Geology Reference
In-Depth Information
Fig. 3.22.
Vertical cross sections showing incorrect projections across discontinuities.
a
Projection across
a fault.
b
Projection across an axial surface. The region below ground level is
patterned
.
δ
:
dip;
t:
thickness
of the interval being projected
bed thickness remains constant throughout the fold for all units being projected
(Fig. 3.21a), in other words, that all the horizons are parallel. This is called parallel
folding, and is only one of the possible fold styles. Deformation can change the thick-
nesses, especially the thicknesses of thick soft units between stiffer units. The direc-
tion of constant thickness is an element of the fold style. A similar fold maintains
constant thickness parallel to the axial surfaces. Constant vertical thickness projec-
tion (Handley 1954; Banks 1993) is strictly appropriate only for similar folds that have
vertical axial surfaces (Fig. 3.21b). The resulting thinning on steep limbs is a common
feature of compressional folds, even in those that maintain constant bed thickness
elsewhere. If the axial surfaces of a similar fold are inclined, the direction of constant
thickness is inclined to the vertical (Fig. 3.21c). Projection of surfaces is probably
best restricted to situations in which bed thicknesses are approximately constant
(Fig. 3.21a).
Projection of thickness is based on the further assumption that the stratigraphy
between the projection point and the composite surface is an unbroken sequence of
uniform dip. If the vertical line of projection crosses a fault (Fig. 3.22a) or an axial
surface (Fig. 3.22b), then the projection will be incorrect. The shorter the projection
distance, the less likely these problems are to occur.
The value of a composite-surface map in structural interpretation is shown by the
composite surface of the Mtfp (Fig. 3.23) in the Blount Springs map area. The pro-
jected points allow a structure contour map to be constructed for the top of a centrally
located stratigraphic horizon (top Mtfp). This map was produced by first interpolating
the dip values between control points and then projecting all the points on each con-
tact to the top of the Mtfp. Projecting using very steep dips provided some unrealistic
results because the long vertical projection distances cross gently dipping axial sur-
faces, invalidating the result as in Fig. 3.22b. The obviously incorrectly projected points
have been removed. The square points in Fig. 3.23a are the locations of dip measure-
ments and approximately indicate the limits of control. The complex structure north-
west of the anticline and the flat surface to the southeast are artifacts of the contouring
process (kriging) and do not represent real structure.
