Geologic Time Earth’s History

Determining Earth’s History Must determine sequence of events Must determine sequence of events To establish chronological framework use: To establish chronological framework use: Relative Age Relative Age Absolute Age Absolute Age

Dating – 2 types Relative DatingRelative Dating Absolute Dating – Place rocks and events in sequence of formation – Specify actual number of years that have passed since the event occurred

Principles of Relative Dating Law of Superposition—StenoLaw of Superposition—Steno Principle of Original Horizontality –StenoPrinciple of Original Horizontality –Steno Principle of Lateral Continuity– StenoPrinciple of Lateral Continuity– Steno Principle of Cross-Cutting Relationships— HuttonPrinciple of Cross-Cutting Relationships— Hutton Principle of InclusionsPrinciple of Inclusions Principle of Faunal Succession—William SmithPrinciple of Faunal Succession—William Smith Principle of Baked ContactsPrinciple of Baked Contacts

Law of Superposition In an undisturbed section, each bed is younger than the bed below it. In an undisturbed section, each bed is younger than the bed below it. – – map formations, beds, etc.

Disturbed Sections

Principle of Original Horizontality Sedimentary layers are mostly deposited horizontally Sedimentary layers are mostly deposited horizontally

Which way is up? Or in which direction are beds younger? Key to establishing sequence of events is to know which way is up. Key to establishing sequence of events is to know which way is up. Vertical beds (quartzite, limestone, and shale) in mouth of Sheep Creek Canyon, probably sec. 5, T. 17 N., R. 6 E., 22 miles northwest of Silver Lake, Crucero Quadrangle, San Bernardino County, California. October 16, ID. Thompson, D.G tdg U.S. Geological Survey Vertical beds (quartzite, limestone, and shale) in mouth of Sheep Creek Canyon, probably sec. 5, T. 17 N., R. 6 E., 22 miles northwest of Silver Lake, Crucero Quadrangle, San Bernardino County, California. October 16, ID. Thompson, D.G tdg U.S. Geological Survey

Principle of Lateral Continuity Layers of sediments extend laterally until pinches out (terminates) Layers of sediments extend laterally until pinches out (terminates)

Principle of Cross-Cutting Relationships Faults and intrusions are younger than the rocks they cut throughFaults and intrusions are younger than the rocks they cut through

Principle of Faunal Succession Evolution is irreversible Evolution is irreversible Therefore, can use fossils to establish order of events Therefore, can use fossils to establish order of events

Principle of Inclusions Fragments of one rock unit that have been enclosed within another are olderFragments of one rock unit that have been enclosed within another are older In each diagram, which inclusion is younger? Older?

Baked Contacts Intrusions bake rocks around it Intrusions bake rocks around it

Relative Dating Youngest  G A B C F D Oldest  E Place these rocks from youngest to oldest

Conformable and Unconformable Beds Conformable BedsConformable Beds Beds that have not been disturbedBeds that have not been disturbed Image of Permian-Jurassic sequence, Colorado PlateauImage of Permian-Jurassic sequence, Colorado Plateau

Uncomformities Substantial break or gap in the rock record All geologic history not evident But part of record remainsBut part of record remains Have uplift and erosion, unconformity produced 3 types of unconformites AngularAngular Disconformity Nonconformity

Angular Unconformity Tilted rocks are overlain by flat-lying rocks Tilted rocks are overlain by flat-lying rocks Siccar Point, England

Disconformity Beds are parallel relative to each other but there is a break in sequenceBeds are parallel relative to each other but there is a break in sequence Implies a cessation of sedimentation or erosion, but no tilting Hard to recognize

Nonconformity Sedimentary layers overlie igneous or metamorphic layers Sedimentary layers overlie igneous or metamorphic layers

Unconformities 1. Surface 1 is an example of ?Nonconformity 2. Surface 2 is an example of ? Angular unconformity 3. Surface 3 and 4? Disconformity 4. Granite is _________ than the Schist.Younger Question:Answer: