Geology Reference
In-Depth Information
tary rocks tend to maintain relatively constant bed thickness, although the thickness
changes that do occur can be very important. If large thickness changes are observed
in deformed sedimentary rocks other than evaporites or overpressured shale, primary
stratigraphic variations should be considered as a strong possibility.
In cross section, folds may be harmonic, with all the layers nearly parallel to one
another (Fig. 1.14), or disharmonic, with significant changes in the geometry between
different units in the plane of the section (Figs. 1.13, 1.15). The fold geometry is con-
trolled by the thickest and stiffest layers (or multilayers) called the dominant members
(Currie et al. 1962). A stratigraphic interval characterized by a dominant (geometry-
controlling) member between two boundary zones is a structural-lithic unit (Fig. 1.15;
Fig. 1.14.
Dip-domain style folds in an
experimental model having
a closely spaced multilayer
stratigraphy. The
black
and
white
layers
are plasticene
and are separated by grease
to facilitate layer-parallel slip.
The
black layers
are slightly
stiffer than the
white layers
.
(After Ghosh 1968)
Fig. 1.15.
Structural-lithic units. (After Currie et al. 1962)
