Geology Reference
In-Depth Information
Two other principles are also used in modern stratigraphy:
Principle of fossil succession:
Fossils
(the remains of previous organisms) occur in
a determinable order, and any time period can be recognized by the fossils
present in rocks formed during that time. This principle applies only to sediment-
ary rocks because they are the only ones that can contain fossils.
Principle of inclusions:
If a rock contains pieces of a different rock (called
inclu-
sions
), the pieces must be from an older rock. This principle is especially useful
for sorting out a sequence of events that includes igneous or metamorphic rocks,
along with sedimentary ones.
Losing time in the layers
Together, the six principles of stratigraphy provide scientists the ground rules for put-
ting rock layers in their correct order for interpretation of the past. However, each prin-
ciple is based on the assumption that the rocks you examine have an undisturbed and
complete record — that the sequence of rock layers is
conformable.
Very often, pages of time are missing in the geologic record. These breaks in the rock re-
cord are called
unconformities.
An unconformity occurs when rock layers have been
eroded, moved, or otherwise changed, and then more rock layers are added above. This
process results in a period of time for which no rock record exists.
Fortunately, when periods of time are missing from the rock layers, certain clues signal
this fact to the stratigrapher. There are three different types of unconformities, and each
offers its own clues:
Angular unconformity:
This unconformity occurs when rock layers are tilted at an
angle by uplift, faulting, or folding (see Chapter 9); the layers are eroded; and then
new horizontal layers are created above them. This process is illustrated in Figure
16-1.
Disconformity:
A disconformity is the most difficult unconformity to recognize.
This is because both layers are sedimentary rock, and both are still horizontal, but
they are separated by a surface that was eroded, removing part of the rock record.
The older rock layers are still positioned horizontally when the new rock layers
form above them, as illustrated in Figure 16-2. A stratigrapher may not recognize
the disconformity in the rock sequence until he or she has applied other strati-
graphic principles (such as the principle of fossil succession) that indicate time is
missing.
