Correlation of tops is the most rapid and economical biostratigraphic technique and is the one most commonly used.
Figure 1 shows how the overlap of species' ranges (between inception and extinction) is used to define zones.
A biostratigraphic zone is a body of rock defined or characterized by its fossil content.
The clustering of fossil extinctions often represents missing or condensed sections.
More than two hundred years ago, before formulation of the theory of evolution, it became apparent that the same general succession of faunas could be recognized in different rocks at widely separated locations.
Trilobites appeared before ammonites, for example, and dinosaurs became abundant before mammals.
The thousands of marine invertebrate fossils studied by Deshayes enabled Lyell to develop a number of subdivisions of the Tertiary of the Paris Basin based on the quantification of molluskan species count and duration.By the mid-1800's the New York Surveys had resulted in the development of a relative stratigraphic zonation based primarily on fossil distribution.New York localities are world famous for Cambrian through Devonian strata and fossils, but of particular importance to this website discussion is the contribution of the Ordovician rocks of central New York State to the establishment of a North American focused biochronology.Species extinctions, often referred to as “tops,” are used as horizons of correlation.The first downhole occurrence (“ ” in the illustration below) in a well section is the datum most commonly used.