GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
v. 2015
LAB 03: Geologic Maps I: Topo Maps & Outcrop Patterns
Objectives:
This lab will develop & exercise the skills of 3D visualization and description of geologic
structures using correct terminology and will acquaint you with:
a) symbols used to describe different structures,
b) the relationships between inclined layers, their true thickness vs. apparent thickness, topography, and
how to predict where planar layers may occur in the subsurface or on a topo map,
c) the outcrop pattern of folds in map view,
d) the ‘rule of V’s’ to discern the orientation of planar features on geologic maps, and
e) the use of geologic cross sections to discern 3-D perspectives on structures.
Background:
You should be familiar with reading topographic maps. Please refer to your Physical
Geology laboratory exercises on topographic maps for review.
Materials:
tracing paper, pencils, ruler, protractor, color pencils, scissors, ALL Ch. 3.
Introduction
To properly assess the shape and orientation of a subsurface structural reservoir in an oil field, or the
extent of a seismically active fault, it is necessary to learn how to visualize such features in 3-D based on
their 2-D representation on geologic maps. This set of laboratory exercises will focus on a critical element
of geologic map analysis: How to recognize the orientation of structures based on outcrop patterns.
0
2000 ft.
D
Map Legend
S
Tertiary dike
K
O
77
18
N
15
K
Cretaceous
D
Devonian
S
Silurian
O
Ordovician
K
S
27
K
S
25
O
contact; unconformity (circles on
younger rocks)
D
O
24°
15
inclined vertical horizontal
bedding bedding bedding
plunging
anticline
syncline
S
Figure 1 shows a geologic
map and oblique relief map
to help visualize the three
important features: faults,
folds, and unconformities.
Also shown are the common
map
symbols
used
to
illustrate a geologic map.
All maps should contain:
• a legend
• scale & geographic
coordinates
• appropriate caption.
In this map, note how the
structure
symbols
correspond to the orientation
of the underlying rock unit
and also indicate attributes
such as the dip of faults, etc.
Fig. 1. Geologic map and
oblique perspective diagram.
O
Page 1 of 7
GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
v. 2015
Topography & Outcrop Pattern Recognition
Outcrop
patterns
of
different
geologic units provide a qualitative means of
understanding the 3D orientation of units
along the uneven surface of the Earth.
Topographic maps display relief, or changes
in elevation, using lines of equal elevation
called topographic contour lines (Fig. 2).
’
600
400’
400’
0’
20
600’
200’
Fig. 2. Shaded relief and resulting
topographic map. Note how contour lines on
the oblique shaded relief map project
upwards onto the 2D topographic map.
800’
800’
200’
A
If an exposed geological unit (e.g.,
a fault, stratigraphic bed, unconformity,
etc.) is planar over some distance, the
resulting outcrop pattern over uneven
topography may be used to predict the
unit’s orientation.
On a geologic map, horizontal
surfaces, such as the contact between
sandstone and shale beds, would appear
as a line parallel to the topographic contour
lines (Fig. 3A) Inclined planar units would
make an angular pattern on a non-flat
topographic surface (Fig. 3B). Note how
the inclined layer dips “towards” the
mountain and makes a “V” that points “up”
the drainage. In contrast, a vertical surface
such as a strike-slip fault would appear as
a line on the geologic map that “cuts
across” the topographic contour lines (Fig.
3C). In map view, such surfaces would be
linear over some spatial distance.
Fig. 3. Oblique perspective shaded-relief
perspectives
and
equivalent
2D
topographic maps depicting the trace of
planar units intersecting topography. A)
horizontal bed; B) inclined bed, dipping 25°
N; C) vertical unit, striking EW/90° dip.
Note structure symbols that depict
horizontal, inclined, and vertical contacts,
respectively.
Oblique View
Map View
N
structure symbols
B
25°
N
C
N
Page 2 of 7
GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
Oblique perspective view
v. 2015
Map View
“Rules of V’s”
A)
300
m
m
200
300 m
200 m
100 m
N
B)
N
30°
m
300
m
N
35°
m
300
N
60°
E) Vertical surface; no “V” in map view. Outcrop
pattern is linear in map view and “cuts across”
topographic contour lines.
200
m
300
m
F) Inclined surface dips upstream, “V” points
upstream.
N
200
300
m
E)
m
N
While one can memorize these rules, it is best
to work on visualizing the geometrical relations
until your understanding becomes second
nature.
Figure 4. A-F various planar beds in oblique
view (left) and the resulting outcrop pattern on a
2D topo map (right). Soccer man in A is climbing
up the drainage wall. Arrows point down the
drainage. Martini glass is on a horizontal
surface.
200
m
12°
m
300
F)
C) Surfaces that dip downstream at the same
gradient as the stream appear parallel to the
stream.
D) Inclined surfaces with dips downstream, “V”
points down stream.
m
200
D)
A) On a geologic map, horizontal planes appear
parallel to contour lines (Fig. 4A).
B) Planes that dip downstream at a shallower
gradient than the stream “V”, or point,
upstream. The outcrop pattern has a “narrow”
profile in map view.
200
C)
It is important to be able to visualize the
intersection of a planar unit and topography.
The “rule of V’s” provides a general means of
interpreting outcrop patterns and determining
the orientation of planar geologic features as
they intersect topography.
Page 3 of 7
GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
v. 2015
Outcrop patterns and folds
Folded, planar beds can make an infinite number of interesting and variably complex patterns. Figure 5
depicts two examples: On the left a horizontal, upright fold, on the right a plunging, upright fold.
A
B
X’
X
slice through fold
D horizontal
(map view with no topography)
horizontal slice through fold
(map view with no topography)
trace of
axial plane
X
X’
X’
X
N
N
0
500 ft.
C map view (with topography)
map view (with topography)
800
E
0
0
60 00
4
0
60
80
40
0
0
20
0
20
Figure 5. Two views of antiforms (“A”-shaped folds). A) Cross section X-X’ depicts the profile shape of the
fold. B) horizontal slice through a horizontal, upright antiform. at the elevation of soccer man (i.e., a map
view with no topography). Note how the map pattern of the limbs of the fold are parallel in the horizontal
slice. C) Map depicting the outcrop pattern of the same fold as it intersects an uneven topographic
surface. Can you visualize and draw a profile of this fold? D) Map view of a plunging, upright antiform. E)
Map depicting the outcrop pattern of the same fold as in D) as it intersects an uneven topographic surface.
Visualize the profile view of the fold.
NOTES:
Page 4 of 7
GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
v. 2015
Predicting Outcrop Patterns
If you know the orientation of a unit at one locality on a topo map, you can predict where the unit will crop
out within the map area, assuming the unit is planar and continuous. Figure 6A depicts a topo map with
some structural data (strike/dip symbols) noted at three localities. At station 5 (St. 5; elevation 240’) the
attitude of a planar bed is 090°/20°. Complete the outcrop pattern of this bed across the map area.
Step 1: draw a profile plane with a vertical scale that includes the lowest to highest elevations in the map
area (Fig. 6B). Project the point at St. 5 from the map onto the profile plane at 240’ (black arrow).
Step 2: draw a line through this point with an inclination of 20° toward the south. You have now drawn the
cross section of the planar bed
(Fig. 6B).
A)
’
240
’
250
0’
3
2
20°
N
0’
’
210
’
260’
240’
220’
100’
0
N
drainage
contact
24
St. 5
220
20°
Step 3: At each elevation on the
profile, project a line across the
topo map. Place a point where the
line of a given elevation crosses
the
equivalent
topographic
elevation. For example, in Fig. 6B
note how the open circle at profile
elevation of 210’ is projected onto
the topo map and two open
circles are drawn where the line
crosses the 210’ topographic
contour interval.
B)
’
N
’
24
0’
220
’
210
20°
’
St. 5
0
23
20°
δ=20°
240
’
250
Step 4: Once all points have been
drawn on the map, sketch the
outcrop pattern by “free-handing”
a line that connects the points.
DO NOT USE A RULER when
connecting points of topography.
Figure 6. A) incomplete geologic
map displaying bedding symbols
and field station, St. 5. B) Method
to complete outcrop trace of
planar, inclined contact.
100’
S
0
fold line
NOTES:
Page 5 of 7
GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
Common Map symbols
v. 2015
Wells
strike/dip of foliation, trend/plunge
of lineation in foliation plane
Strike/dip of bedding,
open circle=upright based on
geopetal structure
oil well
gas well
strike/dip of joint/fracture
oil and gas well
strike/dip of cleavage
strike/dip of overturned
bedding
dry hole
strike/dip of
igneous modal layer
strike/dip of dike
or schileren
strike/dip of magmatic foliation, trend and
plunge of lineation in foliation plane
horizontal bedding
vertical bedding
Contacts
Contacts - solid = exposed & well located, dashed where approximate, dotted
where inferred; U = relative displacement “up”, D = relative displacement “down”
unconformity; circles on younger rocks
contacts
U
D
thrust/reverse fault; “teeth” on hanging wall, tick = dip of fault where measured,
arrow = plunge and trend of striations on fault surface. Dash/dotted as above.
Note that dip value (40°) is greater than plunge of striae (30°).
U
normal fault; ticks on hanging wall, tick = dip of fault where
measured, arrow = plunge and trend of striations on fault surface.
Dash/dotted as above.
D
alternate symbols for low-angle (< 30°) normal fault,
called a detachment fault
D
U
strike-slip fault
trace of fold axis
Folds
plunge direction and
amount
upright anticline
small dome
overturned anticline, axial surface
dips to N
trace of fold axis
syncline
overturned syncline,
axial surface dips to S
72
small basin
N
trend and plunge of asymmetric “z” (left)
and “s” (right) fold axes
plunge direction
and amount
Antiformal syncline, dip of axial plane
72
trend and plunge of small-scale
fold axis
Synformal anticline, dip of
axial plane
Common structure symbols used on geologic maps. Be able to visualize the structures that these
symbols depict.
Page 6 of 7
GEOL 3402 • Structural Geology • Prelab 03:
Topographic maps & geologic outcrop patterns
v. 2015
Cut-out PRIOR lab session to help
visualize outcrop patterns.
100 m
200
m
m
30°
300
fold lines
cut along border of polygon after creasing the paper along the fold lines.
300 m
200 m
100 m
100 m
200 m
300 m
300 m
200 m
100 m
Page 7 of 7
light grey lines = topographic contours in meters above sea level
200 m
300 m