Define biostratigraphic dating
Often biostratigraphic correlations are based on a fauna, not an individual species, as this allows greater precision.Further, if only one species is present in a sample, it can mean that (1) the strata were formed in the known fossil range of that organism; (2) that the fossil range of the organism was incompletely known, and the strata extend the known fossil range.Distribution of benthic forms is frequently restricted by basin configuration or other barriers to migration.Changes in the abundance or species composition of fossil assemblages within a biostratigraphic zone are useful in refining correlations.Biostratigraphy originated in the early 19th century, where geologists recognised that the correlation of fossil assemblages between rocks of similar type but different age decreased as the difference in age increased.The method was well-established before Charles Darwin explained the mechanism behind it—evolution.He named stages after geographic localities with particularly good sections of rock strata that bear the characteristic fossils on which the stages are based.In 1856 German palaeontologist Albert Oppel introduced the concept of zone (also known as biozones or Oppel zone).
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.
A zone includes strata characterised by the overlapping range of fossils.
They represent the time between the appearance of species chosen at the base of the zone and the appearance of other species chosen at the base of the next succeeding zone.
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.
Species extinctions, often referred to as “tops,” are used as horizons of correlation.