Biostratigraphic correlation

If two widely separated rock units contain a sequence of identical zone fossils, then the rocks have the same relative age. Using graptolites, ammonites and some microfossils, rocks can be correlated worldwide. 

Methods of biostratigraphic correlation

1. First or last appearance - of zone fossils, but when fossil groups first appear they can be hard to find at first point, as they maybe very rare initially. The same applies towards the end of a fossil's range 
2. The range - can be very helpful when used with other fossils. Some fossils have a short time range these fossils are good zone fossils.
3. A fossil assemblage - when a number of different fossils are found in one bed 

Problems of bistratigraphic correlation 

• Many fossils are restricted to particular environments
• Some kinds of fossil are very longed- ranged - they dont evolve quickly so are not good zone fossils
• Derived fossils confuse the true sequence of beds
• Not all sedimentary rocks contain fossils. Inparticular, rocks laid down in glacial, fluvial and desert environments on land are unlikely to contain fossils