Geologic Time
Earth’s History
Determining Earth’s History


Must determine
sequence of events
To establish
chronological
framework use:


Relative Age
Absolute Age
Dating – 2 types
•Relative Dating
– Place rocks and events
in sequence of
formation
• Absolute Dating
– Specify actual number
of years that have
passed since the
event occurred
Principles of Relative Dating
•Law of Superposition—Steno
•Principle of Original Horizontality –Steno
•Principle of Lateral Continuity– Steno
•Principle of Cross-Cutting Relationships—
Hutton
•Principle of Inclusions
•Principle of Faunal Succession—William
Smith
•Principle of Baked Contacts
Law of Superposition

In an undisturbed section, each bed is younger
than the bed below it.
– map formations, beds, etc.
Disturbed Sections
http://facweb.bhc.edu/academics/science/harwoodr/geol101/study/structur.htm
http://ceed.utpb.edu/geology-resources/west-texas-geology/folds/
Principle of Original
Horizontality

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.
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,
1917. - ID. Thompson, D.G.
99 - tdg00099 - U.S.
Geological Survey
Principle of Lateral Continuity

Layers of sediments extend laterally
until pinches out (terminates)
Principle of Cross-Cutting
Relationships
• Faults and intrusions are younger than the rocks they cut
through
Principle of Faunal Succession


Evolution is
irreversible
Therefore, can use
fossils to establish
order of events
Principle of Inclusions
• Fragments 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
Relative Dating
Place these rocks from youngest to oldest
Youngest 
Oldest 
G
A
B
C
F
D
E
Conformable and Unconformable Beds
• Conformable Beds
• Beds that have not been disturbed
• Image of Permian-Jurassic sequence, Colorado
Plateau
Uncomformities
• Substantial break or gap in the rock record
• All geologic history not evident
• But part of record remains
• Have uplift and erosion, unconformity produced
• 3 types of unconformites
• Angular
• Disconformity
• Nonconformity
Angular Unconformity

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 sequence
• Implies a cessation
of sedimentation or
erosion, but no
tilting
• Hard to recognize
Nonconformity

Sedimentary layers overlie igneous or
metamorphic layers
Unconformities
4
3
2
1
1.
2.
3.
4.
Question:
Surface 1 is an example of ?
Surface 2 is an example of ?
Surface 3 and 4?
Granite is _________ than the Schist.
Answer:
Nonconformity
Angular unconformity
Disconformity
Younger
http://facweb.bhc.edu/academics/science/harwoodr/Geol101/Study/Images/D041-551.jpg