Geology Reference
In-Depth Information
Fig. 7.33.
Fault-cut maps with stratigraphic separations.
a
Data: elevation of fault cut, amount of strati-
graphic separation (fault cut).
b
Structure contour map on the fault surface; amount of fault cut is
posted. The contour interval is 500 units, and negative contours are below sea level. The fault cut (
open
circle
) does not correlate with the contoured fault
Fig. 7.34.
Stratigraphic separation dia-
gram representative of rela-
tionships seen along the strike
direction of a thrust fault.
FW
curve:
Stratigraphic position
of the thrust in the footwall;
HW curve:
stratigraphic posi-
tion of the fault in the hang-
ingwall.
a
Single unbroken
thrust sheet.
b
Thrust sheet
broken by a lateral ramp
Another method for displaying the variation of the stratigraphic separation along
a fault is by means of a stratigraphic separation diagram (Fig. 7.35; Elliott and Johnson
1980; Woodward 1987). The vertical axis of the diagram is the scaled stratigraphic col-
umn and the horizontal axis is the distance along the fault. The curves on the diagram
show the stratigraphic unit that the fault is in at any particular point. A thrust fault
usually places older over younger units and so the footwall block should be the upper
curve (Fig. 7.35). The opposite is true for a normal fault. For example, line A (Fig. 7.35a)
represents a particular geographic point on the fault, one at which the footwall is at the
base of unit 4 and the hangingwall is in unit 9. If the complete fault has been observed,
the hangingwall and footwall curves will join at the fault tips. Both hangingwall and
footwall curves will be smooth if they are not broken by oblique faults. An oblique
structure will produce a rapid change in the stratigraphic level of the curve of the
block that contains the feature (Fig. 7.35b). If the rapid change is in the footwall, it is
likely to have been caused by a lateral ramp. If the oblique structure is in the hangingwall,
it is likely to be a tear fault that subdivides only the hangingwall. If the feature is present
