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Chronostratigraphic Correlation for dummies

November 22, 2010 Comments off


Summary: organises rocks on the basis of their age

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Biostratigraphic Correlation…

November 13, 2010 1 comment

Basic Overview: Using zone fossils found within rock units to relatively date them

Quote from Wiki (not as concise but explained much better):  focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at some other section. The fossils are useful because sediments of the same age can look completely different because of local variations in the sedimentary environment. For example, one section might have been made up ofclays and marls while another has more chalky limestones, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time.

(Basic explanantionof biostratigraphic correlation-the aim is to use a measured fossil record in sedimentary sections to see previous events in space and time. They organise local records and put them into the best order. There is more than one method to do this and there has been a lot of developments in these methods due to high speed computers.)

First/ Last  Appearance of a fossil

If we know when the fossil was first and last found in other sediments around the world we can say that the rock dates from between those two parameters.

(Parameter- is the word used when their is no actual date, it is a computation from data values recorded.)

Best fossils for the job

  • A fossil which is known to have a small range of existence, this allows you to date it more precisely
  • If there are two or more fossils present in the rock and their ranges are known to overlap this will further increase precision of the dating (see fig1. p282)
  • fossil assemblages are even better as they contain a wide range of fossils to which the above point can be applied

Problems

  • some fossils have very long ranges as their evolution was very slow, therefore they are relatively useless for biostratigraphic  correlation.
  • Good zone fossils such as graptolites are very delicate so specimens are rare as are destroyed except in very low energy enviroments.
  • derived fossils ruin biostratigraphic correlations as it will give the rock a much older age than it actually is
  • not all sedimentary rocks contain fossils particularly terrestrial deposits.
  • Many fossils are resticted to particular environments, for example benthonic invertbrates. So they are found only in some rock types.  Read more…
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