Geologic Structures, Maps
and Block diagrams
ØUnderstand the importance of cross
sections.
Learning Ø Be able to construct cross sections
Objectives from basic geological maps.
Ø Be able to calculate true thickness
of beds.
Geologic map of
Kuwait
• 1. Rock units or geologic strata.
• 2. Bedding planes and structural features
such as faults ,folds , are shown with strike
and dip or by symbols which give three-
dimensional orientations features.
• 3. Stratigraphic contour lines.
Geologic symbols
Structural Geology
• Study the rock deformation ( change
in shape of the rock , volume and
position ) and geologic structure that
resulted from deformation ( fault ,
fracture, joint, fold ) .
• Fault : are fractures or breaks in rocks on which movement
1. Fault ( brittle has occurred.
• The fault block above the fault is the hanging wall .
deformation )
• The fault block below the fault is the footwall.
What type of fault is this ?
Normal fault
2. Fold ( ductile
deformation )
• Upwards , downwards or
sideways bends
of rock layers ( without break ) .
Non- plunging Syncline fold vs Non- plunging Anticline fold
Plunging anticline
Overturned fold
HORIZANTAL AND INCLINED BEDS
Constructing a cross section of horizontal
beds
•First determine the line along which you are going
to draw the section, it should:
1. Be representative of the study area.
2. Cross all major structural features (e.g. faults and folds)
3. have appropriate data on the map or well logs to draw a
complete section.
4. Often be drawn perpendicular to major structural
features.
Complete the geologic cross section
• On some graph paper draw an x- axis with the same scale and length as the line of
the cross section.
• Then add y axes to both ends, that have a relevant scale to the topographic
contours.
• If you join these dots up, you should end up with an accurate scaled topographic
cross section that looks something like the one below
Construct the lithological boundaries
• Now, with your scrap piece of
paper add in where the
lithological boundaries
intercept your cross sectional
line
• Then, overlay your scrap
piece of paper with your cross
section and mark on the
different lithological
boundaries.
• Now, extrapolate from these points the bed boundaries; If they dip, then draw the
lines at the correct angle of dip for each bed.
• However for this example, as the unit boundaries followed the contours, they are
evidently horizontal.
• Now you have drawn an accurate, to scale, cross section.
Calculating true thickness from a map
• True thickness (t) = width of outcrop (w) x sin (θ) (angle of dip)
• Example: If the width of an outcrop is 100m and has a dip of 14˚ then
t = 100m x sin(14˚)
t = 24.2 m
Geological outcrops are shown in the NW
corner of the map.
1. Complete the geological outcrops over the
whole map.
2. Draw a geologic cross section along A-B
showing each of the beds in stratigraphic
order and to scale (use a scale of 1cm =
100m).
3. Indicate the actual thickness of each bed.
Draw a section along the line A-B.
1. Draw legend
• A jagged line is drawn at the bottom of a
stratigraphic column where the thickness of a
unit is unknown.
• As these beds are horizontal the actual
thickness is the same as the outcrop width.
Constructing a cross section
Block Diagram
• Cross section : are vertical into the
earth that are used to interpret the
depth.
• Block diagrams : are a
combination of geologic maps and
cross sections.
Constucting a cross section for inclined beds
