Outline• Stratigraphy:

– Study of the layers of sedimentary rocks

• Unconformities• Correlation• Relative dating• Absolute dating

Optional Field Trip ExerciseChanged from March 18 to

March 4Due March 9 at 1:10 pm

Zuhl Museum

Alumni Center, 775 College Av.

Sedimentation inlake or sea

Sedimentation inlake or sea

Sediments are deposited in horizontal layers and slowly change into rock.

Principle of Original Horizontality

• In stratigraphy:

• Sedimentary layers are originally deposited as horizontal sheets.

• Folded or tilted beds indicates something happened to them later

TIME 1 Beneath the sea, sediments accumulated in beds.

B

C

D

A

Principle of Superposition

• In stratigraphy:

• Sedimentary layers at the bottom of a sequence are OLDER than the ones on top

• If older beds are on top, then the sequence has been turned upside down

Younger

Older

Unconformities: Missing time

Tapeats Sandstone (500 million years)

Cardenas Basalt (1.1 billion years)

Angular Unconformity

Nonconformity: seds over granite

Nonconformity

Sequence of events:

1) Deposition of sediment2) Tilting of beds3) Erosion4) Deposition of sediment

Write down the sequence of events for this location.

1. Deposition of sediment

1. Deposition of sediment2. Folding 3. Erosion4. Deposition of sediment

on top

TIME 1 Beneath the sea, sediments accumulated in beds.

B

C

D

A

TIME 2 Tectonic forces caused uplift, exposing the beds to erosion.

B

C

D

A

Uplift

TIME 3 Erosion stripped away bed D and part of C.

B

C

A

TIME 4 Subsidence allowed a new layer, E, to be deposited.

BC

A

E

Subsidence

Unconformity

TIME 1 Beneath the sea, sediments accumulated in beds.

TIME 2 Tectonic forces caused uplift, folding, and deformation.

Uplift

Compression

TIME 3 Erosion stripped away the tops of the folded layers, leaving portions of several layers exposed.

TIME 4 Subsidence allowed new sediments to be deposited.

Subsidence

Angularunconformity

Plants grow on this surface

10 cm

Principle of Faunal Succession

• In stratigraphy:

• Fossils appear in a definite sequence

• This sequence can be used to CORRELATE beds in one area to another

Outcrop A Outcrop B

III

II

III

Outcrop A Outcrop B

III

II

III

Outcrop A Outcrop B

III

II

III

Outcrop A Outcrop B

III

II

III

Some of the fossils found inoutcrop A are the same as fossils found in outcrop B, some distance away.

Outcrop A Outcrop B

III

II

III

Some of the fossils found inoutcrop A are the same as fossils found in outcrop B, some distance away.

Layers with the same fossils are the same age.

Outcrop A Outcrop B

III

II

III

Some of the fossils found inoutcrop A are the same as fossils found in outcrop B, some distance away.

Layers with the same fossils are the same age.

Stratigraphicsuccession

A composite of the two outcrops.

Younger rocks

Older rocks

I

II

III

The Age of the Earth

4,560,000,000 years

or 4.6 billion years

Geologic Time Scale

• Precambrian (4.6 Ga to 540 Ma)

• Paleozoic (540 Ma to 250 Ma)• Mesozoic (250-65 Ma)

– Triassic– Jurassic– Cretaceous

• Cenozoic (65 Ma to the present)

LIFE ON EARTH

3.5 billion year old bacteria and algae

Cambrian explosion of life:

540 million years ago

Trilobites-Paleozoic arthropods

Today: 80% of species are arthropods

another Trilobite

Crinoids

First land animals:365 Ma

Dimetrodon-Permian reptile

End of the Paleozoic

Major extinction event:96% of marine life was killed70% land vertebrates killed

End of the Paleozoic

Mesozoic-Ammonites

Mesozoic-Ammonites

Mesozoic-Triceratops

Cretaceous-Tyrannosaurus

Rex

End of the Mesozoic

Major extinction event:85% of all species died.

Cenozoic-Mammoth

Cenozoic-Sabre tooth tiger

Summary

Principle of superpositionPrinciple of original horizontality

Principle of faunal successionUnconformities

Time scaleEvolution of life