CIV 185: Maps with inclined strata, part 1
Name 1:
Name 2:
Due Feb. 24, either submit a paper copy, or as pdf via Quercus
ID#:
ID#:
Learning outcome: Identify the intersection between planar features (rock layers or contacts between rock layers)
and a topographic surface using structural and topographic contour lines.
Strike, dip direction, and dip are measurements made in the field to communicate the spatial orientation of any plane (such as a
contact between rock layers, a fault plane, a joint).
-- The strike is the angle between two lines: (i) the intersection of the plane and the horizontal (an imaginary horizontal line), also
called the strike line, and (ii) the direction of true north. In the field the strike can be measured with a compass (corrected for the
magnetic declination); on a map the strike can be measured with a protractor. The strike is always perpendicular to dip direction
-- The dip is the angle between the horizontal plane and the inclined plane. That imaginary line is also the line along which a drop of
water would slide down the plane. The bearing of that line is the dip direction. In other words, the true dip is the steepest angle one
can measure between a plane and the horizontal. In the field dip is measured with a clinometer; on a map the dip of a plane can be
determined using structure contours and cross sections.
On a geologic map, strike and dip of layers is represented by a symbol. One type of symbol plots the bearing of the strike as a long
line, with a short tick at right angles to it showing the dip direction, and a number indicating the value (in degree) of the dip.
Sometimes an arrow is plotted parallel to the dip direction with a number indicating the value.
Part 1: 3-point problem, just an example below – the real problem is on the next page
Outcrop patterns of a planar contact can be determined if that plane is known at three points of space (for example at outcrops or from
boreholes). The figure below shows you an example of how you can construct structure contours for such a 3-point problem. Once
you know the structure contours, you may determine the outcrop pattern of that unit.
On the map on the next page, a thin coal seam is found at points A, B, and C.
(a) Construct the structure contours for this thin layer of coal. [4 pts]
(b) The coal layer has the following dip direction ______________ and dip _______________ [3 pts]
(c) Complete the outcrop of the coal layer across the map. [3 pts]
(d) At point D, how deep would you have to dig to find this coal seam? _________ meters [2 pts]
(e) Would a second coal seam 200 meters vertically below this seam outcrop on the map? If yes, sketch its outcrop. [3 pts]
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CIV 185 Lab: Maps with inclined strata, part 2
Name 1:
Name 2:
Due Feb. 24, either submit paper copy, or pdf via Quercus
ID#:
ID#:
Learning outcome: Identify the intersection between planar features (rock layers or contacts between rock layers)
and a topographic surface using structural and topographic contour lines.
Structure contours, and intersection of contacts with topography
If a plane is horizontal, its outcrop pattern simply follows the topographic contours. One way to
ponder how dipping strata show on a geologic map is to consider structure contours. Like
topographic contours they plot the elevations of a plane above a datum, usually mean sea level.
Surfaces with uniform dip and strike will have straight, parallel, and evenly spaced structure
contours as shown in ex. B (bottom right). If the structure contours are curved the strike varies, or if
the structure contours are unevenly spaced the dip changes (as with folded layers).
Structure contours can be constructed using depth to contacts from borehole data, seismic reflection
profiles, or information from within a mine. In areas of sufficient relief, structure contacts can also
be determined from the intersections between topographic contours and the surface traces of
contacts. Conversely, one can predict the surface traces if one knows strike and dip at one outcrop
location. This is shown on the right in ex. A (top right). The top map shows an outcrop with a
measured and labeled strike/dip value; the bottom map shows the structure contours determined from
this information and the constructed outcrop pattern.
True thickness versus apparent thickness,
true dip versus apparent dip
Only a vertical layer will outcrop at its true
thickness on a map. Similarly, the dip of a
layer is true only if a cross-section shows it cut
perpendicular to strike. Relations between outcrop width W, true dip α, true
thickness T, and apparent thickness A (also known as borehole thickness), can be
determined from trigonometric calculations as T = W sin(α), and A = W tan (α).
These relationships can be determined from cross-sections perpendicular to
strike. If you need to draw a cross section through layers whose dip direction is
not perpendicular to the strike line, you can calculate the apparent dip δ, as long
as you know the angle θ between the strike line and the cross-section line:
tan δ = tan α sinθ.
Part 2: Geologic cross-section
On the map on the next page, find the contact between units C
and unit D.
(a) Draw structure contours for this plane; draw them as solid lines where
this plane is below ground and as dashed lines where this plane is above
ground. [4 pts]
(b) What is the dip direction of this contact? __________ [2 pts]
(c) Calculate the dip of this contact. __________ [3 pts]
(d) If you were located at “X” near the North margin of the map, how deep would you have to drill down vertically to
reach the C/D contact? If you continued drilling, what would be the borehole thickness of unit C? [4 pts]
(e) Using the space provided below the map, construct a geologic cross-section that shows the true dips and true
thicknesses of the units. It should pass through the center of geologic unit E to the south of the map. Which orientation
and vertical exaggeration do you have to use? Label your cross-section line on the map. [5 pts]
(f) Measure the dip angle and vertical borehole thickness of unit C on your cross-section. Does it match your answers
from (c) and (d)? [2 pts]
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Map: Weijermars Structural Geology and Map Interpretation, Albion Press, 2011.
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