Geoscience Reference
In-Depth Information
Keywords Quantitative stratigraphy • Biostratigraphic correlation • RASC •
Ranking • Scaling • Smoothing splines • Geologic timescale • Northwestern
Atlantic margin • Grand banks • North Sea basin • Californian Eocene
nannofossils • Tojeira sections • Cenozoic microfossils • Cambrian trilobites
• Riley formation • Cretaceous Greenland-Norway seaway
9.1 Ranking and Scaling
RASC is an acronym for RAnking and SCaling of biostratigraphic events. Code of
the RASC computer program was originally published in Computers & Geosciences.
During the past 30 years this program has been continuously maintained and updated.
Its purpose is to combine biostratigraphic data from land-based sections or explo-
ratory wells drilled in sedimentary basins to construct a regional biozonation that can
be used for correlation between sections within a study area. The companion program
CASC (Correlation And Scaling) makes use of splining which is a powerful method
of curve-fitting. It is applied to correlate between stratigraphic sections on the basis of
ranking and scaling results for samples taken at irregular sampling intervals. Impli-
cations of sampling of stratigraphic sections are discussed in detail. Several examples
of past successful large-scale RASC/CASC applications will be given. The original
RASC method has been discussed in detail in Gradstein et al. ( 1985 ), Agterberg
( 1990 ), Agterberg and Gradstein ( 1999 ) and Agterberg et al. ( 2013 ). CASC was
introduced in Agterberg et al. ( 1985 ). In this section, calculation of the “optimum”
sequence resulting from ranking will be explained using a simple, artificial example.
Subsequent scaling of the ranked optimum sequence can be useful for the construc-
tion of regional biostratigraphic event zonations.
Regional standard zonations resulting from either ranking or scaling can be used
for correlation between stratigraphic sections. Initially, the RASC/CASC programs
were written in FORTRAN for mainframe computers. Original code has remained
part of all later versions although it was modified and extended repeatedly. An
executable file for RASC & CASC Version 20 with manual and documentation can
be downloaded from a website maintained by the University of Oslo ( http://www.
nhm2.uio.no/norlex/rasc ) . Complete code for Version 20 was made available
together with Agterberg et al. ( 2013 ). This latest software aims to provide easy
access to the RASC outputs in a user-friendly, interactive fashion and incorporates
CASC code for correlation between sections. Additionally, it contains code for less-
known techniques such as RASC analysis of variance and depth scaling. ActiveX
(OCX) Development Environment was chosen to implement all graphic modules
on a Windows platform (Liu et al. 2007 ).
9.1.1 Methods of Quantitative Stratigraphy
Quantitative stratigraphy uses logical and mathematical tools to help define the
stratigraphic framework of the Earth's crust. Biostratigraphy uses observations on
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