Biostratigraphic Units
Biostratigraphic units are defined by the presence of biological markers, usually fossils, which place the rock into a chronostratigraphic sequence.
Biostratigraphic units are defined by assemblages of fossils. Few biostratigraphic intervals are entirely distinctive as to an age of a rock, and often the best chronostratigraphic resolution that can be provided by biostratigraphy can be a range in ages from a maximum to a minimum when that fossil assemblage is known to have coexisted.
Biostratigraphic units can be defined by as little as one fossil, and can be marker beds or members within a formally identified Formation, for instance an ammonoid bearing bed. This can be a valuable tool for orienting oneself within a stratigraphic section or within a thick lithostratigraphic unit.
Biostratigraphic units can overlap lithostratigraphic units, as the habitat of a fossil may extend from areas where sediment was being deposited as sandstone into areas where it was a being deposited as a siltstone. An example would be a trilobite.
Other biostratigraphic markers are restricted to certain environments, for instance, graptolites are generally only found in shales.
Biostratigraphic units can also be used in archaeology, for instance where introduction of a plant species can be identified by different pollen assemblages through time or the presence of the bones of different vertebrate animals in midden heaps.
Read more about this topic: Geological Unit
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