1. Geologic Time
Marble demo
Some Index Fossils
Foam strata

2. 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.

3. 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

4. Superposition illustrated by strata in the Grand Canyon

5. 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)

6. 3rd principle of relative dating
Principle of cross-cutting relationships
If a rock layer is cut by a fault or igneous intrusion, the rock that is cut must be older than the layer that cuts it.
Igneous Dike

7. 3rd principle of relative dating
Principle of cross-cutting relationships (example 2)
Fault cross-cuts limestone and shale

8. 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)

9. Angular unconformity Disconformity Layered (a) sedimentary rocks
8_9
Nonconformity
Igneous
intrusive rock
Metamorphic
rock
(b)
Younger
sedimentary
rocks
Angular
unconformity
Older, folded
sedimentary
rocks
(c)
Disconformity
Trilobite
(490 million years old)
Brachiopod
(290 million years old)

10. Horizontal younger sediments over tilted older sediments
Cambrian Tapeats sandstone over Precambrian Unkar Group
What type of unconformity is this?
Grand Canyon in Arizona

11. Formation of an angular unconformity

12. Sea level rises, new sediment is deposited
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

13. Nonconformity in the Grand Canyon - Sediments deposited over Schist

14. Cross Cutting Relationships in strata
Zoroaster Granite across Vishnu Schist

15. Correlation of rock layers
Matching strata of similar ages in different regions is called correlation

16. Correlation of strata in southwestern United States
Sections are incomplete
Match with fossils and lithology

17. Because of sea-level changes Fossils are more reliable than sequences of sediment facies
However, falling sea level is useful for worldwide correlation. Why?
Sauk Sequence
WEST
EAST
Transgression
Middle
Cambrian
Lower
Cambrian
Note how western BAS is older than eastern BAS

18. Correlation of rock layers with fossils
Correlation of rock layers with fossils
Correlation often relies upon fossils
Principle of fossil succession (Wm. Smith)
fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it
Index Fossils - useful for correlation
Existed for a relatively brief time
Were widespread and common
Most fossils are just impressions. A few may have small amounts of some original tissue

19. How impression fossils form (the most common type)
8_10
Shells
settle on
ocean
floor
Cast forms when mold
is filled in with mineral
water
Rock broken
to reveal
fossil cast
Shells
buried in
sediment
Rock broken
to reveal external mold
of shell
Mold, or cavity,
forms when original
shell material
is dissolved

20. Fossil Succession shown with Index Fossils

21. Determining the ages of rocks using overlap of fossils
Dinosaurs and flowering plant fossils occur in these sediments, therefore sediments “A” are Cretaceous
B contains trilobites, Cambrian to Permian, and Ginkgo leaves, Permian to Recent, therefore B is Permian

22. Overlapping fossil ranges are the data used in Biostratigraphy – correlation with fossils
These are NOT Index Fossils- WHY?

23. Two Measured Sections
Note the use of overlapping fossil ranges in two distant outcrops, even though the sediment facies are different.

24. In the diagram, the presence of three species of index fossils is shown for two measured sections. The limestone marked A on the left is the same age as
a. sandstone 1
b. mudstone 2
c. shale 3
d. sandstone 4

25. Correlation with index fossils in spite of unconformities
Highway cut A
Correlation
Highway cut B

26. Especially useful in well cores
conodonts
pollen
diatoms
foraminifera
radiolaria
In the Grand Canyon, some of the best indicators of relative ages in the rocks are the fossils that we can’t even see.
Microscopic fossils like the single-celled group of foraminifera are extremely valuable in assigning ages to many of the rock layers of the Grand Canyon.
As are the microscopic fossils called conodonts which are found widespread in rocks that formed in marine environments.
Because they are so widespread, and composed of the mineral apatite, they are believed to be the teeth of sea creatures that swam around the oceans losing quite a few teeth,
Conodonts and foraminfera and other plankton like radiolarians and even tiny grains of fossil pollen are widespread in rocks around the world and show a pattern of species evolution through time and are extremely valuable in determining relative ages of rocks in the Grand Canyon and around the world.
are microfossils

27. Magnetostratigraphy dated with fossils
or with absolute (radiometric) dating

28. Geologic time scale Eon, Era, Period, Epoch
The geologic time scale – a “calendar” of Earth history
Subdivides geologic history into units
Originally created using relative dates
Structure of the geologic time scale
Eon, Era, Period, Epoch

29. Divisions based on fossils
Eons
Geologic Timescale
Phanerozoic
PreCambrian
Divisions based on fossils
Eon, Era, Period, Epoch
Eras

30. Origin of Period Names

31. Geologic time scale Structure of the geologic time scale
Names of the eons
Phanerozoic (“visible life”) – the most recent eon, began about 545 million years ago
PreCambrian (Cryptozoic)
PreCambrian subdivisions:
Proterozoic – begins 2.5 billion years ago
Archean – begins 3.8 bya
Hadean – the oldest eon begins 4.6 bya
Read from bottom to top – Oldest to Youngest

32. Geologic time scale Precambrian time
Nearly 4 billion years prior to the Cambrian period (.545 billion= 545 million)
Long time units because the events of Precambrian history are not know in detail – few fossils, most rock modified
Immense space of time (Earth is ~ 4.6 by)
PreCambrian ( )/4.6 ~ 88%

33. Geologic time scale - Eras
Structure of the geologic time scale
Era – subdivision of an eon
Eras of the Phanerozoic eon
Cenozoic (“recent life”) begins ~ 65 mya
Mesozoic (“middle life”) begins ~ 248 mya
Paleozoic (“ancient life”) begins ~ 545 mya
Eras are subdivided into periods
Periods are subdivided into epochs
Eon>Era>Period>Epoch

34. Using radioactivity in dating
Importance of radiometric dating
Allows us to calibrate geologic timescale
Determines geologic history
Confirms idea that geologic time is immense

35. Radiometric Age Determinations
of the Earth
The oldest known minerals ever found on Earth include some from NW Australia. The containing rock (a conglomerate) is about 3.0 billion years old. The rock contains detrital grains of the mineral zircon that are 3.96 billion years old. The dates are based on datable Uranium in the Zircons.
Similar dates are known from Yellow Knife Lake, NWT, Canada

36. Radiometric Age Determinations of the Earth
The age of the Earth is thought to be about (roughly 4.6) billion years
Based on the dates obtained from meteorites and samples collected on the moon, assumed to have formed at the same time.

37. Recall Isotopes
The number of protons in an atom's nucleus is called its atomic number –defines “element”
Mass protons + neutrons called atomic mass or, more loosely, atomic weight
The number of neutrons can vary
Atoms of the same element with different numbers of neutrons are called isotopes. Some are radioactive

38. electron combines with a proton to form a neutron
Radioactive
parent nucleus
Decay process
Daughter
nucleus
8_19 p p p p p p p
Atomic mass decreases
by 4; atomic number
decreases by 2 p p p p p p p
Proton
Neutron p
Alpha particle
(a)
Emission of 2 protons and 2 neutrons (alpha particle)
Alpha decay p p p p p p p p p
Atomic mass not changed
much; atomic number
increases by 1 because
Neutron becomes proton p p p p p p p p p p
Beta particle
(b)
Beta decay
An electron (beta particle) is ejected from the nucleus p p p p p p p p p p p p
Atomic mass unchanged;
atomic number
decreases by 1 p p p p p p p p
Beta particle
electron combines with a proton to form a neutron
(c)
Electron capture

39. Using radioactivity in dating
Parent – an unstable radioactive isotope. Parent atom fits in crystal.
Daughter product – stable isotopes resulting from decay of parent. Doesn’t fit, so none present when crystal formed.
Half-life – time required for one-half of the parent isotope in a sample to decay into stable daughter product

40. A radioactive decay curve
1/2 = 50% parent: 1 half-life has passed
1/2x1/2 = 1/4 = 25% parent: 2 half-lives have passed
1/2x1/2x1/2 = 1/8 = 12.5% parent: 3-half-lives have passed
Marble Demo

41. Used up around Sparta, Byram – Zircons in pegmatites, rule of cross cutting
Used around here, East Africa, Biotite etc. in volcanics

42. U-238 decays to Pb-206

44. How do we actually “date” a rock?
Collect sample
Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids
Pass through extreme heat, electric and magnetic fields.
Measure parent/daughter ratio of target isotopes - mass spectrometer

45. Dating sediments without fossils
Wasatch Fm. younger than 66 my
Mancos Shale and Mesa Verde Fm.
older than 66 my
Morrison Fm older than 160 my
Radiometric Dating with Igneous Rocks
Or Bracket between fossiliferous layers

46. Even better: we get lucky
Even better: we get lucky. A layer we need to date is between two datable beds
So we have and upper and lower bound on the age of this limestone:
Basalt Lava flow 2
200 mya
Lava flow 1
209 mya
We can bracket this
limestone’s age
between 209 and 200
mya

47. Dating a crystal (3) We calculate age based on half-life Mineral1
Mineral crystal
formed in igneous
rock
Parent
atoms 2
Igneous rock
buried beneath
younger rocks;
daughter atoms
formed by
normal decay
Daughter
(3) We calculate age
based on half-life
atoms

48. But IF: Resets the clock3
Deep burial and
metamorphism
during mountain
building causes
daughter atoms
to escape from
crystal
Heat
Resets the clock
Rock looks as if it just formed: it looks young
Age found dates from metamorphic event 4
After mountain
building ends,
accumulation of
daughter atoms
in crystal
resumes
Easily recognized,
useful in studying
metamorphism

49. Dating with carbon-14 (Carbon Dating) Half-life only 5730 years
Used to date organisms
Carbon-14 is produced in the upper atmosphere
Useful tool for geologists who study very recent Earth history

50. Carbon-14 Cosmic rays Atoms split into bombard smaller particles,
among them neutrons
Cosmic rays
bombard
atmospheric atoms
8_24
Carbon-14
Neutrons strike
nitrogen atoms
Nitrogen atoms lose a
proton and becomes
carbon-14
C-14 mixes with atmospheric oxygen
to produce CO2
C-14 absorbed
by living
organisms
CO2 taken up
by plants
CO2 dissolved
in water
C-14 intake ceases when organism
dies; C-14 concentration decreases

51. Tree Rings Years of age 500 Annual-ring similarities show correlation
8_27
500
Annual-ring similarities
show correlation
400
Current year
200
100
150 50
Tree
growth
rings A D B C A
Sediment layers
with tree logs to
be collected for
dendrochronology B C D
Buried tree
logs

52. Lakes south of the glacier track its advance - NH
8_28
Dating with Lake Varves
Very
little
or no
runoff
Heavy
runoff
into
lake
Ice
Summer
Winter
Turbid water
Clear water
Summer layer
(coarse, thick, and
light-colored)
Winter layer
(fine, thin, and
dark-colored)
Modern Lakes, just count back from present. Fossil pollen track climate.

53. Examples of fossil pollen

55.
Hanneke Bos

56. End of Geologic Time Lecture