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Lecture 4: Mapping Fundamentals
GEO3Z03GEO3Z03STRUCTURAL GEOLOGYSTRUCTURAL GEOLOGY
Lecture 4: TopicsLecture 4: Topicsgeologic mapsstructure contour and structure mapsthree-point problems, cross-sectionsstereonets
Representation of Geologic StructuresRepresentation of Geologic Structures
Structural orientation data are displayed and analysed using various types of graphical aidsgeologic mapsstructure mapscross-sectionsstereonetsrose diagramshistograms
Geologic MapsGeologic MapsShows surface distribution of rock types and contacts
structures portrayed using symbols (strike and dip of beds, fold axes, faults etc.)‘read’ and interpret map to infer subsurface structure
Outcrop PatternsOutcrop PatternsOutcrop patterns controlled by attitude (strike and dip) of beds
and topographic reliefpredictable for inclined beds
Rule of VRule of V’’ssOutcrop pattern of dipping planar bedding is predictable in
valleysbeds dipping downstream V-downstreambeds dipping upstream V-upstream
dip upstream
dip downstream
Rule of VRule of V’’ssOutcrop of vertical bed will always parallel the direction of strike, regardless of terraine.g. vertical dike intruded into older stratavertical structures usually easy to spot on satellite imagery, air photos
Rule of VRule of V’’ssInclined bedding dipping at same gradient as stream willParallel stream valley contours
Outcrop PatternsOutcrop PatternsWhich direction are beds dipping relative to stream valley?
Outcrop PatternsOutcrop PatternsWhich direction are beds dipping relative to stream valley?
Block Models/DiagramsBlock Models/DiagramsRelations between outcrop pattern and subsurface structure are
visualized using block models or diagramsconstruct cross-sections along map edges
Bryce 3Bryce 3--DDBlock models now constructed using 3-D modelling sofware
slice and dice stratigraphy interactively
Structure Contour MapsStructure Contour MapsMap showing the relief on a geologic surface
e.g. top or bottom of bedding plane, fault, folded surfaceconstructed from borehole data
Structure Structure ContoursContoursStructure contour lines are lines of equal elevation
show elevation relative to a horizontal datumvalues are often negative since subsurface elevations are commonly below sea level
Folded surface (antiform)
Projection of map plane
-10
-20
0
-30
-10
-20
-30
Elevation
metres
Folded surface (antiform)
Projection of map plane
-10
-20
0
-30
-10
-20
-30
Elevation
metres
Datum SurfaceDatum SurfaceDatum is a horizontal reference surface (e.g. sea level)
commonly use subsurface datum - usually regional stratigraphic surface with low relief (e.g. top of shale)
BH-1 BH-2
Unit B - Shale
Unit A
Unit C
Datum = 0 m
Depth
100 m
BH-3 BH-4
Elevation =
- 100 m b.d.s.
BH-1 BH-2
Unit B - Shale
Unit A
Unit C
Datum = 0 m
Depth
100 m
BH-3 BH-4
Elevation =
- 100 m b.d.s.
Constructing Structure ContoursConstructing Structure ContoursStructure contours defined by finding points of equal elevation
along a bed contactfind intersections of contact with topo contourdraw structure contours through points of equal elevation
100 m a.s.l.
90
80
80
90100
Unit A
Unit B
Example:Example:
100 m a.s.l.
90
80
80
90100
Unit A
Unit B
Planar SurfacesPlanar SurfacesFor uniformly dipping plane, the structure contours are parallel linescontours equally spaced for surface of constant dip
45
INCLINED BED WITH
CONSTANT DIP ANGLE STRUCTURE CONTOUR MAP
- 10 m
- 2045
- 30
- 40
- 50
4545
INCLINED BED WITH
CONSTANT DIP ANGLE STRUCTURE CONTOUR MAP
- 10 m
- 204545
- 30
- 40
- 50
CurviCurvi--planar Surfacesplanar SurfacesContours lines are curvilinear with variable spacing
e.g. folded surface, erosion surface with valleys, ridgesdip direction and magnitude changes across map
FOLDAXES
COMPLEXLY FOLDED DIPPING SURFACE
STRUCTURE CONTOUR MAP
- 20
- 30
- 40
- 50
- 10
FOLDAXES
COMPLEXLY FOLDED DIPPING SURFACE
STRUCTURE CONTOUR MAP
- 20
- 30
- 40
- 50
- 10
Rules of ContouringRules of Contouring
The general ‘rules’ of contouring also apply to all structure maps:
1) contours cannot cross or bi-furcate2) contours cannot end in the middle of the map, except at a fault or other discontinuity3) same contour interval must be used across the map and elevations must be labelled4) elevation is specified relative to datum (e.g. m above sea level)
Determining Determining Dip Angle, Dip Angle, ααDip direction and angle can be determined from structure
contour mapmeasure horizontal separation X, find difference in Ztan α = Z/X, α = tan-1 (Z/X)e.g. α = tan-1 (10 m/100 m), α = 6º
α100 m
Distance between structure contours (X)
Change in
elevation (Z)
30 m
20 m
STRUCTURE CONTOUR MAP
6º6º
- 20
- 30
- 40
- 50
- 10
ThreeThree--point Problempoint ProblemMinimum of three points is required to uniquely define a
orientation of a planecan construct structure contours given minimum of three boreholes
40
50 m a.s.l.20
40
50203040
4050 30 20
40
50 m a.s.l.20
40
50203040
4050 30 20
40
50203040
4050 30 20
ThreeThree--point Problempoint Problem1. Find minimum and maximum values2. Draw line between max, min elevations and subdivide into equal distance intervals3. Connect points of equal elevation to define structure contour
40
50203040
4050 30 20
40
50203040
4050 30 20
IsochoreIsochore MapMapMap showing change in thickness of stratigraphic interval
constructed from borehole datadoes not take into account dips of surfaces and is an apparent thickness
BH-1
BH-2
Apparent
thickness
Unit A
Unit B
Unit C
BH-1
BH-2
Apparent
thickness
Unit A
Unit B
Unit C
Zero ThicknessZero ThicknessAreas where stratigraphic unit is absent (eroded or not
deposited) are bounded by a zero contourzero contour useful in defining edges of geologic units e.g. oil-bearing sandstones
00
0
20 30
40
2030
33
34
N
0 500
metres
0
11
8
25 0
0
1231
45
722
25
0
0
0
0
05
0
00
0
0
0
0
0 10
0
0
146
20
0
0
21 25
8
0
4
48
36
326
15
38 5
14
7
0
10
0
0
ISOPACH OF FURNACE CREEK UPPER SAND (THICKNESS IN METRES)
00
0
20 30
40
2030
33
34
N
0 500
metres
0
11
8
25 0
0
1231
45
722
25
0
0
0
0
05
0
00
0
0
0
0
0 10
0
0
146
20
0
0
21 25
8
0
4
48
36
326
15
38 5
14
7
0
10
0
0
ISOPACH OF FURNACE CREEK UPPER SAND (THICKNESS IN METRES)
IsopachIsopach MapMapMap showing thickness of unit taken perpendicular to bed
sometimes difficult to estimate true thickness when there is lots of relief on bounding surfacescalculate using trig
α
BH-1
BH-2
α
BH-1
BH-2
Structure CrossStructure Cross--sectionssectionsCross-section is a 2-D ‘slice’ through stratigraphy
construct by projecting elevations of structure contours onto profileprocedure called “orthographic projection”
