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Transcript
OTHER NUMERICAL DATING TECHNIQUES
VARVE CHRONOLOGY
Lakes can produce annual layers.
Usually occur in glacial lakes or those that freeze
over in winter.
Coarser sediments are deposited in summer.
Winter-summer layers are called COUPLETS.
Couplets in lakes are known as VARVES.
Count the couplets back from the sediment surface
to determine numerical age.
OTHER NUMERICAL DATING TECHNIQUES
VARVE CHRONOLOGY
OTHER NUMERICAL DATING TECHNIQUES
LICHENOMETRY
Lichens are plant-like organisms
that grow on rocks.
Grow at a measurable rate.
By measuring size on items of
known date, the size is
plotted against size on
unknown aged objects.
Good for the last 9000 years.
OTHER NUMERICAL DATING TECHNIQUES
LICHENOMETRY
OTHER NUMERICAL DATING TECHNIQUES
SURFACE EXPOSURE DATING
Numerical dating can also be applied to landscape
features.
Features such as moraines or alluvial fans can make use
of COSMOGENIC ISOTOPES.
These isotopes are produced on the surface of newly
exposed rocks by bombardment of cosmic rays.
COSMIC RAYS are high velocity neutrons.
Penetrate rocks to a depth of 2-3 m. Go right thru you.
OTHER NUMERICAL DATING TECHNIQUES
Alluvial fan
Moraine
OTHER NUMERICAL DATING TECHNIQUES
COSMOGENIC DATING
Cosmic rays produce new “cosmogenic” radioactive
isotopes.
Si, Mg, Al, Fe are converted to 10Be.
K, Ca, Cl are converted to 36Cl.
Once formed they begin to decay.
OTHER NUMERICAL DATING TECHNIQUES
COSMOGENIC DATING
10Be has t1/2 of ~ 300,000 years.
36Cl has t1/2 of ~ 1.5 million years.
Fills the gap between 14C and K-Ar.
Problem exists that new cosmogenic isotopes are always
being created.
Not constant number like non-cosmogenic isotopes.
Also dealing with very small amounts of daughter
isotopes.
May be lost due to erosion.
AGE OF THE EARTH
Oldest Earth materials are zircons found in sandstone
in Australia
4.1-4.2 by
Metamorphic Rocks in NWT, Canada
Granites in Greenland
3.96 by
3.7 by
Meteorites have U-Pb and Rb-Sr dates of 4.6 by
AGE OF THE EARTH
Moon rocks dated between 3.3 and 4.6 by
Original lunar crust is 4.6 by
Moon and Earth formed at about the same time.
Earth is 4.6 by old.
SEDIMENTARY ARCHIVES
GE 142 focuses on sedimentary rocks
What makes them different than igneous and
metamorphic rocks?
SEDIMENTARY ARCHIVES
With sedimentary rocks we can consider:
Source area
Mode of transport
Physical processes active
Chemical processes active
Biological processes active
Climate
Lithification and Diagenesis
These may all influence a single unit
SEDIMENTARY ARCHIVES
Tectonic Influence
Deformation of large areas of the Earth’s crust over
large intervals of time
SEDIMENTARY ARCHIVES
Uplift
Size of particles?
Thickness of units?
Sediment supply?
SEDIMENTARY ARCHIVES
Subsidence
Size of particles?
Thickness of units?
Sediment supply?
SEDIMENTARY ARCHIVES
Low Topography
Size of particles?
Thickness of units?
Sediment supply?
SEDIMENTARY
ARCHIVES
Craton
Composed of two
parts
Shield and Platform
Shield - exposed ancient
crystalline rocks
Core of continent
Canadian Shield in North
America
SEDIMENTARY
ARCHIVES
Craton
Platform – covering on shield
Flat to gentle dip
Undisturbed since
preCambrian
Stable interior of
continents
SEDIMENTARY
ARCHIVES
Orogenic Belts
Elongate regions that border
craton
Usually highly deformed
Mountain belt
Earthquakes, volcanoes in
younger belts
Highly deformed or meta.
rocks in older belts
Exposed igneous intrusive
bodies
SEDIMENTARY
ARCHIVES
Accretionary Wedges
(Coastal Plains)
Large wedges of sediment
accumulate on margins
of craton
Later deformed
Accumulate on trailing edge
Deformed on leading edge
SEDIMENTARY
ARCHIVES
Tectonic setting determines
type of sedimentary
rocks
Sedimentary rocks can then
infer tectonic setting
SEDIMENTARY ARCHIVES
Environments of Deposition
All of the physical, chemical, biologic and geographic
conditions under which sediments are deposited.
Type of sediment influenced by climate and geography
The type of sediment is key to the environment
Use modern depositional environments to interpret past
UNIFORMITARIANISM or ACTUALISM
SEDIMENTARY ARCHIVES
Environments of Deposition
Sedimentary rocks may be:
Extrabasinal in origin
Sediments formed from the weathering of pre-existing
rocks outside the basin, and transported to the
environment of deposition
Intrabasinal in origin
Sediments form inside the basin; includes chemical
precipitates, most carbonate rocks, and coal.
By comparing modern sedimentary deposits with ancient
sedimentary rocks, the depositional conditions can be
interpreted.