Geology Reference
In-Depth Information
Fig. 5.28.
Alternative interpretation of geological data in Fig. 5.27a.
a
Tangent diagram of bedding atti-
tudes.
Open circle
is the 34, 212 point. Plunge of the fold is 30S.
b
Geological map of syncline with a
coaxial minor fold on the limb. (After Stockwell 1950)
Potential interpretation problems associated with minor folds are illustrated by the
map in Fig. 5.27a. The observed contact locations and bedding attitudes could be ex-
plained by the maps in either Fig. 5.27b or 5.27c. The shape of the first-order fold honors
the 34SW dip in Fig. 5.27b and ignores it in Fig. 5.27c. The dip oblique to the contact
could be justified as being either a cross bed or belonging to a minor fold.
Plotting the data from Fig. 5.27 on a tangent diagram (Fig. 5.28a) shows that all the
points, including the questionable point (34, 212), fall on the same line, indicating a
cylindrical fold plunging 30° due south. This result leads to rejection of the cross-bed
hypothesis and indicates that the oblique bedding attitude is coaxial with the map-
scale syncline. If the map of Fig. 5.27c is supported by the contact locations, then the
structure has the form given by Fig. 5.28b.
5.7
Growth Folds
A growth fold develops during the deposition of sediments (Fig. 5.29a). The growth
history can be quantified using an expansion index diagram, where the expansion
index,
E
, is
E
=
t
d
/
t
u
,
(5.13)
with
t
d
= the downthrown thickness (off structure) and
t
u
= the upthrown thickness (on
the fold crest). The thicknesses should be measured perpendicular to bedding so as not
to confuse dip changes with thickness changes (Fig. 5.29a). The expansion index given
here is the same as for growth faults (Thorsen 1963; Sect. 7.6). Different but related equa-
tions for folds have been given previously by Johnson and Bredeson (1971) and Brewer
