Laboratory Manual for Introductory Geology 4e

12.2.5 Unconformities: Evidence for a Gap
in the Geologic Record
When rocks are deposited continuously in a basin without interruption by tectonic activity,
uplift, or erosion, the result is a stack of parallel beds. Beds in a continuous sequence
are said to be conformable because each conforms to, or has the same shape and orientation
as, the others, as in the photograph of the Grand Canyon in Figure 12.1a.
Tilting, folding, and uplift leading to erosion interrupt this simple history and
break the continuity of deposition. In these cases, erosion may remove large parts
of the rock record, leaving a gap in an area’s history. We may recognize that deposition
was interrupted or that erosion took place, but we can’t always tell how long the
interruption lasted—it could have been a million, a hundred million, or a billion
years—or what happened during that time. As we noted in Section 10.3.4, a contact
indicating a gap in the geologic record is called an unconformity because the layer
above it was not deposited conformably and continuously on those below.
There are three kinds of unconformities. An angular unconformity forms
when rocks are tilted or folded, eroded, and covered by younger horizontal strata
(FIG. 12.6a). The layers above the angular unconformity are at an angle to the
older folded or tilted beds below. A nonconformity forms when a pluton intrudes
host rock and crystallizes, and erosion then removes the host rock and part of the
pluton (FIG. 12.6b). Later deposits of sedimentary rock sit directly on the eroded
intrusive rock; the absence of contact metamorphism in this instance shows that
the pluton had cooled before the sedimentary rock was deposited. A disconformity
can be the subtlest type of unconformity because the strata above and below may
be parallel. Disconformities form when a sequence of sedimentary rock is deposited
and remains horizontal, undergoes erosion if sea level drops, and is later covered
by new sedimentary rock when the area is once again submerged (FIG. 12.6c, d).
FIGURE 12.6 How the three kinds of unconformities form.
Mountains
1
Mountains form and layers
fold, then erosion removes
the highland.
1
Erosion removes cover, so
crystalline rock lies exposed
at the Earth’s surface.
Future erosion surface
Future erosion surface
Erosion
surface
2
Time
Erosion
surface
2
Time
3
3
New, horizontal
layers
Angular unconformity
Nonconformity
Old, folded
layers
(a) An angular unconformity: (1) layers undergo folding; (2) erosion
produces a flat surface; (3) sea level rises and new layers of sediment
accumulate.
(b) A nonconformity: (1) a pluton intrudes; (2) erosion cuts down
into the crystalline rock; (3) new sedimentary layers accumulate
above the erosion surface.
(continued)
12.2 PHYSICAL CRITERIA FOR DETERMINING RELATIVE AGE
307