Geology Reference
In-Depth Information
Fig. 11.45.
Unconformity flattening pitfall.
a
Cross section of an undeformed unconformity and over-
lying valley fill.
b
Unconformity restored to horizontal showing an anticline that never existed
fault displacements in Fig. 11.44a must still be removed by a subsequent restoration.
Based on the principles of simplicity and structural reasonableness, the restoration
using active faults is a better representation of the geology at the time that the datum
was horizontal.
Unconformities are not usually horizontal when they form, making the restoration of
an unconformity a potential flattening pitfall (Calvert 1974). Flattening an unconformity
to horizontal converts the original topography on an unconformity surface (Fig. 11.45a)
into an anticline (Fig. 11.45b). The anticline indicated by the restoration was never
present. An analogous problem occurs if the thickness variations of the unit directly
above an unconformity are interpreted as representing a paleostructure when they, in
fact, represent paleotopography. Interpretation of thickness variations in the shale
overlying the unconformity in Fig. 11.45a as a paleostructure would lead to the inter-
pretation of a growth syncline and the adjacent thinner areas as being growth anti-
clines, an incorrect interpretation of the true geometry in this example.
11.7.1.3
Oblique Simple Shear
The vertical simple shear concept can be generalized to shear in any direction (White
et al. 1986). An oblique simple-shear restoration follows the same procedure as the
vertical simple-shear restoration, except that the measurement lines are inclined to the
regional at an angle other than 90°. The oblique lengths measured on the deformed-
state cross section (Fig. 11.46a) are restored by translation in the shear direction to
return the reference horizon to the regional (Fig. 11.46b). The spacing between any
two measurement lines in the direction of the regional (
S
), and the shear angle (
),
remains constant from the deformed state to the restored cross sections in both verti-
cal and oblique simple-shear restorations.
The procedure for restoring a faulted section by oblique simple shear is analogous
to the procedure for vertical simple shear except that the shear angle is less than 90°.
The shear angle is measured downward from the regional. The fault displacement
parallel to the regional is constant and equal to the hangingwall block displacement,
D
.
α
