Geology Reference
In-Depth Information
Fig. 4.14.
Interpretation of a thickness map.
a
Thickness data.
b
Triangulation contouring.
c
Kriged
map on a 10
10 grid.
d
Paleostructural interpretation produced by making the thicknesses negative
on the kriged map. The plane of zero elevation is shown above the map and would represent the upper
surface of the unit at the end of deposition
×
As an example, an isopach map is constructed from the data in Fig. 4.14a. For the
purpose of discussion, the points are contoured by both triangulation (Fig. 4.14b) and
kriging (Fig. 4.14c). The paleogeographic implications of the map should be consid-
ered before either map is accepted. The triangulated map suggests a stream channel
whereas the kriged map suggests an isolated depocenter. Both computer contouring
methods close the contours within the map area. Re-examination of the data reveals
that if the unit represents a channel, it could be extended off the map to both the north
and south and still be consistent with all the control points. Accepting the depocenter
interpretation, it can be visualized in 3-D as a paleostructure map by reversing the sign
on the contours so that the thickest part plots as the deepest (Fig. 4.14d).
Tthickness trends on isopach maps could alternatively represent unrecognized faults
that are too small to be identified directly. A normal fault will cause a thinning of the
isopachs and a reverse fault will cause a thickening. Section 8.5 discusses faults on
isopach maps. Figure 4.14 could represent a reverse fault that is too small to repeat the
top and base of the unit.
4.3.2
Isocore Maps
Isocore maps are particularly valuable for determining the volume of a unit present in
the area of interest. The area enclosed by each isocore contour is multiplied by the
