biostratigraphic

• Microfossils are useful zone fossils used to date layers of rock, macrofossils are too large and would break during borehole extraction. 
• Foraminifera - small, marine and calcareous. Thousands of species can be identified to indicate environment. 
• Calcareous nannofossils - coccolithophore, widespread. 
• Dinoflagellates - planktonic, species vary due to temperature, salinity and depth. 
• Palynomorphs - pollen and spores, resistant to weathering and indicate environment 
• 1. Matching units to be of the same age using initial and final appearance of a zone fossil. First appearance is not always detected as new species are rare, the final appearance can be difficult due to dwindling numbers for a long period of time. 
• 2. Fossil assemblage - a range of fossils found in a bed. 
• Disadvantages:

  • Many fossils are restricted to certain environments and rock types 
  • Some fossils have long time ranges and little evolution 
  • Some fossils are fragile 
  • Derived fossils are confusing 
  • Not all sedimentary rocks contain fossils 
• The fossil is the key factor not the rock type.