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Geologic Time
Correlation and Dating of the Rock
Record

Time
• Relative
– Order of deposition of a body of rock based on
position
• Absolute
– A number representing the time a body of rock
was deposited

Relative Time
• Determining of sequence of events

• Which came first?

In what order did these events occur in
American History?
A B C

D E

Relative Time
• Tools
– Smith
• Fossil Succession
– Steno
• Superposition
• Original horizontality
– Lyell
• Cross-cutting relationships
• Intrusions
• Inclusions

Principle of fossil succession
• Fossils occur in a consistent vertical order in
sedimentary rocks all over the world.
(William"Strata Bill" Smith, late 1700's, England).

• This principle is valid and does not depend on any
pre-existing ideas of evolution. (In fact, Charles
Darwin's ideas on evolution did not appear until
50 years later - 1858).

Geologists interpret fossil succession to be the result of
evolution - the natural appearance and disappearance of
species through time.

"Fossil species succeed one another in a definite and
recognizable order"

Fossils at the base of a thick sequence of sediments
(so older, by previous principles) are less like
present-day species than those near the top

* Fossils unlike present-day species, but like
each other, are found in widely separated
sites
* A fossil species which is observed to occur
above (and so younger than, by previous
principles) a second fossil species in one
locality will always occur above that second
species, wherever found.

Unconformities
• Unconformities are buried surfaces of
erosion or non-deposition

Unconformities
1.Angular unconformities
Implies tectonic deformation and erosion of underlying
strata.

2.Nonconformity
Sedimentary strata overlying igneous or metamorphic
rocks (in an erosional - not intrusive- contact)

3.Disconformity
An irregular surface of erosion between two units of
parallel strata

Nonconformity
Brahma Schist underlies Tapeats Sandstone

http://www.physci.mc.maricopa.edu/Geology/FieldTrips/ColoradoRiver/ColoradoRiver_2006_Summer/ColoradoRiver_2006_Summer_Images_1024/DSC00512a_1024c.jpg

Relative Dating
• Principle of Cross-cutting relationships

• Principle of Intrusions

• Principle of Inclusions

Principle of Cross-cutting
relationships

Principle of Inclusions

In which picture is the Granite older?

Intrusions vs Unconformities
• A xenolith is a fragment of country rocks
which has been broken off during an
intrusion, and has become surrounded by
magma. The xenolith is older than the
igneous rock which contains it.
• Through erosion and resedimentation,
younger rocks will often have pieces of the
older rock included (sedimentary).

Test Your Knowledge
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History of
Geologic Time
• Geologic Systems
– Body of rock that
contains fossils of
diverse animal life
– Corresponds to
geologic period
• Sedgewick
– Named Cambrian
• Murchison
– Named Silurian

Stratigraphy
• Study of stratified rocks, especially their
geometric relations, compositions, origins, and
age relations
• Stratigraphic units
– Strata
• Distinguished by some physical, chemical, or paleontological
property
• Units of time based on ages of strata
– Geologic Systems
• Correlation
– Demonstrate correspondence between geographically
separated parts of a stratigraphic unit
• Lithologic
• Temporal

Units of Time
• Time-rock unit
– Chronostratigraphic unit
– All the strata in the world deposited during a particular interval
of time
• Erathem, System, Series, Stage
• Time unit
– Geochronologic unit
– Interval during which a time-rock unit is formed
• Eras, Period, Epoch, Age
• Boundary stratotype
– Boundary between two systems, series or stages, formally
defined at a single locality

Geologic Time Scale
• Chronologic units - Time/Age

– Eons (largest):
– Era
– Periods
– Epochs
– Ages

Geologic Time Scale
• Geochronologic Units = Place

– Eon (largest) = Eon
– Era = Era
– System = Period
– Series = Epoch
– Stage = Age

Biostratigraphy
• Biostratigraphic unit
– Defined and characterized by their fossil content
• Stratigraphic range
– Total vertical interval through which that species
occurs in strata, from lowermost to uppermost
occurrence

Biostratigraphy
• Index fossil
– Abundant enough in the stratigraphic record to be
found easily
– Easily distinguished from other taxa
– Geographically widespread and thus can be used to
correlate rocks over a large area
– Occurs in many kinds of sedimentary rocks and
therefore can be found in many places
– Has a narrow stratigraphic range, which allows for
precise correlation if its mere presence is used to
define a zone

Magnetic Stratigraphy
• Use of magnetic
properties of a rock to
characterize and
correlate rock units
• Magnetic field
– Reversals in polarity of
field are recorded in
rocks when they
crystallize or settle
from water

Magnetic
Stratigraphy
• Chron
– Polarity time-rock unit
– Period of normal or
reversed polarity
• Normal interval
– Same as today
– Black
• Reversed interval
– Opposite to today
– White

Lithostratigraphy
• Subdivision of the stratigraphic record on the basis of
physical or chemical characteristics of rock
• Lithostratigraphic units
– Formation
• Local three-dimensional bodies of rock
– Group
– Member
• Stratigraphic section
– Local outcrop of a formation that displays a continuous vertical
sequence
• Type section
– Locality where the unit is well exposed, that defines the unit

Lithologic Correlation
• Cross-sections of
strata
– Establish geometric
relationships
– Interpret mode of
origin

Lithologic Correlation
• Grand Canyon
– McKee
– Used Trilobite
biostratigraphy to
determine age
relationships
– Eastern portion of
units is younger than
western

Facies
• Transgression
– Landward migration of
shoreline
– Grand Canyon
• Cambrian transgression
• Facies
– Set of characteristics of a body
of rock that presents a
particular environment
• Facies changes
– Later changes in the
characteristics of ancient strata

Absolute Age
• 4.6 billion years old
• Early estimates
– Salts in the ocean
• 90 million years old
– Accumulation of sediment
• 100 m.y. or less
• Gaps in stratigraphic record
• Unconformities represent large breaks in accumulation
• Didn’t include metamorphosed sedimentary rocks
– Earth’s temperature
• Kelvin
• 20-40 million years old

Absolute Ages
• How old is the Earth?

• 4.6 billion years (4,600,000,000 years)

• Radiometric dating (Uranium, Thorium).
Mass spectrometer.

Early Attempts
• 1654 Archbishop Usher (Ireland), genealogy
in Bible Earth was created October 22, 4004
BC,
• 9:00 am was added later
• Earth was 6000 years old.
• Led to the Doctrine of Catastrophism:
• Earth was shaped by series of giant disasters.
• Many processes fit into a short time scale.

Early Attempts
• 1770's, 1780's "Revolution"
• James Hutton, Father of Geology (Scotland)
1726-1797.
• Published Theory of the Earth in 1785.

Hutton
• Hadrian's Wall built by Romans,
after 1500 years no change.
Suspected that Earth was much
older.

• Slow processes shape earth.

• Mountains arise continuously as
a balance against erosion and
weathering

Hutton
• Doctrine of Uniformitarianism: "Present is key to
the past".

• The physical and chemical laws that govern nature
are uniform
•
• Unconformity at Siccar Point, Scotland

• "No vestige of a beginning, no prospect of an end"

Charles Lyell
• Charles Lyell 1800's compared amount of
evolution shown by marine mollusks in the
various series of the Tertiary System with the
amount that had occurred since the beginning
of the Pleistocene.

• Estimated 80 million years for the Cenozoic
alone.

Various Geologists
• Thickness of total sedimentary record
divided by average sedimentation rates (in
mm/yr).

• In 1860, calculated to be about 3 million
years old.

• In 1910, calculated to be about 1.6 billion
years old.

Lord Kelvin
• In 1897, Lord Kelvin
assumed that the Earth was
originally molten and
calculated a date based on
cooling through conduction
and radiation.

• Age of Earth was calculated
to be about 24-40 million
years.

Lord Kelvin
• Problem: Earth has an internal heat source
(radioactive decay)

• Discovery of radioactivity by Henri
Becquerel in 1896.

John Joly
• In 1899 - 1901, John Joly (Irish) calculated
the rate of delivery of salt to the ocean. River
water has only a small concentration of salts.
Rivers flow to the sea.
• Evaporative concentration of salts.

• Age of Ocean = Total salt in oceans (in grams)
divided by rate of salt added (grams per year)

• Age of Earth = 90-100 million years.

John Joly

• Problems: no way to account for recycled
salt, salt incorporated into clay minerals, salt
deposits.

von Helmholtz and Newcomb

The German physicist Hermann von Helmholtz and the American
astronomer Simon Newcomb joined in by independently
calculating the amount of time it would take for the Sun to
condense down to its current diameter and brightness from the
nebula of gas and dust from which it was born.

100 million years, consistent with Thomson's calculations.
However, they assumed that the Sun was only glowing from the
heat of its gravitational contraction. They knew of no other ways
for it to produce its energy.

Rutherford and Boltwood

• In 1905, they used radioactive decay to
measure the age of rocks and minerals.

• Uranium decay produces He, leading to a
date of 500 million years.

Rutherford and Boltwood

In 1907, Boltwood suspected that lead was the
stable end product of the decay of uranium.
Published the age of a sample of urananite
based on Uranium-Lead dating.

Date was 1.64 billion years.

Age of Earth
• So far, oldest dated Earth rocks are 3.96 billion
years.
• Canadian Shield. (NW Territories near Great
Slave Lake, 3.96 byr).
• Detrital Zircons in sedimentary rocks are 4.1 - 4.4
byr
• Older rocks include meteorites and moon rocks
with dates on the order of 4.6 billion years.

Geologic Time Scale.
The age for the base of
each division is in
accordance with
recommendations of the
International
Commission on
Stratigraphy for the
year 2000.

The standard geologic
time scale for the
Paleozoic and other eras
developed without
benefit of a grand plan.
Instead, it developed by
the compilation of “type
sections” for each of the
systems.

• Cretaceous

• Kreta = Chalk

• White Cliffs of Dover

Absolute Age
• Radioactive decay
– Becquerel, 1895
• Uranium undergoes spontaneous decay
• Atoms release subatomic particles and energy
• Change to another element
– Parent isotope decays/daughter isotope
produced

Principles of Radiometric Dating
• Naturally-occurring radioactive materials
break down into other materials at known
rates. This is known as radioactive decay.

• Radioactive parent elements decay to stable
daughter elements.

What is an Isotope?
• Nuclide of an element with different masses

Absolute Age
• Three modes of decay
– Loss of alpha particle
• Convert parent into
element that has nucleus
containing two fewer
protons
– Loss of beta particle
element whose nucleus
contains one more proton
by losing an electron
– Capture of beta particle
element whose nucleus
has one less proton

Absolute Age
• Radiometric dating
– Radioactive isotopes
decay at constant
geometric rate
• After a certain amount
of time, half of the
parent present will
survive and half will
decay to daughter
• Half-life
– Interval of time for half
of parent to decay

Absolute Age
• Useful isotopes
– Uranium 238 and thorium 232
• Zircon grains
– Uranium 238 and lead 206
• Fission track dating
– Rubidium-Strontium
– Potassium-Argon, Argon-Argon
– Radiocarbon dating
• Produced in upper atmosphere
• Half life = 5730 years
• Maximum age for dating: 70,000 years
• Bone, teeth, wood

Absolute Age
• Fission-Track Dating
– Measure decay of
uranium 238 by
counting number of
tracks
– Tracks formed by
subatomic particles
that fly apart upon
decay

Radioactive parent isotopes and
their stable daughter products
Each radioactive isotope has its own
unique half-life.

A half-life is the time it takes for half
of the parent radioactive element to
decay to a daughter product.

Radioactive parent isotopes and
their stable daughter products
Radioactive Parent Stable Daughter Half Life

Potassium 40 Argon 40 1.25 billion yrs

Rubidium 87 Strontium 87 48.8 billion yrs

Thorium 232 Lead 208 14 billion years

Uranium 235 Lead 207 704 million years

Uranium 238 Lead 206 4.47 billion years

Carbon 14 Nitrogen 14 5730 years

Radioactive Decay
• Radioactive decay occurs by releasing
particles and energy.

• Alpha particles
• Beta particles
• Neutrons
• Gamma rays (high energy X-rays) are also
produced.

Radioactive Decay
• Alpha particles (He)
• large, easily stopped by paper
• charge = +2
• mass = 4

Radioactive Decay
• Beta particles
• penetrate hundreds of times farther than
alpha particles, but easily stopped compared
with neutrons and gamma rays.
• charge = -1
• mass = negligible

Radioactive decay
series of uranium-
238 (238U) to lead-
206 (206Pb).

Datable Minerals
• Most minerals which contain radioactive isotopes
are in igneous rocks. The dates they give indicate
the time the magma cooled.
• Potassium 40 is found in:
• potassium feldspar (orthoclase)
• muscovite
• amphibole
• glauconite (greensand; found in some
sedimentary rocks; rare)
•

Datable Rocks
• Radioactive elements tend to become
concentrated in the residual melt that
forms during the crystallization of
igneous rocks. More common in
SIALIC rocks (granite, granite
pegmatite) and continental crust.

Datable Rocks
• Radioactive isotopes don't tell much about
the age of sedimentary rocks (or fossils).
The radioactive minerals in sedimentary
rocks are derived from the weathering of
igneous rocks. If the sedimentary rock were
dated, the age date would be the time of
cooling of the magma that formed the
igneous rock. The date would not tell
anything about when the sedimentary rock
formed.

14
How does Carbon dating work?

• Cosmic rays from the sun
strike Nitrogen 14 atoms in
the atmosphere and cause
them to turn into
radioactive 14C, which
combines with oxygen to
form radioactive CO2.

14

• Living things are in
equilibrium with the
atmosphere, and the
radioactive CO2 is absorbed
and used by plants. The
radioactive CO2 gets into the
food chain and the carbon
cycle.

14
• All living things contain a constant ratio of 14C to 12C (1
in a trillion).

• At death, 14C exchange ceases and any 14C in the tissues
of the organism begins to decay to Nitrogen 14, and is
not replenished by new 14C.

14
• The change in the 14C to 12C ratio is the basis for
dating.

• The half-life is so short (5730 years) that this
method can only be used on materials less than
50,000 years old.

• Assumes that the rate of 14C production (and
hence the amount of cosmic rays striking the
Earth) has been constant.

Deviation of carbon-14 ages to true ages from
the present back to about 5000 B.C. Data are
obtained from analysis of bristle cone pines from
the western United States. Calculations of carbon-
14 are based on half-life of 5730 years.
(Adapted from Ralph, E. K., Michael, H. N., and Han, M. C. 1973. Radiocarbon
dates and reality. MASCA Newsletter 9:1.)

Absolute Age
• Best candidates for
most radiometric
dating are igneous
– Not necessarily useful
for sediments
• Error in age estimate
can be sizable

Absolute Age
• Absolute ages change
– Error increases in older
rocks
– Techniques change
• Biostratigraphic
correlations are
usually more accurate
– Radiometric dates used
when fossils not
present

How old is the Old Red Sandstone?
a. Older than 425 myr
b. Younger than 370 myr
c. Between 425 and 370 myr
d. Have no idea

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