How do fossils help in determining the relative age of strata?

Question

Are the three different localities around the same relative age? Explain your response.

Answer 1

Fossils can be used to help determine the age of strata, although an absolute global chronology based on fossils in not feasible without intensive "help" from chemical dating techniques.  This is because while some species can be proved to have gone globally extinct at a certain time, the majority of species are likely to evolve and go extinct at different times in different locations.

Biostratigraphy (as it is called) is very useful for creating localized timezones, but if you compare the accepted biozones from, say, China to those in the U.S.A. you would find that the zones had different names, and different start and end points.

Biostratigraphy is also limited in that it is possible that an ancestor species may predate its descendent by millions of years, but due to limited preservation potential, its descendent may precede it in the rock record, making dating very confusing.

This is not to say that biostratigraphy is useless at determining relative strata age.  On a local (e.g. basinal) scale, it can give a very good idea of relative strata age, by comparing the fossil assemblages in each strata.

Answer 2

Normally sedimentary rock only contains the fossil remains of past life and is the principal source of coal,oil and gas due to its preserving nature.Also,because sedimentary rock is made up of the remains of earlier rock,it provides clues to the Earth's geological history.

Fossils of species that lived for just a short span of geological time,and are widespread around the world,allow geologists to date the rocks in which they are found to a reasonably specific time period.Such fossils are known as 'index fossils'.