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

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