Transcript Angular unconformity

Determining geological ages
• Relative ages – placing rocks and geologic
events in their proper sequence, oldest to
youngest.
• Absolute dates – define the actual numerical
age of a particular geologic event. For example,
large dinosaurs died out 65 mya. The Lavas
along Rt 22 and Rt 78 were deposited about 205
mya.
Relative Age Dating assigns a
non-specific age to a rock, rock
layer or fossil based on its
position in the Strata relative to
other rocks, rock layers or
fossils.
Relative Age Dating is based on a
list of principles or rules.
First principle of relative dating
• Law of superposition
• Developed by Nicolaus Steno in 1669
• In an undeformed sequence of
sedimentary or volcanic rocks the oldest
rocks are at the base; the youngest are at
the top
-Superposition
Principle of Superposition
Superposition illustrated by strata
in the Grand Canyon
2nd principle of relative dating
• Principle of original horizontality
• Layers of sediment are originally
deposited horizontally (flat strata have
not been disturbed by folding, faulting)
3rd principle of relative dating
• Principle of cross-cutting relationships
3rd principle of relative dating
• Principle of cross-cutting relationships (example 2)
Cross-Cutting
An Igneous rock is always
younger than the rock layer that
it has intruded or cut across.
Principle of Cross-Cutting Relationships
The dike is youngest
because it cuts across
layers 1-4
Layer 1 is the
oldest rock layer
Key to Rocks Used in Diagrams
Limestone
Igneous
Metamorphic
Cross-cutting Relationship with
multiple overlapping intrusions
Erosional Features and Faults that
cut across rock layers are always
younger.
Example of Law of Cross-Cutting Relationships
Which came first, the rock layers or the faults?
Cross-cutting Normal Fault
The Law of Embedded
Fragments, or Law of Inclusion,
states that rocks that are
embedded in another rock must
be older than the rock in which it
is found.
Inclusion
Examples of Law of Inclusions
Inclusion- Conglomerate
fragments in overlying Shale
Inclusion-Granite fragments
included in overlying Shale
Inclusion- Shale fragments
imbedded in Granite intrusion
Another method of examining
the Geologic Record involved
examining instances where rock
layers are missing
(Unconformities).
The processes that would bring
about the removal of these
missing layers require large
amounts of time.
Unconformities
(loss of rock record)
• An unconformity is a break in the rock record
produced by erosion and/or nondeposition
• Types of unconformities
– Nonconformity – sedimentary rocks
deposited above metamorphic or igneous
rocks (basement) with time lost
– Angular unconformity – tilted rocks overlain
by flat-lying rocks
– Disconformity – strata on either side of the
unconformity are parallel (but time is lost)
Layered
sedimentary
rocks
(a)
8_9
Nonconformity
Metamorphic
rock
Igneous
intrusive rock
(b)
Younger
sedimentary
rocks
Angular
unconformity
Older, folded
sedimentary
rocks
(c)
Disconformity
Brachiopod
(290 million years old)
Trilobite
(490 million years old)
Formation of an angular unconformity
Angular Unconformity
Angular Unconformity
Erosional Surface
Horizontal younger sediments over tilted older sediments
Cambrian Tapeats sandstone over Precambrian Unkar Group
What type of unconformity is this?
Grand Canyon in Arizona
Angular Unconformity
Angular Unconformity, Siccar Point, Scotland
Disconformity
Development of a Nonconformity
An intrusion occurs
The overburden is eroded away
Pennsylvanian sandstone over
Precambrian granite is a
nonconformity
Sea level rises, new
sediment is deposited
Nonconformity- Sedimentary
Rock layers over older Igneous
or Metamorphic
Nonconformity in the Grand Canyon - Sediments deposited over Schist
Cross Cutting Relationships in strata
Zoroaster Granite across Vishnu Schist
Rock Layer Correlation
• Correlation is the matching of rock layers from
one area to another.
– Matching rocks in different locations due to
their similar characteristics
– Key Beds
– Stratigraphic Matching
– Using Index Fossils (fossils that lived and
died in one particular geologic time) to
match rock layers
Correlating Rock age using Index
Fossils and Stratigraphic Matching
Correlation of rock layers
• Matching strata of
similar ages in
different regions is
called correlation
Correlation of strata in
southwestern United States
Sections are incomplete
Match with fossils and lithology
Matching Rock Layers in Africa and South America
Index Fossil Requirements
Index Fossils must
– be easy to identify
– have been very abundant
– have lived in a wide geographic area
– have existed for a short geologic time
(ie: someone’s picture in a yearbook)
NYS Regents Exam diagram
Absolute Age Dating
Radiometric DatingProportion of Parent to Daughter
Isotopes
To get amount of parent material for each half-life, know that after one
Radioactive Dating- Half Life
Half Life
• The original isotope is called the parent
• The new isotope is known as the daughter isotope
– Produced by radioactive decay
– All parent isotopes decay to their daughter isotope at a
specific and unique rate
– Based on this decay rate, it takes a certain period of
time for one half of the parent isotope to decay to its
daughter product
– Half life – the time it takes for half of the atoms in the
isotope to decay
Tree Ring Chronology
(Dendrochronology)
Comparison with known tree ring
sequences
Can go back 10,000+ years
Based on living and fossil wood
Paleoclimate information
Paleohydrology
Archeology.
• EX: The half life of C-14 is 5,730 years
– So it will take 5,730 years for half of the C-14
atoms in an object to change into N-14 atoms
– So in another 5.730 years, how many atoms will be
turned into N-14?
• HALF LIFE
• In another 5,730 years, another half of the remaining
atoms will degrade to N-14, and so on.
• So after 2 half lives, one forth of the original C-14 atoms
remain
• After 3 half lives, one eighth of the original c-14 atoms
still remain
• Keeping cutting in half
Radiocarbon Dating
– C-14 is useful for dating bones, wood and
charcoal up to 75,000 yo
– Living things take in C from the environment to
make their bodies
– Most is C-12 but some is C-14
• The ratio of these two types in the enviro is always
the same
• By studying the ratio in an organism it can be
compared to the ratio in the environment presently