Biostratigraphy is the branch of
stratigraphy which 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 of
clays 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.
