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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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