Sedimentary rocks and their depositional
environments
• How can sedimentary rocks be used to
figure out what the environment was in the
past?
Depositional environments
*How does energy of the environment relate to grain size???
Sedimentary environments
• Sea level rising, shoreline moves inland
transgression
• Sea level falling, shoreline moves off
land regression
• What can cause changes in sea level?
• Transgression and regression animation
Sea level rising: transgression
Transgression
How to recognize a transgression
Fining upward
Transgression facies
From: http://www.purdue.edu/eas/mesozoic/lab_05.html
Sea level dropping: regression
Regression
How to recognize a marine regression
Coarsening upward
How to recognize a
transgression
Regressive facies
From: http://www.purdue.edu/eas/mesozoic/lab_05.html
Geologic time: absolute dating
• relative dating animations
• We’ve talked about rocks and how to tell the relative
age of them using Steno’s laws and sedimentary
structures.
• Radioactive decay of certain elements allows us to
date rocks more precisely.
Absolute dating
• Radioative elements are geologic
“clocks”
• Amount of decay=age of the element
and age of rock (have to be careful
here!)
Isotopes of Hydrogen
e-
• Hydrogen
• 1 proton, 0 neutrons
• Deuterium
• 1 proton, 1 neutron
en
• Isotope are elements with different
atomic masses (neutron #’s differ)
• When atoms are unstable they eject
particles from their nucleus and become
an ISOTOPE!
Radioactive decay
• Decay rate in terms of half-life (time)
• Half-life: time for 1/2 of all atoms to
decay from parent element to
daughter element
Radioactive Decay
• Mineral forms
with only parent
atoms
• Through time, the
parent decays to
form daughter
product
How to use half-life (example)
• Half-life of potassium 40 is 1.3 billion years
• After 3.9 billion years,how many half-lives
have occurred
• What fraction of potassium 40 is left?
• 1/2 * 1/2 * 1/2 = 1/8
All rocks are not created equal
for radiometric dating use!
• Expensive and may not have good
minerals to date
• Sedimentary rocks are almost
impossible to date…why? (think of what
sed rocks are made of)
• Best rocks: igneous
• Complicated rocks: metamorphic
Problems with dating
• Errors
• Usually 0.5% of age, as good as 0.1%
• E.g. -- 2 billion year rock is +/- 2-20 million
• Dating detrital sed rocks
• Is the age found of the sedimentary deposit
or igneous formation?
• Dating metamorphic rocks
• Did isotope ratio survive metamorphism?
Common isotopes used to date
rocks!
Parent
Daughter
1/2 life
Use
238U
206Pb
4.5 billion
Old rocks
10Ma - 4.6Ga
40K
40Ar
1.3 billion
Young rocks
<10 Ma
14C
14N
5730 years
Young
artifacts
100-70,000
years
Carbon-14 dating
• How does Carbon-14 dating work?
• Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere
and cause them to turn into radioactive Carbon 14, which combines with
oxygen to form radioactive carbon dioxide.
• Living things are in equilibrium with the atmosphere, and the radioactive
carbon dioxide is absorbed and used by plants. The radioactive carbon
dioxide gets into the food chain and the carbon cycle.
• All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in
a trillion).
• At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues
of the organism begins to decay to Nitrogen 14, and is not replenished by
new C-14.
• The change in the Carbon 14 to Carbon 12 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 70,000 years old. Archaeological dating uses this
method.) Also useful for dating the Pleistocene Epoch (Ice Ages).
• Assumes that the rate of Carbon 14 production (and hence the amount of
cosmic rays striking the Earth) has been constant (through the past
70,000 years).
Carbon-14 Dating
• For more see this site carbon-14
animations and info
• And this site pdf article
Structure of the Carbon Atom
• Nucleus
• Protons (+)
• Neutrons (n)
• Electrons (-)
• # protons: atomic #
• # protons + neutrons:
atomic mass
• Isotope: element with
different # neutrons