Geology Reference
In-Depth Information
Faults may die out laterally or vertically into folds. During interpretation of the fault
location, wells should be examined for evidence of continuity across the proposed fault
trace. An unfaulted well (or outcrop) at an intermediate elevation and between the
upthrown and downthrown blocks would be evidence for a fold. No such evidence is
found for the fault in Figs. 8.7or 8.8, hence it is mapped as extending across the entire
stratigraphic section, e.g., across the top of the upper map horizon (Fig. 8.8) originally
mapped as being continuous (Fig. 8.6a).
8.3.2
Joining Offset Marker Surfaces to a Fault
The final step in constructing a complete structure contour map of a faulted surface is
to extend the marker surfaces until they join the fault. The basic principle is that the
offset marker must join the fault along cutoff lines where the elevations exactly match
those of the fault as exemplified by the faulted surfaces in Fig. 8.1. Rarely are there
enough data close to the fault plane to eliminate the need for interpretation (Fig. 8.9).
The data gaps adjacent to the faults in Fig. 8.6c are typical.
The marker-surface geometries on opposite sides of the fault may or may not be
directly related, depending on the nature of the fault. If the fault is curved or if the
structures on opposite sides of the fault developed independently using the fault as
a displacement discontinuity, then the marker surfaces may be completely unrelated
on opposite sides of the fault (Fig. 8.10a,b) and the two sides should be contoured
Fig. 8.9.
Cross section showing typical subsurface data available for contouring a marker horizon across
a fault.
Thin dotted lines
show area of uncertain marker bed location.
Thick dashed line
shows area of
uncertain fault location
Fig. 8.10.
Effect of fault curvature on the relationship between hangingwall and footwall bed geometry.
a
Listric fault before displacement.
b
Listric fault after displacement: the hangingwall shape is changed.
c
Planar fault before displacement.
d
Planar fault after displacement: the hangingwall shape is unchanged
