Geology Reference
In-Depth Information
a tangent diagram. A preliminary plot of the data indicated that 83° dip attitude is
inconsistent with the rest of the data and exerts too much control on the result, as
mentioned above. Removing the 83° point results in a smooth best-fit curve that indi-
cates a nonplunging fold with a slightly conical curvature, opening to the southwest
and having a crestal trend of 0, 226 (Fig. 5.8b). This agrees with the geometry of the
composite structure contour map (Fig. 5.8a).
5.2.5
Crest and Trough on a Map
A consistent definition of the fold trend, applicable to both cylindrical and conical
folds, is the orientation of the crest or trough line (Fig. 5.9). In both cylindrical and
conical folds (Figs. 5.3, 5.4) the crest line is a line on a folded surface along the struc-
turally highest points (Dennis 1967). The trough line is the trace of the structurally
lowest line. In cross section, the crest and trough traces are the loci of points where the
apparent dip changes direction. In cylindrical folds the crest and trough lines are parallel
to the fold axis and to each other, but in conical folds the crest and trough lines are not
parallel. Crest and trough surfaces connect the crest and trough lines on successive
horizons. The trace of a crest or trough surface is the line of intersection of the surface
with some other surface such as the ground surface or the plane of a cross section. The
crests and troughs of folds are of great practical importance because they are the
positions of structural traps. Light fluids like most natural hydrocarbons will migrate
toward the crests and heavy liquids will migrate toward the troughs.
The U-shaped trace of bedding made by the intersection of a plunging fold with a
gently dipping surface, such as the surface of the earth, is called a fold nose. Originally
a nose referred only to a plunging anticline (Dennis 1967; Bates and Jackson 1987) with
a chute being the corresponding feature of a plunging syncline (Dennis 1967). Today,
common usage refers to both synclinal and anticlinal fold noses. The term nose is also
applied to the anticlinal or synclinal bend of structure contours on a single horizon
(Fig. 5.9). The dip of bedding at the crest or trough line is the plunge of the line. The
Fig. 5.9.
Crest (
C
) and trough (
T
) lines on a structure contour map.
a
Plan view.
Arrows
point in the
down-plunge direction.
b
3-D oblique view to NE
