# Relative Dating and Sequencing Relative vs. Absolute Time Laws Unconformities Correlating Geologic Cross Sections.

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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 17 th 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.

Superposition- Superposition- youngest to oldest Superposition- Superposition- youngest to 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.

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 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. 1.Limestone deposited 2. 2.Sandstone deposited 3. 3.Shale Deposited 4. 4.Uplift 5. 5.Erosion

Sequence 2: Faulting 1. 1.Limestone deposited 2. 2.Sandstone deposited 3. 3.Shale deposited 4. 4.Faulting

Time for another worksheet Sequence 3 & 4

Sequence 3: Folding 1. 1.Limestone deposited 2. 2.Sandstone deposited 3. 3.Shale deposited 4. 4.Folding

Sequence 4: Igneous Intrusion 1. 1.Limestone deposited 2. 2.Sandstone deposited 3. 3.Shale deposited 4. 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 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 area folding faulting, etc.

Erosion wears away the uppermost layers

Area submerges and deposition begins again. Here’s the unconformity

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