The RASC method for ranking and scaling of biostratigraphic events

The RASC method for ranking and scaling of biostratigraphic events was developed by the authors and associates during the past 20 years. The purpose of ranking is to create an optimum sequence of events observed in different wells or sections subject to stratigraphic inconsistencies in the direction of the arrow of time. These inconsistencies, which result in crossovers of lines of correlation between sections, are due to various sampling errors and other sources of uncertainty including reworking and misclassification. They can be resolved by statistical averaging combined with stratigraphic reasoning. Subsequent scaling of the events can be carried out by estimating intervals between successive events along a relative time-scale. This results in the scaled optimum sequence. Either the ranked optimum sequence or the scaled optimum sequence can be used for biozonation and for correlation between sections with error bars to denote precision of the correlation. The observed positions in the sections of different biostratigraphic events can have different degrees of precision. These differences can be evaluated by analysis of variance. Other types of stratigraphic events including logmarkers can be integrated and incorporated with the biostratigraphic events. The purpose of this review paper is to provide a succinct description of RASC as it is today using the Cretaceous microfossil example of Gradstein et al. [Gradstein, F.M., Kaminski, M.A., Agterberg, F.P., 1999. Biostratigraphy and paleoceanography of the Cretaceous Seaway between Norway and Greenland. Earth-Science Reviews] for illustration.