geologic time, unconformities and relative dating uniformitarianism appendix a
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Geologic Time, Unconformities
and Relative Dating
Uniformitarianism
Appendix A
Law of Original Horizontality, Law of Superposition
• All sedimentary rocks are deposited in an original, horizontal position (gravity)
• When undisturbed: Oldest rocks deposited first on bottom, youngest on top
Youngest
Oldest
Middle
Cross Cutting Relations – 2 Types
• Whatever is doing the “cutting” is younger: Fault or intrusion
Cross Cutting Relations: Faults
Cross Cutting Relations: Faults• Faulting: An offset in rocks caused
by tectonic motions, mountain building, etc.
• Normal Fault: Tension– Headwall moves DOWN, footwall moves
UP
• Reverse Fault: Compression– Headwall moves UP, footwall moves down
• Thrust Fault: Low angle reverse fault
Cross-Cutting Relationships: Igneous Intrusions
• NOTE: Contact Metamorphism
Unconformities – 3 Types• Unconformity – “missing time” in the rock
record
• A boundary (“Squiggly line”) separating two types of rocks; erosion or nondeposition
Erosion,
time passes
Original strata Deposition along erosional surface
Nonconformity• Boundary separates younger sedimentary
rocks, from much older, underlying igneous or metamorphic rocks.
• Contact Metamorphism
Sed
Ig/Met
Angular Unconformity
• The older strata below has been folded or tilted (usually steeper angle) due to tectonic motion
Sed
Sed
Disconformity• The surface of “missing time” is parallel to
bedding in two sets of sed rocks
• Hardest unconformity to recognize in the field – how do you tell the surface is erosional and there really is missing time?
Sed
Sed
Today’s lab
• Pages 34 to 42
• Complete Figures 2.11 to 2.22– Which event came first, second, etc– List type of unconformity (angular,
nonconformity, disconformity) and type of fault (normal or reverse)
• Remember: FOLDS, Unconformities, intrusions and deformation are events!
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