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
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.
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