Download Geochronology - The Grand Canyon Association

Lava Dams and Lepidodendrons
Techniques used to date rocks and geologic
events in the Grand Canyon
• The Purpose – Why use dating techniques?
• The Science – “Clocks in the Rocks”
• The Results – Examples of key dates and
relationships used to understand the origin of the
Grand Canyon and its history
by Brian F. Gootee
Research Geologist – Arizona Geological Survey
GCFI Instructor
Adjunct faculty – Scottsdale Community College
Photo by Brian F. Gootee
Questions beget Questions
How do we know how old those rocks are?
How old is the Grand Canyon?
How did the Grand Canyon form?
How old is the Colorado River?
“Geologic Clocks” in the Rocks
Incremental dating
(tree rings, lake deposits, ice, etc.)
Radioactive decay
Fission track
Magnetostratigraphy
Tephra correlation
Cosmogenic nuclide dating
What is radioactive dating?
• It is a technique used to measure the amount of radioactive decay of
certain isotopes within a rock.
• An isotope is a species of an element, e.g. Carbon has 15 species!
• An isotope can be stable or unstable. Unstable ones decay
• Popular isotopes known to decay include:
– U238 Pb207
•
•
•
•
Ar40  Ar39
C14C12
– Parent isotope  daughter isotope
The clock starts when atoms crystallize from magma into minerals to make
an igneous rock. Isotopes are locked inside crystals when magma
solidifies. And parent isotopes begin to decay into daughter isotopes
By measuring the ratios of parent to daughter isotopes, one can
determine the age of the mineral
Ages such as 1,840 ± 1 million years are achievable
What can be dated? IGNEOUS ROCKS AND THEIR MINERALS
Zircon
Photo by Carl Zeiss
Monazite
Apatite
Parents & Their Daughters
 Over time, parent atoms decay & daughter atoms accumulate
all parent atoms
(one half-life)
(two half-lives)
more daughter atoms
time
Radioactive Decay
Time
Example: for an isotope with a half-life of 5,730 years (Carbon-14)
# of atoms
Parent
Daughter
P:D ratio
Animal dies and decays
1000 (100%)
0 (0%)
-
After 5,730 years (1 HL)
500 (50%)
500 (50%)
1:1
After 11,460 (2 HL)
250 (25%)
750 (75%)
1:3
After 17,190 (3 HL)
125 (12.5%)
875 (87.5%)
1:7
After 57,300 yrs (10 HL)
1 (<1%)
999 (>99%)
1:999
What Does An Isotopic Date Tell Us?
Can you date this rock?
Can you date this rock?
Yes.
No.
What Does An Isotopic Date Tell Us?
 Can radioactive dating tell us the age of this sedimentary rock?
 This does NOT tell us the age of the ROCK!
Example: detrital zircon analysis
Zhao and others, 2004
Dickinson and Gehrels, 2003
Dating
Pros ….….. and …...... Cons
• A reliable age
• Accurate (<1 – 10 %)
• Methods in tandem
provide the best
solution
• Brackets geologic
events, including
fossils
• Consilience
• Loss of daughter
• Gaining >1 parent
• Inherent problems
within the sample
• Expensive $$$
• Flawed assumptions
• Misinterpretation
Intermission & Poll
Dating the Grand Canyon
• Lava Dams at Vulcans
Throne
• Dating the oldest rocks
in the Canyon
• What about all the
fossils?
• DZ work in Yuma
• 4.8 Ma tephra
Lava Dams below Vulcans Throne
“What a conflict of water and fire there must have been here! Just
imagine a river of molten rock running down a river of melted snow.
What a seething and boiling of waters, what clouds of steam rolled into
the heavens!"
John Wesley Powell, August 25, 1869
Photo by Brian F. Gootee
Photo by Balsley, J.R.
Using chronology to understand process
Field Observations
• More than 150 individual lava flows built 13 individual lava dams
across the Colorado River (~22 km3 of lava spilled into the Canyon)
• At least one to five large lakes formed behind the dam, thought to
have backed up as far as Moab, Utah
• 5 catastrophic dam failures and resulting outburst floods
• Outburst flood deposits formed up to 200 m (660 ft) above river
level, as much as 53 km downstream. Boulders as large as 15
meters (50 feet) in length
Geochronology
• Recent Ar40/Ar39 and cosmogenic nuclide dates revised the ~1 My
age to individual flows between 525,000 to 100,000 years
• Dams may have formed rapidly within hours/days
• Formed short-lived lakes no longer than ~10 to 100 years
• Dates of lava flows also used to calculate incision rates for the
Colorado River
Outburst-flood deposit
165,000 +- 18,000 yr
Grey Ledge flow
Up river
After Crowe and others, 2008
The Oldest in the Southwest?
• Oldest rocks in the Grand Canyon belong to the Elves
Chasm pluton, 1,840 ± 1 Ma (U/Pb) (zircons)
• The younger Vishnu rocks (metamorphosed sand
and mud) contain detrital zircons from an older
province ~2,200 to 3,300 My age
• This was the “basement” for the Vishnu
• Grains in the Vishnu derived from Mohave desert
area and/or Australia, China or Antarctica
Elves Chasm pluton
Age-Dating Fossils?
• Fossils older than about
70,000 years cannot be
dated …. Yet
• Paleontologists study
enough fossils to bracket
the age of the rock.
• Nearly the entire
Paleozoic section from
Kaibab to Tapeats is dated
by correlating fossils with
other places on other
continents
Fossilized Lepidodendron trees
found in the Surprise Canyon
Formation along estuary
channels
How old is the Colorado River?
• The Colorado River
empties into the Salton
basin and Gulf of
California, The Colorado
River Delta
• By dating zircon sand
grains within the CR delta,
you can determine the
ages of source terrains
which supply the detrital
sands
• This method allows to
date the terrains within a
river’s watershed, like a
genetic “fingerprint”
Salton basin
Colorado River Delta
Paleolakes – a Top-Down System
• The Colorado River fed
paleolakes between
Lake Mead and Gulf of
California
• Each paleolake ended
by breaching a bedrock
paleodivide
• Timing of these events
is constrained by
Ar40/Ar39 dating of an
ash to 4.8 Ma
• Compared with a
database of over 5,000
dated ash samples
• We now know that the
Colorado River
completed its course to
the Pacific ocean 4.8
million years ago!
Grand Canyon factoids
• The oldest grains in the Coconino Sandstone go
back as far as 3,700 million years!
• In contrast, the oldest grains of sand in the base
of the Supergroup (1,000 My older) is “only”
1,750 My!
• The Paleozoic sequence in the Grand Canyon is
almost entirely dated by correlating fossils to
worldwide assemblages without numerical age
dating.
• Is there life on Mars? Or How old is the Grand
Canyon?
Questions beget Questions
How do we know how old those rocks are?
•
•
By measuring the amount of radioactive decay in rocks
By measuring the amount of decay of parent isotopes into
daughter isotopes within a rock
How old is the Grand Canyon?
• Depends on what part and depth
• Between 70 and 6 Ma
How did the Grand Canyon form?
• Mass wasting (99%) and erosion (1%)
• We don’t really know yet
• From the top below Lake Mead
How old is the Colorado River?
• Greater than 4.8 million years
Some common constraints of isotopes