Geologic TimeGeologic Time

Earth’s HistoryEarth’s History

Determining Earth’s HistoryDetermining Earth’s History

Must determine Must determine sequence of eventssequence of events

To establish To establish chronological chronological framework use:framework use: Relative AgeRelative Age Absolute AgeAbsolute Age

Dating – 2 typesDating – 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 DatingPrinciples of Relative Dating

• Law of Superposition—StenoLaw of Superposition—Steno• Principle of Original Horizontality –Principle of Original Horizontality –StenoSteno

• Principle of Lateral Continuity– StenoPrinciple of Lateral Continuity– Steno• Principle of Cross-Cutting Principle of Cross-Cutting Relationships—HuttonRelationships—Hutton

• Principle of InclusionsPrinciple of Inclusions• Principle of Faunal Succession—Principle of Faunal Succession—William SmithWilliam Smith

• Principle of Baked ContactsPrinciple of Baked Contacts

Law of SuperpositionLaw of Superposition In an In an undisturbedundisturbed section, each bed is section, each bed is

younger than the bed below it.younger than the bed below it.– map formations, beds, etc.

Disturbed SectionsDisturbed Sections

http://ceed.utpb.edu/geology-resources/west-texas-geology/folds/http://facweb.bhc.edu/academics/science/harwoodr/geol101/study/structur.htm

Principle of Original Principle of Original HorizontalityHorizontality

Sedimentary layers are mostly deposited Sedimentary layers are mostly deposited horizontallyhorizontally

Which way is up? Or in which Which way is up? Or in which direction are beds younger?direction are beds younger?

Key to establishing Key to establishing sequence of events is to sequence of events is to know which way is up.know which way is up.

Vertical beds (quartzite, Vertical beds (quartzite, limestone, and shale) in limestone, and shale) in mouth of Sheep Creek mouth of Sheep Creek Canyon, probably sec. Canyon, probably sec. 5, T. 17 N., R. 6 E., 22 5, T. 17 N., R. 6 E., 22 miles northwest of miles northwest of Silver Lake, Crucero Silver Lake, Crucero Quadrangle, San Quadrangle, San Bernardino County, Bernardino County, California. October 16, California. October 16, 1917. - ID. Thompson, 1917. - ID. Thompson, D.G. 99 - tdg00099 - D.G. 99 - tdg00099 - U.S. Geological SurveyU.S. Geological Survey

Principle of Lateral ContinuityPrinciple of Lateral Continuity

Layers of sediments extend laterally Layers of sediments extend laterally until pinches out (terminates)until pinches out (terminates)

Principle of Cross-Cutting Principle of Cross-Cutting RelationshipsRelationships

• Faults and intrusions are younger than the rocks Faults and intrusions are younger than the rocks they cut throughthey cut through

Principle of Faunal SuccessionPrinciple of Faunal Succession

Evolution is Evolution is irreversibleirreversible

Therefore, can use Therefore, can use fossils to establish fossils to establish order of eventsorder of events

Principle of InclusionsPrinciple of Inclusions

• Fragments of one rock unit that have been enclosed within Fragments of one rock unit that have been enclosed within another are olderanother are older

In each diagram, which inclusion is younger? Older?

Baked ContactsBaked Contacts

Intrusions bake rocks around itIntrusions bake rocks around it

Relative DatingRelative Dating

Youngest G A

BCFD

Oldest E

Place these rocks from youngest to oldest

Conformable and Unconformable Conformable and Unconformable BedsBeds

• Conformable BedsConformable Beds• Beds that have not been disturbedBeds that have not been disturbed• Image of Permian-Jurassic sequence, Colorado Image of Permian-Jurassic sequence, Colorado

PlateauPlateau

UncomformitiesUncomformities

• 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 UnconformityAngular Unconformity Tilted rocks are overlain by flat-lying rocksTilted rocks are overlain by flat-lying rocks

Siccar Point, England

DisconformityDisconformity

• Beds are parallel Beds are parallel relative to each relative to each other but there is other but there is a break in a break in sequencesequence

• Implies a cessation of sedimentation or erosion, but no tilting

• Hard to recognize

NonconformityNonconformity

Sedimentary layers overlie igneous or Sedimentary layers overlie igneous or metamorphic layersmetamorphic layers

UnconformitiesUnconformities

1. Surface 1 is an example of ? Nonconformity2. Surface 2 is an example of ? Angular unconformity3. Surface 3 and 4? Disconformity4. Granite is _________ than the Schist. Younger

Question: Answer:

1

2

3

4

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