F
C
This method assumes constant bed thickness during
folding and that all folds are parallel. In this method, the
curvature of a folded bed varies uniformly from one dip
measurement to the next. Bed form lines may be extended
as tangent planes to concentric circles. For example, in
Figure A, dip measurements at points A, B, and C connect
as a single curved surface that is tangent to circles with
radii of p1 and p2. One can imagine how to construct such a
cross section with a drawing compass.
e
ing
p6
p5
p3
tra
G
ce
h
of
H
I
p4
Dip-Domain Method
This method assumes regions (or “domains”) of equal dip separated by narrow hinge regions. This method may
be used in regions of thick, well-layered sedimentary packages such as in foreland fold and thrust belts,
commonly associated with similar type folds (Fig. 3). Figure 3A (below) shows a geologic strip map along a river
drainage with dip, dip direction data. Note how distinct domains with similar dip values may be defined and
separated by dip domain boundaries. Project these boundaries onto a profile line along with dip ticks on either
side (Figure 3B). Dip domain boundary lines may be extended into the subsurface (and air) with an inclination
equal to the line that bisects the two dip domains (Fig. 3B). Figure 3C represents the completed dip domain cross
section, including a pair of dark form lines
average dip in each domain
N
defining bedding.
A
dip domain boundary
10°
20°
40°
55°
45° 30°
18°
X
8°
5°
27° 7°
10°
38°
1000 ft.
42°
70°
35°
70°
70° 67° 35°
10°
20°
40°
55°
30°
inclination of dip domain
boundary = bisecting angle between adjacent domains
C
5°
15°
domain
cross
70°
60°
30°
45°
Figure 3. Dip
construction.
X’
30°
Free-Hand Method
project dip domain
boundaries down to profile line
B
X
17°
30°
500
15° 30° 60°
15°12°
4°
52°
20°
X
0
60°
70°
35°
This is the most common method
of cross section construction. As in
all other methods, section lines are
perpendicular
to
X’ constructed
regional structural grain and dip
values, contacts and other features
are projected onto the profile line,
being careful to calculate apparent
dips where necessary. Features in
the subsurface (and air) are
sketched
“free-hand”
as
X’ constrained by all data sets
available, including map data,
seismic data, well data and
regional stratigraphy and geology.
The shape of structures interpreted
in the subsurface (and air) should
mimic the shape of structures as
seen in outcrop and map view.
section
GEOL 4334
Lab 06: Geologic Cross Sections
v. 2015
Constructing a Cross Section, continued.
2. Plot a topographic profile along the cross section. Using the map scale as a guide, construct a profile
grid with no vertical exaggeration (V=H); in other words, the horizontal scale is the same as the vertical
scale of your cross section.
a. This can be done by taking gridded paper and placing the paper parallel to your profile line (Figs. A and
B). Scale and label the elevations on the profile grid. Graph paper with 10 or 20 boxes per inch is ideal for
1:24,000 topo sheets because the scale for these maps is 1 inch = 2,000 feet.
b. Where the profile line crosses a topo contour, place a point on the equivalent elevation on the profile
grid. Once all elevation control points have been transferred, using a free-hand style, connect the dots to
form the topographic profile (Fig. D).
GEOL 4334
Lab 06: Geologic Cross Sections
v. 2015
c. Place dip ticks above the profile line where they occur on the map. Transfer these dip ticks to the profile
line (Fig. D). Use a protractor to accurately plot the dip of the planar feature. At this point it’s important to
remember that you must calculate apparent dip values if the profile is not perpendicular to the strike of the
given planar feature. If the trend of the dip direction is within ~ 5° of the profile line you can use the true
dip value. (For some sections, however, you need to calculate all apparent dips.)
3. Mark contacts on the topographic profile where the particular feature crosses the line of the section.
Project dip data onto the topographic profile plane (Fig. C and D). You can project structural data onto the
profile plane from some distance from the actual
cross section line depending on the scale of
Y
Y’
heterogeneity of the structures. There is no good
rule for how far dips may be projected, but as long
as you feel that there is no good reason for a dip to 500’
500’
change between its location and the line of cross
section, it may be projected.
400’
400’
To the right is a completed profile ready to be 300’
drawn and interpreted. Be sure to leave enough
space below and above the topographic profile to 200’
draw in your structures. Make multiple copies of the
profile so that you can sketch out different 100’
interpretations. Remember, constructing cross
SL
sections is an iterative process that takes much
0’
time, thought, and reflection.
300’
200’
100’
100’
200’
400’ 500’
300’
600’
700’
800’
SL
4. Draw in structures (faults, folds) and the lithologic units on the cross section so that unit thicknesses
are preserved or vary within the context of the map patterns. Use the map patterns and outcrop
exposures, if available, to guide your depiction of structures in the subsurface. For example, if the field
area is dominated by kink-style, fault-bend folds with no apparent layer thickening, then be sure to depict
this geometry in your cross section.
5. Extend structures and lithologic units above the present level of erosion and to depth, if sufficient
information is present to do so. You may want to complete several alternative cross sections. The most
important thing to keep in mind is that your cross section should be geologically, geometrically and
physically reasonable. E) Completed cross section
6. Finish the cross
section by drawing in
the traces of contacts,
form lines, faults, the
axial
surfaces
of
anticlines
and
synclines, etc. In some
cases you may use
form lines to illustrate
the structure or fabric
within units in the
cross section (Fig. E).
Lightly color the cross
section corresponding
to the map units. Be
sure to include a legend.
Y
Y’
500’
500’
400’
400’
300’
300’
200’
200’
100’
100’
SL
V=H
0’
100’
200’
300’
400’ 500’
600’
700’
800’
SL
sandstone
shale
thrust fault
bedding
bedding form lines