Geology Reference
In-Depth Information
Fig. 10.7.
Geologic map. Units from
youngest to oldest are Mh,
Mpm, Mtfp. Topographic el-
evations are in feet and the
scale bar is 1 000 ft. a Structure
contours ( heavy lines ) deter-
mined from the intersection of
mapped formation boundaries
with the topographic contours.
b Revised geologic and struc-
ture-contour map
Fig. 10.8.
Structure contours on the top
and base of an outcropping
unit on a topographic map.
Structure contours drawn be-
tween closest corresponding
elevations across the V formed
by the outcrop trace. Solid
dots are control points; long
dash contours are on top of
the bed; dotted contours are
on the base of the bed. Ques-
tion marks show where struc-
ture contours indicate that the
bed surface should intersect
the topographic surface but
no intersection is observed
boundaries of the Mpm (Fig. 10.7a). The 800-ft and 600-ft contours are parallel, but
the 700-ft contour is quite different, suggesting that the contact locations producing
the 700-ft contour could be wrong. The map is improved by changing the least certain
outcrop trace to make the 700-ft contour to be parallel to the others (Fig. 10.7b). The
revised map becomes a new working hypothesis which should be field checked.
The consistency between a structure contour map and bedding attitudes from out-
crop measurements or dipmeters provides a powerful test of the interpretation. The
fold axis trend must be consistent with the structure contour trend. The potential
problem is illustrated with the map in Fig. 10.8. If the formation boundary elevations
are sampled at individual points around the structure, they will be contoured as shown
(previously discussed in Sect. 5.5). A problem will be recognized only if compared to
the trend of the fold axis, which trends north-south with zero plunge. The correct
structure contours are shown in Fig. 10.9a. This interpretation would be clear with
good exposure in the field (Fig. 10.9b), but in the subsurface could not be interpreted
without knowledge of the fold axis.
A few locations with dip information can be used to test and control the structures.
The information provided by a small number of formation tops alone can be con-
toured in numerous ways as evidenced by the two maps (Fig. 10.10a and 10.10b)
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