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Biostratigraphic correlation : simplified

November 23, 2010

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.
Zone fossils can include
 • Graptolites
• Ammonites
• Some microfossils

 Methods
• Using the first or last appearance of each zone fossil.
 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 the end of the fossil range.
• Range of a zone fossil can be very helpful when used with other fossils.
Some fossils have a short time range, others have a long one, but where 2 or more fossil ranges overlap, a particular biozone can be determined. Zones are often named after an index fossil. The shorter the time ranges the better the zone fossil.
• Fossil assemblages When a number of different fossils are found in one bed.

Problems
• Many fossils e.g. benthonic invertebrates are restricted to particular environments, for example, lime-mud sea floor, so are just found in a few rock types.
• Some fossils have a very long range, their rates of evolution were slow, and so are no uses at establishing biozone.
• Good zone fossils e.g. graptolites are delicate so only preserve in quiet environments, being destroyed by more turbulent conditions.
• Derived fossils confuse the true sequence of beds. They will give an age far older than the rock they have ended up in after erosion and redeposition
• Not all sedimentary rocks contain fossils. In particular, rocks laid down in glacial, fluvial and desert environments on land are unlikely to have any fossils.

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