Quantitative stratigraphy uses logical and
mathematical tools to help define the stratigraphic
framework of the Earth’s
crust. Biostratigraphy uses observations on fossil taxa. Biostratigraphic events commonly used for
this purpose are the observed first and last occurrence (abbreviated to FO and
LO) of each fossil taxon considered. Co-occurrences of fossil taxa in the biostratigraphic record can be used as
well. Methods for the integration and long-distance correlation of observed biostratigraphic events include the RASC
method for RAnking and SCaling. The main difference between RASC and other methods of
regional biostratigraphic correlation is that RASC
estimates the relative positions of average fossil events instead of maximal
time-stratigraphic ranges, although maximal ranges also can be obtained by
using RASC. Different methods of quantitative stratigraphy are briefly reviewed in
this chapter. Initially, ranking is illustrated by application to a simple,
artificial dataset. Scaling is explained as a refinement of ranking.
Implications of techniques of sampling stratigraphic sections are discussed.
RASC probable positions with error bars can be determined
in different sections
for CASC correlation over long distances. This process makes use of spline-curve fitting (splining). For method comparison,
several datasets published by others are re-analyzed, not only to establish
regional biostratigraphic standards but also to
perform correlations between stratigraphic sections. These datasets
include FOs and LOs of Eocene nannofossils in wells drilled in
California and trilobites from the Cambrian Riley Formation in central Texas.
Large-scale RASC/CASC applications
involving many thousands of observations
results for well data
from the Cenozoic North Sea basin, northwestern Atlantic margin and the
Cretaceous seaway between Norway and Greenland. Paleoceanographic interpretations of RASC biozonations supplemented by analysis
of variance to study diachronism and correlations between wells are exemplified as well.
The international
numerical geologic time scales have been and continue to be partially based on spline-curves fitted to relate
age determinations on rock samples to their positions in the relative geologic
time scale that is based on classifications of rock units that can be
correlated worldwide. Methods of time scale construction are discussed at the
end of this chapter.
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