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 organismsthat grow on rocks.
Grow at a measurable rate.By measuring size on items of
known date, the size isplotted 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 useof COSMOGENIC ISOTOPES.
These isotopes are produced on the surface of newlyexposed 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
Moraine
Alluvial fan
OTHER NUMERICAL DATING TECHNIQUES
COSMOGENIC DATING
Cosmic rays produce new “cosmogenic” radioactiveisotopes.
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 DATING10Be 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.
OTHER NUMERICAL DATING TECHNIQUES
Oldest Earth materials are zircons found in sandstonein Australia 4.1-4.2 by
Metamorphic Rocks in NWT, Canada 3.96 by
Granites in Greenland 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 byOriginal lunar crust is 4.6 by
Moon and Earth formed at about the same time.
Earth is 4.6 by old.
AGE OF THE EARTH
SEDIMENTARY ARCHIVES
GE 142 focuses on sedimentary rocks
What makes them different than igneous andmetamorphic rocks?
SEDIMENTARY ARCHIVES
With sedimentary rocks we can consider:
Source areaMode of transportPhysical processes activeChemical processes activeBiological processes activeClimateLithification and Diagenesis
These may all influence a single unit
SEDIMENTARY ARCHIVESTectonic 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 ARCHIVESCraton
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 shieldFlat to gentle dipUndisturbed since
preCambrianStable interior of continents
SEDIMENTARY ARCHIVES
Orogenic Belts
Elongate regions that bordercraton
Usually highly deformedMountain belt
Earthquakes, volcanoes inyounger belts
Highly deformed or meta.rocks in older belts
Exposed igneous intrusivebodies
SEDIMENTARY ARCHIVES
Accretionary Wedges(Coastal Plains)
Large wedges of sedimentaccumulate on margins of craton
Later deformed
Accumulate on trailing edge Deformed on leading edge
SEDIMENTARY ARCHIVES
Tectonic setting determinestype of sedimentary rocks
Sedimentary rocks can theninfer tectonic setting
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 originSediments 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.
SEDIMENTARY ARCHIVES
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