Dating

Question 1

Outline stratigraphy

Answer 1

• Principle of original horizontality by Nicholas Steno
• Sedimentary units are deposited essentially horizontal
• Principle of superposition
• Younger layers are on top of older layers – stratigraphic succession
Basic:
• Beneath the sea, sediments are arranged in order of age
• Tectonic activity then uplifts and reveals them and erosion takes place on the youngest sediments

Unconformities (break in time):
• Beneath the sea, sediments are arranged in order of age
• Tectonic activity then uplifts and reveals them and erosion takes place on the youngest sediments
• Erosion removes an entire layer and part of another
• Subsidence beneath the sea allowed a new layer to be deposited over the new top layer
• The irregular surface above is now an unconformity

Question 2

Types of unconformity:

Answer 2

Nonconformity: Separates igeneous or metamorphic rocks from sedimentary rocks
Disconformity: Lower (older) sediments are not tilted
Angular Unconformity: Lower (older) sediments are tilted
Paraconformity: strata are parallel with little apparent erosion

Question 3

What are lateral variations and facies?

Answer 3

• Facies is the sum total of all physical and biological and chemical characteristics of a rock at the time of deposition
• Facies are the many different sediments and resulting rocks that form at the same time, but in different depositional environments
• Sea level lowers = regression = sediments coarsen upward
• Sea level raises = transgression = sediments fine upward

Implications:

• Rocks of a certain age may not be the same age as the same rock in a neighbouring area
• Lithostratigraphy = correlation based on rocks

Question 4

What is sequence stratigraphy?

Answer 4

• Seismic mapping of sedimentary strata
• Seismic stratigraphy, Basic unit is called a ‘sequence’
• Beds are bounded above and below by an unconformity

Question 5

Calculating time using accumulation?

Answer 5

Time=H/R

T = height / sedimentation rate

Question 6

Dating principles?

Answer 6

Principle of crosscutting relationships
• A cross-cutting feature (e.g. fault / dyke) is younger than the thing it cuts
Principle of included fragments
• If something is included in something else it must be older than the thing in which it is included

Question 7

What is absolute dating?

Answer 7

• Geochronology
• Based on radisotopic dating
• Calibrated to a numerical time scale
Most common – parent isotope U(238 or 235) – daughter – Pb(206 or 207) – covers all geological time – found in Zircon

Question 8

Pb/U Geochronology?

Answer 8

The best example of this dating method is U/Pb dating of zircons.
•When zircon crystallises U4+ can readily substitute for Zr4+
•Pb2+, which has a very different ionic radius, is excluded from the crystal
lattice.
•The result is that almost 100% of the Pb in a zircon is derived from the radioactive decay of U.
• Zircon is also exceptionally resistant to disturbance so it retains it’s age
information far better retained than for isotope systems and minerals.
• U comprises two radioactive isotopes 238U and 235U, which decay to 206Pb
and 207Pb, respectively, but at very different rates. Thus zircons have two
radioactive clocks
•The pair of equations above can be used to define the locus of points for
which 238U-206Pb and 235U-207Pb ages concur, the locus being called the Concordia

Question 9

Other Geochronology?

Answer 9

Re-Os molybdenite Geochronology