Geology Reference
In-Depth Information
Properties of Faults
7.1
Introduction
This chapter focuses on the recognition of faults, their geometrical properties, displace-
ment distributions, and the correlation of multiple fault cuts into continuous fault
surfaces. Unconformities are treated here because, by truncating units at the map scale,
they share characteristics with faults and need to be distinguished from faults.
7.2
Recognition of Faults
Faults are recognized where they cause discontinuities in the traces of marker horizons
on maps and cross sections, discontinuities in the stratigraphic sequence, and anomalies
in the thicknesses. Faults may also be recognized from the diagnostic shape of drag folds
adjacent to the fault and by the distinctive rock types caused by faulting.
7.2.1
Discontinuities in Geological Map Pattern
At the map scale, a fault is inferred where it causes a break in the continuity of the units
on a geologic map. A time slice through a 3-D seismic-reflection volume is similar to a
geologic map in a region of low topographic relief and will also show faults as discontinu-
ities. As an example, the coal seams in the north half of Fig. 7.1 are abruptly truncated
along strike where they intersect faults (points A). Fault dips and hence fault separations
(Sect. 7.4.2) cannot be determined from the near-horizontal map surface, but these are
known to be normal-separation faults. The truncation of units by a contact (B, Fig. 7.1)
indicates that the contact is a fault or that the beds are cut by an unconformity. The base
of the continuous bed that crosses the truncated beds (C, Fig. 7.1) is either a fault contact or
an unconformity. The contact indicated by B and C in Fig. 7.1 is a fault because the trun-
cated beds to the north (bc, ml) are younger than the cross-cutting unit to the south (by)
and so should be above the unconformity, not below it. At D, E, and F, the contacts are parallel
and so provide no direct evidence of faulting, although the absence of stratigraphic units
at the contacts suggests the presence of faults. The contact at D can be traced into a loca-
tion (C) where it is faulted; hence it is probably a fault. The contact at F is a reverse fault
because the dips of bedding on both sides of the contact show that the older units to the
south lie on top of the younger units to the north. The contact indicated by E places upright
lower Cambrian rocks adjacent to overturned younger units to the north, suggesting re-
