Relative Dating and Sequencing

• Relative vs. Absolute Time

• Laws

• Unconformities

• Correlating Geologic Cross Sections

Relative Time: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: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 SuperpositionLaw of Superposition

1. The Law of Superposition - in a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom.

Superposition- youngest

to

oldest

Superposition- youngest

to

oldest

Grand Canyon

2. 2. The law of cross-cutting relationships The law of cross-cutting relationships – – anything that cuts across layers of anything that cuts across layers of

rock is younger than the rocks that it rock is younger than the rocks that it has intruded into. This applies to has intruded into. This applies to

faults and igneous intrusions. faults and igneous intrusions.

Cross Cutting

3. 3. The law of included fragments The law of included fragments - - the fragments that make up a rock the fragments that make up a rock are older than the entire sample.are older than the entire sample.

4. 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.

6. Cole’s Law-

Thinly sliced cabbage.

Folds/Tilts

More Folds/Tilts

Other Guidelines for figuring out a Other Guidelines for figuring out a sequence:sequence:

• Sedimentary rocks are usually formed under water.

• Weathering and erosion usually happen above water (on dry land).

Contact Metamorphism!Contact Metamorphism!

• Contact metamorphism shows that the rock that was changed was there first when the intrusion happened.

Time for a worksheetTime for a worksheet

• Sequence 1 & 2

Sequence 1: Uplift & ErosionSequence 1: Uplift & Erosion

1. Limestone deposited

2. Sandstone deposited

3. Shale Deposited

4. Uplift

5. Erosion

Sequence 2: FaultingSequence 2: Faulting

1. Limestone deposited

2. Sandstone deposited

3. Shale deposited

4. Faulting

Time for another worksheetTime for another worksheet

• Sequence 3 & 4

Sequence 3: FoldingSequence 3: Folding

1. Limestone deposited

2. Sandstone deposited

3. Shale deposited

4. Folding

Sequence 4: Igneous IntrusionSequence 4: Igneous Intrusion

1. Limestone deposited

2. Sandstone deposited

3. Shale deposited4. Igneous Intrusion

Note: contact metamorphism is the same event as the intrusion.

Formation of an unconformityFormation 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

Taconic Unconformity

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 areafoldingfaulting, etc.

Erosion wears away the uppermost layers

Area submerges and deposition begins again.

Here’s the unconformity