Geology Reference
In-Depth Information
A stratigraphic template (Woodward et al. 1989) greatly speeds up the reconstruc-
tion process for beds having uniform thickness or a constant thickness gradient. The
template is a series of lines representing the restored bed geometry. For constant bed
thickness the lines are parallel. A constant bed-length restoration can then be done by
measuring bed lengths and marking the appropriate lengths on the template. This saves
the time of drawing each bed segment separately. Regional thickness gradients can be
shown on the template by diverging lines.
Units that have primary stratigraphic thicknesses changes may be restored by a
modification of the constant line-length method. Begin with the uppermost unit that
is to be restored, the top of which is the reference horizon. The thickness of this unit
is measured at multiple points along the layer (Fig. 11.34a). The positions of the thick-
ness measurements are recorded with respect to the pin line on the top of the unit. The
top of the unit is restored and the distances of the thickness measurement points from
the pin line are marked. The thicknesses are marked on the restoration in the direction
perpendicular to the upper horizon (Fig. 11.34b). The lower horizon is drawn to main-
tain its original length but it may “slip” laterally past the points where the thickness
was measured. The restoration preserves the lengths between segments on the base of
the unit but not necessarily their positions. The total line length is preserved on the top
and base. For a section containing faults (Fig. 11.34c,d), thickness measurements are
taken at fault cutoffs and restored with respect to the upper cutoff if possible, other-
wise with respect to the lower cutoff. Each stratigraphic unit on the cross section is
measured and restored separately to the base of the unit above it until the restoration
is complete.
Difficulties may be encountered in piecing the section back together across faults.
The problems are usually related to uncertainty in the correct restoration of the fault
shape. The best solution is to subdivide the stratigraphy into thinner units and then
restore the thinner units as just described. In the limit, each unit being restored is a
single line of variable thickness that is stretched out and placed along side the unit
above it.
Fig. 11.34.
Bed-length restoration of variable thickness units (after Brewer and Groshong 1993).
Dots
represent points at which original thickness measurements are made.
W
t
: segment width at top;
W
b
: segment width at base.
a
Deformed state fold.
b
Restored fold.
c
Deformed-state faulted fold.
d
Restored faulted fold
