A QUANTITATIVE RE-ANALYSIS OF THE LATE CRETACEOUS NANNOFOSSIL ZONATION, WESTERN INTERIOR SEAWAY

In 2014, Corbett et al. published a Late Cretaceous high-resolution biostratigraphic framework, using 48 nannofossil bioevents in the Greenhorn, Eagle Ford and basal Austin Chalk formations of Colorado, Kansas and Texas. The Ranking and Scaling (RASC) method with PAST software was used to calculate the most likely optimum sequence of first and last occurrences, with literature data added from eighteen GSSP and other classical sections worldwide.

We have re-evaluated the Corbett et al. optimum zonation, using the orginal RASC version 20 method and its software, which has superior error analysis and graphics. This study directly evaluates the RASC method in two different (both public) software packages. Data handling and formatting issues were easily addressed using QSCreator software bundled with RASC20.

The only real difference in optimum sequence (zonation) results is in placement of L.maleformis FAD and P.fenestrata LAD, one of which has extreme variance. RASC is more conservative in stratigraphic placement of these ‘low value markers’, and not so sensitive to stratigraphic outliers noise. The removal of one taxon and 9 records of 8 other noisy taxa improved the Corbett et al. zonation, which in its final format includes 43 nannofossil bioevents. RASC variance analysis is an elegant and rapid procedure to weed out outliers in stratigraphic datasets.