Geology Reference
In-Depth Information
derived by applying different computer contouring techniques to identical data.
If dip information is available from even a few locations, the derived contour direc-
tions and spacings (Sect. 3.6.1) greatly constrain the interpretation. The map in
Fig. 10.10c has been produced by extending the known dips in 3-D until they inter-
sect to define boundaries with the adjacent domains, then contouring the resulting
structure by triangulation. The map so constructed (Fig. 10.10c,d) agrees perfectly
with the known dips.
Any conflict between the structure contours and the local bedding attitudes should
be investigated. Local attitude measurements with a compass in outcrop or from a
dipmeter in the subsurface can be accurate yet not reflect the map-scale structure. Small-
scale bedding variations, cross bedding, or minor folding, can all lead to bedding atti-
tudes that diverge from the trend that should appear on the structure contour map.
The regional trend will be clearer, in spite of any small-scale variability of the bedding
attitudes, if the fold axis is found using multiple attitude measurements (Sect. 5.2).
10.4
Bed Thickness Anomalies
Many quality-control measures are ultimately based on the concept that bed thick-
nesses should remain constant or be smoothly varying. This concept is applied as a
quality control tool to the compatibility between multiple horizons on maps and cross
sections, the internal consistency of composite-surface maps, and the geological like-
lihood of the growth history given by the expansion index.
10.4.1
Compatibility between Structure Contour Maps
The structure of nearby stratigraphic horizons is usually fairly similar. If unit thick-
nesses are approximately constant, the shapes of nearby horizons will be nearly the
same. The compatibility of maps on closely spaced horizons is indicated by structure
contours that are nearly parallel (Fontaine 1985) and separated by approximately con-
stant distances (in gently dipping beds). A significant difference in trends on adjacent
surfaces suggests a possible misinterpretation of one or both of the surfaces. The struc-
ture contours on different horizons cannot intersect without implying a structural or
stratigraphic discontinuity. Only faults and unconformity surfaces can cut across strati-
graphic boundaries.
Mapping different horizons independently can easily lead to incompatible surfaces.
Figure 10.11 shows structure contour maps on two closely spaced horizons on the same
part of the Sequatchie anticline. In this example more control data were generated for
the lower surface (Fig. 10.11b) by means of vertical projection (Sect. 3.6.2). To deter-
mine if the maps are compatible, they are superimposed (Fig. 10.12). The superposed
contours are approximately parallel and so are compatible by this criterion, except in
the shaded region of second-order folding on the southeast limb of the major anticline
(Fig. 10.12a). Disharmonic folding is implied by the folds confined to the Mtfp. This
may be geologically reasonable if the mechanical stratigraphy is appropriate (Sect. 1.5.1)
but should be checked.
