geologic time, unconformities and relative dating uniformitarianism appendix a

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

Principle of Components

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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!