Geology Reference
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Fig. 1.32. Vertical cross section showing the relative fault displacement terminology with horizontal as
the reference plane
Fig. 1.33.
Vertical cross section showing
the relative fault displacement
terminology with bedding as
the reference plane
separation fault. Using the horizontal as the plane of reference (i.e., originally horizon-
tal bedding), a normal-separation fault extends a line parallel to bedding and a re-
verse-separation fault shortens the line.
Using bedding as the frame of reference is not the same as using a horizontal plane,
as illustrated by Fig. 1.33 which shows the faults from Fig. 1.32 after a 90° rotation. With
bedding vertical, a reverse displacement (Fig. 1.33) extends the bedding while short-
ening a horizontal line. The fault might have been caused by reverse slip on a fault
formed after the beds were rotated to vertical or by the rotation of a normal fault. Using
bedding as the frame of reference (Norris 1958), an extension fault extends the bed-
ding, regardless of the dip of bedding, and a contraction fault shortens the bedding.
A fault cut is the point at which a well crosses a fault. A fault with a component of
dip separation has the effect of omitting or repeating stratigraphy across the fault at
the fault cut (Fig. 1.34). With respect to a vertical line or a vertical well, a normal fault
causes the omission of stratigraphic units (Fig. 1.34a) and a reverse fault causes the
repetition of units (Fig. 1.34b). Opposite-sense omissions or repetitions may occur in
a well that is not vertical (Mulvany 1992). For example, a well drilled from the footwall
to the hangingwall of a normal fault will show repeated section down the well (Fig. 1.34a)
and will show missing section down the well if the fault is reverse (Fig. 1.34b).
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