Geoscience Reference
In-Depth Information
Chapter 9
Quantitative Stratigraphy, Splining
and Geologic Time Scales
Abstract Quantitative stratigraphy uses logical and mathematical tools to help
define the stratigraphic framework of the Earth's crust. Biostratigraphy uses obser-
vations 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 posi-
tions 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 include 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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