Geology Reference
In-Depth Information
7.2.6
Rock Type
Rock types diagnostic of faulting (Fig. 7.11) include the cataclasite suite, produced pri-
marily by fragmentation, and the mylonite suite, produced by large crystal-plastic strains
and/or recrystallization (Sibson 1977; Ramsay and Huber 1987). A cataclastic fault sur-
face, or slickenside, is usually grooved, scratched, or streaked by mineral fibers or may
be highly polished. The parallel striations or mineral elongation directions are slicken-
lines which indicate the slip direction at some stage in the movement history. A mylonite
typically has a thinly laminated compositional foliation and a strong penetrative min-
eral lineation which is elongated in the displacement direction. Soft-sediment fault
zones typically have macroscopic textures similar to those of mylonite zones, although
no metamorphism has occurred. At the microscopic scale, the grains in a soft-sedi-
ment fault zone are usually undeformed.
A fault zone may be recognizable in well logs. An uncemented cataclastic fault zone
is mechanically weak and very friable. This is the reason why an outcropping fault zone
may erode to a valley. Where a well encounters a mechanically weak unit, such as an
uncemented fault zone, an enlargement of the well-bore diameter occurs. A caliper log
is produced by a tool that measures the size of the well bore and a fault may be re-
corded as an expansion in the size of the borehole, generally short in length. Well
enlargement may occur in any mechanically weak unit such as coal, uncemented clay
or sand, salt, etc. If the normal stratigraphic reasons for well enlargement can be elimi-
nated, however, then the presence of a fault zone is a good possibility. A high-resolu-
tion dipmeter provides a log that resembles a photograph of the wall of the borehole.
A fault zone may be inferred from the same observational criteria that would apply to
Fig. 7.11.
Representative fault-rock textures. The scale is approximate.
a
Cataclasite.
b
Mylonite. Drawn
from thin section photographs in Ramsay and Huber (1987)
