Presentation on theme: "Relative Dating and Sequencing Relative vs. Absolute Time Laws Unconformities Correlating Geologic Cross Sections."— Presentation transcript:
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
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.
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.
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.
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
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