Friday, October 5, 2012

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.
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).
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
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.


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