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

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

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 strataDeposition 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

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

Todays 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!