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==Correlation of “tops”==

==Correlation of “tops”==

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. The inception (“*” in the illustration below), or lowest occurrence of a species or lineage, is a reliable datum only in core or outcrop samples because caving is virtually unavoidable in cutting samples; however, it can help refine the stratigraphy. The overlap of species extinctions and inceptions allows the development of range zones (see figure below), which can be correlated from site to site.

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. The inception (“*” in the illustration below), or lowest occurrence of a species or lineage, is a reliable datum only in core or outcrop samples because caving is virtually unavoidable in cutting samples; however, it can help refine the stratigraphy. The overlap of species extinctions and inceptions allows the development of range zones (see figure below), which can be correlated from site to site.

A biostratigraphic zone is a body of rock defined or characterized by its fossil content.<ref name=ch17r65>North American Commission on Stratigraphic Nomenclature, 1983, North American Stratigraphic Code: AAPG Bulletin, vol. 67, p. 841–875.</ref> The clustering of fossil extinctions often represents missing or condensed sections. Correlation of tops is the most rapid and economical biostratigraphic technique and is the one most commonly used.

A biostratigraphic zone is a body of rock defined or characterized by its fossil content.<ref name=ch17r65>North American Commission on Stratigraphic Nomenclature, 1983, North American Stratigraphic Code: AAPG Bulletin, vol. 67, p. 841–875.</ref> The clustering of fossil extinctions often represents missing or condensed sections. Correlation of tops is the most rapid and economical biostratigraphic technique and is the one most commonly used.

The illustration below shows how the overlap of species' ranges (between inception and extinction) is used to define zones.

[[:file:applied-paleontology_fig17-14.png|Figure 1]] shows how the overlap of species' ranges (between inception and extinction) is used to define zones.

 

==Planktonic vs. benthic “tops”==

==Planktonic vs. benthic “tops”==