Relative Dating and Sequencing
•
•
•
•
Relative vs. Absolute Time
Laws
Unconformities
Correlating Geologic Cross Sections
Relative Time:
• Ages of events are placed in
order of occurrence.
• No exact date is identified.
– Ex. WWI and WWII
– I am the second child in my family.
– Mr. Williams is mad old, yo
Absolute Time:
• identifies the exact date of an
event.
– Ex. 65 Million Years Ago
– 1990
Nicholas Steno
• First Things First…or…
“How’d that get there?”
• In the 17th C., Nicolas Steno made an important
observation:
"Sediments are usually deposited in horizontal
layers."
He called this
“ORIGINAL HORIZONTALITY”
Law of Superposition
1. The Law of Superposition - in a
sequence of undisturbed
sedimentary rocks, the oldest
rocks will be at the bottom.
SuperpositionSuperpositionyoungest
youngest
to
to
oldest
oldest
Grand Canyon
2. The law of cross-cutting
relationships – anything that
cuts across layers of rock is
younger than the rocks that it
has intruded into. This applies to
faults and igneous intrusions.
Cross Cutting
3. The law of included fragments
- the fragments that make up a
rock are older than the entire
sample.
The Law of Folds or Tilts: tilts in
rocks are younger than the rocks
themselves.
5. The Law of Original
Horizontality- Rocks are usually
deposited flat and level.
4.
6.
Cole’s LawThinly sliced cabbage.
Folds/Tilts
More Folds/Tilts
Other Guidelines for figuring out a
sequence:
• Sedimentary rocks are usually
formed under water.
• Weathering and erosion usually
happen above water (on dry
land).
Contact Metamorphism!
• Contact metamorphism shows
that the rock that was changed
was there first when the
intrusion happened.
Time for a worksheet
• Sequence 1 & 2
Sequence 1: Uplift & Erosion
1.
2.
3.
4.
5.
Limestone deposited
Sandstone deposited
Shale Deposited
Uplift
Erosion
Sequence 2: Faulting
1. Limestone
deposited
2. Sandstone
deposited
3. Shale deposited
4. Faulting
Time for another worksheet
• Sequence 3 & 4
Sequence 3: Folding
1. Limestone deposited
2. Sandstone
deposited
3. Shale deposited
4. Folding
Sequence 4: Igneous Intrusion
1. Limestone
deposited
2. Sandstone
deposited
3. Shale deposited
4. Igneous Intrusion
Note: contact
metamorphism is
the same event as
the intrusion.
Formation of an unconformity
• An unconformity is an old buried surface that was eroded
• Over time, new rock layers were deposited on it
• Sometimes layers of rock are missing
• There is a break or gap of geologic time not represented
by the layers in an area. The gap represents an
unknown length of time
• No way of knowing exactly what happened but we do
know UPLIFT exposed rocks to weathering and erosion.
• Rocks above unconformity are younger – rocks below
older
Upper Silurian
Carbonates
Tilted
Ordovician
Shales and
Sandstones
unconformity
Taconic Unconformity
How is an unconformity formed?
1. Uplift – area of crust uplifted above sea
level (deposition – under water)
2. Erosion – some time after
3. Submergence (subsidence) below sea
level
4. Deposition – new sediments deposited
on top of the buried eroded surface
Layers are formed according to superposition.
Something happens to uplift the area
folding
faulting, etc.
Erosion wears away the uppermost layers
Area submerges and deposition begins again.
Here’s the
unconformity