Archive for November, 2010

Biostratigraphic correlation : simplified

November 23, 2010 Comments off

Summary: This can be used to correlate rocks by looking at the sequence of zone fossils, if 2 widely separated rock units contain a sequence of identical zone fossils; they have the same relative age. Read more…

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Chronostratigraphic Correlation- Simple

November 23, 2010 Comments off

-This is where the rock is dated by linking the events which may have occured over large areas. e.g. sea level change Read more…

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biostratigraphic correlation: simples

November 22, 2010 Comments off

If two widely separated rock units contain a sequence of identical zone fossils, then the rocks have the same relative age. Using Graptolites, Ammomites and some micro fossils can be correlated worldwide.
there are 3 methods

1) first or last appearance, (extinction) of zone fossils, but when fossil groups first appear they can be difficult to find at the first point, as they may be very rare initially, the same applies towards he end of a fossil’s range> The correlation on the top diagram has been done using the first and last appearances for each of the fossil zones.

2) the range of a zone fossil can be very helpful when use with other fossils. Some fossils, have a short time range, others have a longer time range, but where 2 or more fossil ranges overlap, a particular biozone can be precisely determined. Zones are often named after an index fossil. The shorter time range ,the better the zone fossil.

3) a fossil assemblage is when the number of different fossils are found in one bed.

Problems of biostratigraphic correlation:

* many fossils, especially benthonic invertebrates are restricted to certain environments, e.g. sandy sea floor, so are found in only a few rock types.

* some kinds of fossils are very long ranged. Their rates of evolutionary change were very slow, so are no use in establishing biozones.

* good zone fossils such as the Graptolites are delicate, and only preserved in quiet environments , as they are destroyed in more turbulent conditions.

* derived fossils confuse the true sequence of beds. They will give a age far older than the rock, after erosion and redeposition.

* not all sedimentary rocks contain fossils, in particular, rocks laid down in glacial, fluvial and desert environments on l;and are unlikely to have any

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Oxygen Isotope Level: Second Round.

November 22, 2010 1 comment

summary These can help tell geologists temperature changes through time.

Oxygen has 3 stable isotopes (16O,17O,18O), but the majority(99.76%) of the worlds oxygen is 16O. There is also a noticeable percentage of 18O(0.2%)

When the water from the sea evaporates the lighter of the 3 isotopes 160 is removed easier than the others so there is more 180.

The ratio (relative amount) of these two types of oxygen in water changes with the climate. By determining how the ratio of heavy and light oxygen in marine sediments, ice cores, or fossils is different from a universally accepted standard, scientists can learn something about climate changes that have occurred in the past. The standard scientists use for comparison is based on the ratio of oxygen isotopes in ocean water at a depth of 200-500 meters.

Oxygen becomes trapped in the ice caps, this alters the balance of 160 and 18 This is noticeable when the global temperatures drops. More 180 is present during these periods.

Graph of oxygen 18 depletion as a function of temperature

As air cools by rising into the atmosphere or moving toward the poles, moisture begins to condense and fall as precipitation. At first, the rain contains a higher ratio of water made of heavy oxygen, since those molecules condense more easily than water vapor containing light oxygen. The remaining moisture in the air becomes depleted of heavy oxygen as the air continues to move poleward into colder regions. As the moisture reaches the upper latitudes, the falling rain or snow is made up of more and more water molecules containing light oxygen. So the oxygen levels go back to there original ways. This also happens when the ice melts as the amount of 160 returns to normal.

If there is a lot of 180 this will indicate glaciation and also less water if there is more ice. So sea levels are lower to than usual.

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Sea Level Curves: Simples

November 22, 2010 Comments off


1977 – Introduced by Dr Peter Vail and his team on behalf of ESSO Oil Company as a method for interpreting seismic cross sections. This means that they were taking a section of the sea bed, in this case a section used for drilling for petroleum, firing in seismic waves and measuring where they are reflected in the rock by unconformities. In the seas beds, this represents a change in sea level. This could be isostatic or eustatic. Read more…

Categories: Group Work

Chronostratigraphic Correlation for dummies

November 22, 2010 Comments off

Summary: organises rocks on the basis of their age

Read more…

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Lithostratigraphic Correlation – Simple

November 22, 2010 Comments off
  • Mostly based on using rock types in a series of beds.
  • However there is a greater chance it is a sequence of beds in succession, usually with a marker horizon.
  • A very distict bed that can be followed across all others is ideal to be used.

There are 3 methods of doing this:

  •  Corelation using a sequence of beds- if there are 2 layers of different rock together you can assume they were formed at the same time. If another area has the the same rock you can assume that they formed at the same time.
  • Bed thickness- if different areas have the same thicknesses of different beds in the same pattern if can be asummed they formed at the same time in the same conditions.
  • Rare minerals- if a rare mineral if present in 2 areas you can assume the rocks in the area formed at the same time.

Problems with this:

  • Lateral variation – Sedimentary deposits change in thickness laterally, sediment beds may be thicker but others may be thinner.
  • Diachronous Beds – Where one sediment type is laid down at different times
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