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Figure 17.8
Formation of a Disconformity
Deposition
Disconformity
Uplift and erosion
Jurassic rocks
Mississippian rocks
Deposition
b
Disconformity between Mississippian and Jurassic strata in
Montana. The geologist at the upper left is sitting on Jurassic
strata, and his right foot is resting on Mississippian rocks. This
disconformity represent approximately 165 million years.
a
Formation of a disconformity.
cross-cutting relationships, must be younger than all of the
rocks that it intrudes (Figure 17.12e).
The entire area was then uplifted and eroded; next, beds P
and Q were deposited, producing a disconformity (N) between
beds L and P and a nonconformity (O) between the igneous
intrusion M and the sedimentary bed P (Figure 17.12f, g). We
know that the relationship between igneous intrusion M and
the overlying sedimentary bed P is a nonconformity because
of the inclusions of M in P (principle of inclusions).
At this point, there are several possibilities for recon-
structing the geologic history of this area. According to
the principle of cross-cutting relationships, dike R must
be younger than bed Q because it intrudes into it. It could
have intruded anytime
after
bed Q was deposited; however,
we cannot determine whether R was formed right after Q,
right after S, or after T was formed. For purposes of this
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