A quantitative analysis of calcareous nannofossil bioevents of the Late Cretaceous (Late Cenomanian–Coniacian) Western Interior Seaway and their reliability in established zonation schemes

• Nannofossils in Cretaceous Western Interior (8) compared to other global (18) sites
• Key bioevents from nannofossil zonations and recent literature tested quantitatively
• Quantitatively derived optimum sequence of events produced with Ranking and Scaling
• Sequence reveals reliability of older zonations and changes from new/revised taxonomy.
• Results in high-resolution biostratigraphic framework for Late Cenomanian–Coniacian

Calcareous nannofossil assemblages from 8 localities in the Upper Cenomanian–Coniacian strata of central (Colorado and Kansas) and southern (Texas) Western Interior Seaway, USA were analyzed to produce a new high resolution biostratigraphic framework for the Greenhorn Formation, Eagle Ford Formation, and basal Austin Chalk. Eighteen datasets from coeval successions around the world, including the Global Stratotype Section and Point (GSSP) at Pueblo, CO and proposed reference sections for the base of the Turonian and candidate GSSPs for the base of the Coniacian in Germany, Poland, and New Mexico, were incorporated from literature sources. Additional discussion is devoted to the basal Turonian and Coniacian boundaries as they reflect paleoceanographic phenomena (e.g. OAE2, widespread Late Turonian regression) where high-resolution biostratigraphy has academic, as well as industrial, application.Ranking and Scaling of 48 selected bioevents produces a quantitatively derived optimum sequence of first and last occurrences. Scaling of events along a relative time-scale reveals an 8–9% evolutionary turnover during OAE2. Comparison with the qualitatively developed nannofossil zonations known as the CC zonation of Perch-Nielsen and UC zonation of Burnett reveals inconsistencies, largely due to the recognition of new/amended species concepts. Discrepancies near the Turonian/Coniacian boundary reflect changes to its placement in recent revisions to the geologic timescale. A recently documented sequence of bioevents provides an accurate proxy for the boundary in the absence of the inoceramid index fossil Cremnoceramus deformis erectus (Meek). Fifteen bioevents from the optimum sequence are given greater priority based on their interpreted reliability between the sections examined and the CC and UC zonations.