- SUPERPOSITION:
Newer sediments are deposited on top of older sediments. The newer ones are always on top.
This principle is called the Law of Superposition.
When the sediments are compressed into rocks, the oldest rocks are on the bottom and the newer ones on top.
- Relative dating of sedimentary rocks:
The principle of superposition allows us to put local rock strata into a relative sequence.
This is called relative dating; it allows us to determine what is older and what is newer, but not to assign any numerical age in years.
- Additional principles used in relative dating:
a. Any igneous intrusion, or any fault, that cuts across sedimentary layers is younger (more recent) than the layers it cuts across.
b. Any folding or tilting of sedimentary layers happened after (more recently than) the sedimentary layers affected.
c. An unconformity represents a missing time period (no deposition, but usually including erosion) that occurred after the layers beneath were deposited.
d. Contact metamorphism adjacent to a hot igneous body can only occur to rock formations that are older than the igneous body.
THESE ADDITIONAL PRINCIPLES can then be used to insert igneous rocks into a time sequence that includes sedimentary rocks as well.
If the igneous rocks are then
dated by radioactive dating, it establishes absolute dates (in years) that can pin down the timing of a sedimentary sequence.
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