Download Relative Time and Correlation

Geologic Time
Concepts and Principles
I. Relative Time vs. Absolute Time
Relative Age
A. _____________________
Does not determine the number years involved
but is concerned with the sequence of events
(rock layers, erosion, structures formed, etc.)
Absolute Age
B. ______________________:
Determines the actual age of the rock in years
II. PRINCIPLES USED TO
DETERMINE RELATIVE AGE
Principle of Original Horizontality
A. ____________________________
1.Sedimentary rock layers, or strata,
were originally deposited as relatively
horizontal sheets of sediment.
2.Strata that do not retain their original
horizontality have been displaced by
movements of Earth’s crust.
Sediment is deposited in essential horizontal layers
–Therefore, a sequence of sedimentary rock layers
that is steeply inclined from horizontal must have been
tilted after deposition and lithification
Principle of Lateral Continuity
B. _____________________________
1. Sedimentary rock layers or lava flows extend laterally
(geographically) in all directions until they thin to their
termination or ends of their basins of deposition.
2. Another term for this is that they “pinch out.
Gradual Terminations
– A rock unit becomes
progressively thinner until it
pinches out
– or where it splits into thinner
units
• each of which pinches out,
• called intertonging
•
•
•
•
where a rock unit changes
by lateral gradation
as its composition and/or texture
becomes increasingly different
Principle of Cross-Cutting Relationships
C. ______________________________
• Any feature (fracture, fault, intrusive
mass) that cuts across a body of
sediment or rock is younger than the
body of sediment or rock that it cuts
across.
Cross-cutting
Relationships
• North shore of
Lake Superior,
Ontario Canada
• A dark-colored
dike has intruded
into older light
colored granite.
• The dike is
younger than the
granite.
Cross-cutting Relationships
• Templin
Highway,
Castaic,
California
• A small fault
displaces
tilted beds.
• The fault is
younger than
the beds.
Folds
Principle of Superposition
D. ___________________________
• In an undisturbed
succession of
sedimentary rock layers,
– the oldest layer is at the
bottom
– and the youngest layer is at
the top
Principle of Superposition
• Illustration of the principles of
superposition and original horizontality
• Superposition: The youngest
– rocks are at the top
– of the outcrop
– and the oldest rocks are at the bottom
Overturned Folds
• Can cause older rocks to be located above younger rocks.
Thrust Faults
•Can push older rocks above younger rocks.
Chief Mountain, Glacier National Park, Montana
• Erosional remanant of a thrust fault
• Cross section shows older Precambrian rocks thrust over
younger Cretaceous rocks
Principle of Inclusions
E. ____________________
Inclusions or fragments in a rock are older than the rock itself
Principle of Inclusions
• Light-colored
granite
– in northern
Wisconsin
– showing basalt
inclusions (dark)
• Which rock is older?
– Basalt, because the
granite includes it
Age of Lava Flows, Sills
• Determining the relative ages of lava flows, sills
and associated sedimentary rocks
– uses alteration by heat
– and inclusions
– A lava flow forms in
sequence with the
sedimentary layers.
• Rocks below the lava
will have signs of
heating but not the
rocks above.
• The rocks above may
have lava inclusions.
Sill
– A sill will heat the rocks above and below.
– The sill might also have inclusions of the rocks
above and below,
– but neither of these rocks
will have inclusions of
the sill.
Unconformities
F. __________________
1. Surfaces that represent a gap in the geologic
record.
2. Like pages in a book, they are surfaces on
which sediment was not deposited for a period
of time, or surface on which erosion has
occurred.
3. Unconformities can range in size from bedding
planes (surfaces between strata) to continentwide surfaces
The Sequence of Events in the
Formation of an Unconformity
Siccar Point, Scotland
Hutton’s Unconformity
Siccar Point Scottland
Near Edinburgh
I don’t know who this guy is!
5. Types of Unconformities
Nonconformities
a) _________________
• An erosional surface that separates older igneous or metamorphic
(nonsedimentary) rock from younger overlying sedimentary strata. Note
the absence of contact metamorphism in the rocks immediately above
the igneous or metamorphic rocks.
Sequence of Events in the Formation
of a Nonconformity
Nonconformity
Angular Unconformities
b) ________________________
• An erosional surface that separates tilted or folded strata from overlying
beds of different attitude. This implies that an area has undergone
uplift, and that the uplift was accompanied by either folding or tilting with
erosion of the strata prior to later subsidence and continued deposition.
The Formation of an Unconformity
An erosional surface buried
by younger sedimentary
rocks
Grand Canyon
Disconformities
c) __________________
• An erosional surface that separate essentially parallel sedimentary
strata. These are probably the most common types of unconformities
and are often the most difficult to recognize. The is especially so when
the rock types are similar above and below the erosion surface.
Disconformity
Disconformity
Disconformity
Along a roadcut in Gladeville, TN
III. Correlation
Determining the age relationships between rock units
or geologic events in separate areas
A.
SIMILARITY OF ROCK TYPES
Lithostratigraphic Correlation
1. __________________________________
Being able to trace physically the course
of a rock unit.
2. “Walking the Outcrop”
Lithostratigraphic Correlation
• Correlation of lithostratigraphic units
such as formations
– traces rocks laterally across gaps
Correlation Using Lines Drawn to
Points of Equivalence
Disconformities
Fossils
Index (Guide) Fossils
1. ____________________
a) Short lived species
b) Easily identifiable
c) Wide geographic distribution
Index (Guide) Fossils
• The brachiopod Lingula
– is not useful because,
– although it is easily identified
– and has a wide geographic
extent,
– it has too large a geologic
range
• The brachiopod Atrypa
– and trilobite Paradoxides
– are well suited
– for time-stratigraphic
correlation,
– because of their short ranges
• They are guide fossils
Fossil Assemblage
2. ______________________
• Several different fossil species in a rock
layer
• All species existed when the sediment
was deposited.
• Use a fossil “range chart.”
Fossil Ranges
Fossil Assemblages