Unconformities

Warm-up: Put the following rock layers
in order from OLDEST to YOUNGEST
Notes: Steno’s Laws
• Essential Question: How can you tell the
relative age of geologic events (faults,
erosion) and rocks from a cross section?
Which is older, 1 or 6?
How do you know?
• Sedimentary rocks stack from oldest on
bottom to youngest on top
• First look how each layer is stacked from
bottom to top (oldest to youngest)
• The deeper a layer is, the older it is
• The bottom rock is usually the oldest
• In sedimentary rocks the oldest layers are at
the bottom
1. Law of Superposition
• This rule is called the Law of Superposition:
• In an undeformed sequence of sedimentary
rocks, each bed is older than the one above it
and younger than the one below.
Did the layers underneath A form at
that angle?
How do you know?
2. The Law of Original Horizontality
• This rule is called the Law of Original
Horizontality:
• Layers of sediment are generally deposited in
a horizontal position.
If we observe rock
layers that are flat, it
means they have not
been disturbed and
thus still have their
original horizontality.
2. The Law of Original Horizontality
However, if rock layers
are folded or inclined at
a steep angle, they must
have been moved into
that position by crustal
disturbances sometime
after their deposition.
Which is older, 3 or 1? Which is older,
4 or B?
How do you know?
• If the rock cuts through the rest of the rocks
straight up then it is younger.
• If a rock has a fault, unconformity, or igneous
going THROUGH it, then that layer is older
than the fault, etc.
• Faults cannot split rocks in half unless the
rocks are already there.
• If it overlaps more than one layer, count that
layer 1st before the layers it overlapped
3. The Law of Cross-Cutting
Relationships
• This Rule is called the Law of Cross-cutting
Relationships:
When a fault cuts
through older rocks, or
when magma intrudes
and crystallizes, we can
assume that the fault or
intrusion is younger than
the rocks affected.
In other words, a
younger feature cuts
through an older feature.
Which is older, 2 or 6?
How do you know?
• Any layers above an unconformity are most
likely younger than layers below
• Look for unconformity lines: any layers that
are on top have been erased
• Look for any igneous rocks: where the igneous
rock stops is how old it is
Line A is called an unconformity
An unconformity represents a long period
during which deposition ceased, erosion
removed previously formed rocks, and then
deposition resumed.
In other words, it’s a break in the rock record.
There are three basic types of unconformities:
Angular Unconformities
Disconformities
Nonconformities
What happened here?
Unconformities
Angular
Unconformities
An angular unconformity consists
of tilted or folded sedimentary
rocks that are overlain by
younger, more flat lying strata.
An angular unconformity
indicates that during the pause in
deposition, a period of
deformation (folding or tilting)
and erosion occurred.
• https://www.classzone.com/books/earth_scien
ce/terc/content/visualizations/es2902/es2902
page01.cfm?chapter_no=visualization
What
happened
here?
Unconformities
Disconformities
In a disconformity, the strata on either side are essentially
parallel.
Unconformities
Disconformities
In a disconformity, the strata on either side are essentially
parallel.
What happened here?
Unconformities
Nonconformities
In a nonconformity, the
break separates older
metamorphic or intrusive
igneous rocks from
younger sedimentary
strata.
Here, younger strata rest
upon older, metamorphic
or igneous rocks.
Unconformities
Nonconformities
In a nonconformity, the
break separates older
metamorphic or intrusive
igneous rocks from
younger sedimentary
strata.
Here, younger strata rest
upon older, metamorphic
or igneous rocks.
Unconformities
The three basic
types of
unconformities
illustrated in the
Grand Canyon.
Put it all together: What happened here?
Figure 11.10 (bottom)
angular unconformity
disconformity
nonconformity