Fence diagrams, page 1 Geology 109L Fence ... - MyGeologyPage

Geology 109L
Fence Diagrams and Isopach Maps
Fence Diagrams
Panel or fence diagrams are used for representing stratigraphic data in three dimensions.
They are similar to cross sections, but rather than interpolating subsurface geology from a map, you
interpolate the geology between stratigraphic sections or cores drilled into the subsurface. Fence
diagrams are effective at demonstrating changes in facies, pinchouts and truncations of units,
unconformities, and other stratigraphic relationships occurring in a region. The two first steps in
constructing a fence diagram are to 1) mark the locations of each section on your paper as if it was
a map and 2) choose a vertical scale. You then draw a vertical line representing the length of the
section, and you mark off the stratigraphic boundaries along the line. The next step is to choose
pairs of sections between which to draw the “fence” or panels, i.e. the facies and stratigraphic
relationships. The selection of panels should be based on the relative locations of sections and the
lithologic and stratigraphic variations. Where a choice is possible between several sections, select
those which will present the panel in the most advantageous orientation and will show the widest
variation in lithologic and stratigraphic relationships. Most sections will be connected to two other
sections with panels. Some will be connected to three and those on the edges may be connected to
only one. In cases were a section is connected to three others, one of the panels will be partially
hidden behind another one. Once all of the useful panels are completed, the fence diagram will
show the three dimensional geometry of the various stratigraphic units. Here is a simple example:
Note that unconformities are present where units are missing (thick dashed lines). Since rocks can
only be eroded after they are deposited, the units thin from the top and the contact lines approach
the unconformity from below. Contacts above an unconformity never intersect the unconformity
(unless the unit was not deposited in part of the area).
Fence diagrams, page 1

Isopach Maps:
A second good visualization tool for three dimensional stratigraphic reconstructions is a
map of the thickness of a unit of interest, i.e. an isopach (same thickness) map. These maps consist
of contours like a topographic map, but they represent the thickness of a unit rather than the
elevation of the surface. Various other maps can be constructed. For example, the depth to a
certain unit can be very useful. Sometimes % clay in a sandstone is useful, etc. They are all
constructed by plotting the desired data on a base map and contouring it to identify systematic
variations. This type of work is extremely common in both hydrology and resource exploration
(gas, oil, metals, etc.), and you will hopefully see why as you work through this lab.
The basic principles of contouring are simple. A contour is a line of constant value. The
spacing of contours on a map reflects the steepness of a gradient or slope. The path of a contour is
the expression of the variations on the surface that the contours represent. A group of equally
spaced contour lines would represent an inclined plane, whereas concentric rings would denote a
domal or basinal feature. There are, as with most techniques, a few rules to remember.
1. A contour never crosses over itself or another contour because a single point can not have two
values.
2. Contours must not touch each other except where the gradient is VERY steep relative to your
contour interval. For example, if you have two data points very close to each other with
radically different values, the lines may touch. On a topographic map, points at the top and
bottom of a cliff can have a very different elevation, but be at essentially the same place on
the map. There are no points like this in this lab. Therefore, your contour lines must not
touch each other.
3. Where a slope or gradient reverses direction, as on a ridge or in a valley, the highest or lowest
contour, respectively, must be repeated in map view; i.e., it forms a u-shape or a loop.
4. Closed contours around small areas represent isolated values which are anomalous to the local
slope or gradient. If the anomaly is lower than the normal gradient, the contour is
characterized by short hachured lines pointing inward.
Your Job:
You have been hired to help a small city evaluate locations to place another water supply
well. The city is built on a sequence of lacustrine and fluvial rocks in an area that was tectonically
active. Due to previous tectonic activity, specifically regional tilting and folding, several
unconformities are locally developed that truncate the potential aquifer. The city has hired you to
map out target areas and depths for drilling. To site a good well, you will need a three dimensional
picture of the subsurface distribution of the aquifer, creatively called Unit 6, as well as a map of its
thickness to estimate potential water volume. In this case, thicker is better. Your job is to construct
a fence diagram showing the distribution of all of the geological units, construct maps showing the
depth to the top of Unit 6 and the thickness of Unit 6, and write a short report recommending
potential drill sites.
The following table shows 18 well sections, which are your sources of data. The wells that
intersect Unit 6 at depths shallower than 1500 feet are already being used for water sources. You
will need to recommend new locations where less data are available. Each well penetrates several or
all ten units in the sequence of rocks. Luckily, the previous geologist accurately identified all of the
units before being offered a much higher paying job elsewhere because of the skills she learned
citing water wells when she was in your job. Unit 1 is the youngest and Unit 10 is the oldest. Unit
10 consists of metamorphic rather than sedimentary rocks. An "X" in the table represents the
formation in which the well started. Numbers in the columns represent the depths at which the top
of each unit was encountered in feet (most wells in the US are still reported in feet!). Certain units
are absent in some wells. There is no actual faulting in the region so all of the missing units are
truncated against unconformities. Also, the previous geologist demonstrated that all of the units
were initially deposited over the entire area.
Fence diagrams, page 2

Well
No.
Unit
1
Unit
2
Unit
3
Unit
4
Unit
5
Unit
6
Unit
7
Unit
8
Unit
9
Unit
10
Total
Depth
of
Well
1 X 600 1800 2010
2 X 150 520 900 1120
3 X 1100 2100 2750 3650 4500
4 X 950 1000 1720
5 X 550 625
6 X 1100 2020 2040
7 X 800 1700 2200 2320
8 X 450 1200 1850
9 X 800 1450 2320
10 X 650 1400 1950
11 X 400 2650 3250 3800 4600
12 X 600 1100 2400 2700 3500 4020
13 X 750 1220 1800 2000
14 X 1100 1500 2500
15 X 900 1550
16 X 700 2100 2370
17 X 550 2100 3100 3900 3940
18 X 1100 1600 2700
Your Fence Diagram
Construct a colored stratigraphic fence diagram on a vertical scale of 1" = 2000', showing
the distribution and relationships of the formations penetrated in the wells. Draw a vertical scale bar
on your base map (included here). For each well, plot the depths at which each formation is
encountered and the total depth. Then connect the formation tops using the guidelines above to
develop a panel system along the connecting lines on the base map. A few lines at the surface
between wells have already been drawn for you. You may choose the rest of the panels making
sure that you include each well. When constructing the panels, draw a wavy line where
unconformities are present. Label and color each unit. Expect to have to erase a few lines, so draw
them lightly until you are fairly confident in your interpretation. Reading the following questions
might help you identify the tougher parts early.
General Conceptual Questions
1. Why might the thickness of some units change from well to well? Think of at least 2 reasons.
What conditions might account for the decrease in section thickness between wells 3 and 2?
2. Indicate where any unconformities might be present and how you identified them. What is the
fewest number of erosional events that can account for all of the truncated units?
Fence diagrams, page 3

3. What objection would there be to drawing a panel between wells 13 and 8? Would a panel
between wells 18 and 8 improve the stratigraphic picture?
Isolith and Isopach Maps
Now construct two maps showing the thickness of Unit 6 and the depth to the top of Unit 6.
Start with the isopach map. Calculate and then mark the thickness of Unit 6 near the point marking
each well. Next contour the thickness with a 250 foot contour interval. Use your fence diagram to
help with your interpretation. Pay particular attention to the area near wells 3, 16, and 18 and
implications for the geometry of the unconformity. Make the map consistent with your fence
diagram by putting the 0 foot contour line where Unit 6 is truncated by the unconformity in your
fence diagram.
Next work on the iso-depth map. First outline the area in which Unit 6 occurs, which
should correspond to your 0 thickness contour on the isopach map. It will be helpful to make this
line a different color than your depth contour lines. Next, write the depth to the top of Unit 6 by
each well and contour the depth of Unit 6 in the area in which it occurs. Where Unit 6 is missing,
there should be no contours, not even at 0 feet (think about why). A contour interval of 500 feet is
appropriate for your data because the city would like the well to be at least 500 feet deep for
cleanliness and less than 1500 feet deep to save drilling and pumping costs. The contour lines will
meet the line marking the extent of the area that Unit 6 occurs. Once again, reading the following
questions will help you identify some of the tricky points.
General Conceptual Questions
4. How did you deal with the isopachs for wells 11 and 12 which did not reach the bottom of Unit
6? Why did you make this choice?
5. Did you include the 0 and 500 feet contour intervals on the iso-depth map? Why or why not?
Thought Question (will not be graded): What do you think happened tectonically after deposition
of Unit 6 and before development of the unconformity under Unit 3? How would your tectonic
interpretation influence the way you draw the contours near well 3?
City Report: Write a short report for the municipality discussing: 1) the stratigraphic factors
controlling the distribution of Unit 6; 2) your recommended drilling locations; and 3) why other
locations are poor candidates for drilling. Refer to your fence diagram and maps. Bureaucrats do
not read long reports, so make yours brief and to the point, but be sure to include enough
information to be convincing. Feel free to make lists of points and refer to your diagrams to help
reduce the length of your report.
Fence diagrams, page 4