varve chronology

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


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.


LICHENOMETRY


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.


Moraine

Alluvial fan


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.


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.


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?


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


Uplift

Size of particles?Thickness of units?Sediment supply?


Subsidence



Low Topography


SEDIMENTARY ARCHIVESCraton

Composed of two parts

Shield and Platform

Shield - exposed ancient crystalline rocks

Core of continent

Canadian Shield in North America


Craton

Platform – covering on shieldFlat to gentle dipUndisturbed since

preCambrianStable interior of continents


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


Accretionary Wedges(Coastal Plains)

Large wedges of sedimentaccumulate on margins of craton

Later deformed

Accumulate on trailing edge Deformed on leading edge


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


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.


varve chronology

Documents

numerical age

bymetamorphic rocks

earthmoon rocks

sedimentary rockswhat

sediment supply

new cosmogenic isotopes

surface of newlyexposed

thickness of units