Biostratigraphic dating relies on
Figure 3 shows how a comparison of the presence and abundance of different components of the assemblage (e.g., in situ vs.reworked faunas and floras) may enhance local correlations and help us identify sediment source.Figure 2 shows how variations in the relative proportion of individual species within assemblages can be used to characterize correlatable fossil “populations.” Ratios or percentages of in situ vs.reworked calcareous nannofossil or palynomorph species may differentiate among distinct sediment packages.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.
Figure 1 shows how the overlap of species' ranges (between inception and extinction) is used to define zones.
The use of multiple fossil groups in the same sections can greatly increase both the resolution and the level of confidence in the analysis.
In the absence of index fossils (e.g., within areas of high clastic dilution or when extending a chronostratigraphic horizon across environmental boundaries), correlation methods other than the traditional use of fossil extinctions play an important role.
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.