Quiz Three (9:30-9:35 AM)
UNIVERSITY OF SOUTH ALABAMA
GY 112: Earth History
Lecture 6:
Clever Thinking 3: Stratigraphy
Instructor: Dr. Douglas W. Haywick
Last Time
1. Relative vs. Absolute Dating Techniques
a) Magnetostratigraphy
b) Fission Track Dating
2. Radiometric Dating
3. Mass spectrophotometers
(Web Lectures 7 & 8)
Geological Dating Techniques
Relative Techniques: Assigns an age to a rock that puts it
into a narrow range (e.g., mid-Devonian; Late Cretaceous,
upper Pliocene).
Absolute Techniques: Assigns an age to a rock that is a
number (e.g., 354.7 +/- 21.3 MA; 1,453 KA +/- 67 KA).
Magnetostratigraphy
• Chron
– Polarity time-rock unit
– Period of normal or
reversed polarity
• Normal interval
– Same as today
– Black
• Reversed interval
– Opposite to today
– White
Absolute Techniques
• Fission-Track Dating
– Measure decay of
uranium 238 by
counting number of
tracks
Radiometric Dating
Uranium (and others) are unstable
Radioactive Decay
Three modes of decay
1) Alpha Decay
Loss of alpha particle
•
Convert parent into element
that has nucleus containing
two fewer protons
2) Beta Decay
Loss of beta particle
•
Convert parent into element
whose nucleus contains one
more proton by losing an
electron
3) Gamma Decay
Capture of beta particle
•
Convert parent into element
whose nucleus has one less
proton K40→ Ar40
Alpha Decay (Uranium)
238U
→ 206Pb + 8α
http://mike.gamerack.com/science/halflifeu238.gif
Radioactive Decay
Radioactive Decay
• 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 Lives
Parent Isotope
14C
(Carbon-14)
235U
40K
(Uranium-235)
(Potassium-40)
238U
(Uranium-238)
232Th
87Rb
(Thorium-232)
(Rubidium-87)
147Sm
(Samarium-147)
Daughter Isotope
14N
(Nitrogen-14)
207Pb
40Ar
Half Life (years)
5,730
Datable Material(s)
Wood, shells and organic material
(Lead-207)
700,000,000
Metamorphic, igneous rocks,
Zircon, U-bearing minerals
(Argon-40)
1,300,000,000
Metamorphic, igneous &
sedimentary rocks; feldspar-bearing
minerals
206Pb
(Lead-206)
4,500,000,000
Metamorphic, igneous rocks,
Zircon, U-bearing minerals
208Pb
(Lead-208)
14,000,000,000
Metamorphic, igneous rocks,
Zircon, U-bearing minerals
(Strontium-87)
48,600,000,000
Various rocks and minerals
87Sr
143Nd
(Neodymium-143)
106,000,000,000
Very old rocks, REE bearing
minerals
Age Determination
The all important age equation:
-λt
e
N=No
No is the number of atoms of parent isotope remaining in a substance
N is the number of atoms of daughter isotope produced through decay,
λ is the decay constant (which depend on the isotope in question)
t is the amount of elapsed time.
Age Determination
A more useful equation for age determination:
Rock age= 1/λ x ln[(Do-D) + 1]
N
Do is the original amount of daughter isotope in the sample
N is the amount of current parent isotope in the sample
D is the amount of current daughter isotope in the sample
λ is the decay constant
Today’s Agenda
1. William Smith and Water
2. Stratigraphic Principles
3. Geological Time
(Web Lecture 6)
Clever thinkers:
1769-1839
William “Strata” Smith: the Father of Stratigraphy (and English Geology)
Actually he was a “hydrologist”
Clever thinkers:
1769-1839
William “Strata” Smith: He recognized the importance of fossils in
distinguishing different geological or “stratigraphic” units.
Clever thinkers:
1769-1839
Affectionately called “The Map”
William “Strata” Smith: He used fossils and his knowledge of geology
to make the first detailed geological map of Great Britain
Stratigraphic Principle
Stratigraphy: the study, classification and
correlation of rock layers and layering
Stratigraphic Nomenclature
A typical “stratigraphic:
column
Source: 3dparks.wr.usgs.gov/coloradoplateau/images/bryce_strat.jpg
Stratigraphic Nomenclature
Formation: a lithologically
distinct rock unit that possesses
recognizable upper and lower
contacts with other units and
which can be traced across the
countryside from place to place.
Source: 3dparks.wr.usgs.gov/coloradoplateau/images/bryce_strat.jpg
Stratigraphic Nomenclature
Formation: a lithologically
distinct rock unit that possesses
recognizable upper and lower
contacts with other units and
which can be traced across the
countryside from place to place.
Member: an important “unit”
but one not quite meeting the
requirements of a formation
Source: 3dparks.wr.usgs.gov/coloradoplateau/images/bryce_strat.jpg
Stratigraphic Nomenclature
Formation: a lithologically
distinct rock unit that possesses
recognizable upper and lower
contacts with other units and
which can be traced across the
countryside from place to place.
Member: an important “unit”
Corso
Group
but one not quite meeting the
requirements of a formation
Group: a collection of similar
type formations
Source: 3dparks.wr.usgs.gov/coloradoplateau/images/bryce_strat.jpg
Stratigraphic Correlations
correlation lines
Pinchout
Facies
Change
http://www.sciencedirect.com/science/article/pii/S0195667111000449
Geological Time
Geological Time
The problem is that we have to deal with a lot of time.
4.6 GA = 4,600,000,000 years
Geological Time
Time
0 MA (today)
4.6 GA
Geological Time
Time
Eon
0 MA (today)
Eons: the largest division
of geological time
4.6 GA
Geological Time
Time
Eon
Time
0 MA (today)
4.6 GA
Hadean
4.6 GA to 4.1 GA
Geological Time
Time
Eon
Time
0 MA (today)
4.6 GA
Archean
4.1 GA to 2.5 GA
Hadean
4.6 GA to 4.1 GA
Geological Time
Time
Eon
Time
0 MA (today)
Proterozoic 2.5 GA to 550 MA
4.6 GA
Archean
4.1 GA to 2.5 GA
Hadean
4.6 GA to 4.1 GA
Geological Time
Time
Eon
0 MA (today)
Phanerozoic
550 MAto 0 MA
Proterozoic
2.5 GA to 550 Ma
Archean
4.1 Ga to 2.5 Ga
Hadean
4.6 Ga to 4.1 Ga
4.6 GA
Time
Geological Time
Time
Eon
0 MA (today)
Phanerozoic
Era
Time
Proterozoic
Archean
Hadean
4.6 GA
Eras: the 3-fold division of the eons
Geological Time
Time
Eon
0 MA (today)
Phanerozoic
Era
Time
Proterozoic
Archean
4.6 GA
Hadean
Early, Middle, 4.1 GA to 2.5 GA
Late
4.6 GA to 4.1 GA
Geological Time
Time
Eon
0 MA (today)
Phanerozoic
Proterozoic
Archean
4.6 GA
Hadean
Era
Time
Neoproterozoic
900 MA to 550 MA
Mesoproterozoic
1.6 GA to 900 MA
Paleoproterozoic
2.5 GA to 1.6 GA
Early, Middle,
Late
4.1 GA to 2.5 GA
4.6 GA to 4.1 GA
Geological Time
Time
Eon
0 MA (today)
Phanerozoic
Proterozoic
Archean
4.6 GA
Hadean
Era
Time
Cenozoic
65 MA to 0 MA
Mesozoic
245 MA to 65 MA
Paleozoic
550 MA to 245 MA
Neoproterozoic
900 MA to 550 MA
Mesoproterozoic
1.6 GA to 900 MA
Paleoproterozoic
2.5 GA to 1.6 GA
Early, Middle,
Late
4.1 GA to 2.5 GA
4.6 GA to 4.1 GA
Geological Time
Time
Eon
0 MA (today)
Phanerozoic
Proterozoic
Era
Time
Cenozoic
65 MA to 0 MA
Mesozoic
245 MA to 65 MA
Paleozoic
550 MA to 245 MA
Neoproterozoic
900 MA to 550 MA
Mesoproterozoic
1.6 GA to 900 MA
Paleoproterozoic
2.5 GA to 1.6 GA
Archean
4.1 GA to 2.5 GA
Hadean
4.6 GA to 4.1 GA
4.6 GA
Geological Time
Periods: the most
useful subdivisions of
(mostly) the
Phanerozoic eon
65 MA
245 MA
550 MA
Today’s Homework
1. Review online notes
2. Read War and Peace
Next Time
1. Stable Isotope geochemistry (8a) Hard core science!
GY 112: Earth History
Lecture 6: Stratigraphy
Instructor: Dr. Doug Haywick
dhaywick@southalabama.edu
This is a free open access lecture, but not for commercial purposes.
For personal use only.