Quantitative biostratigraphy of Upper Albian to Lower Cenomanian (Cretaceous) calcareous nannofossils of mid-latitude regions

Abstract

Late Albian - early Cenomanian calcareous nannofossil assemblages from five mid-latitude localities (23-35 degrees) were analyzed using probabilistic and deterministic methods to produce comprehensive and quantitative biostratigraphic sequences that were compared to the widely used 'CC' and 'UC' calcareous nannofossil zonation schemes. This was done to examine their fidelity and general applicability for mid-latitude biostratigraphy and determine whether these quantitative methods can increase the resolution of said schemes. Four biozones spanning the upper Albian through middle Cenomanian were determined using unitary associations (UAs) and ranking and scaling (RASC) optimum sequence from which last appearance datum (LAD) of – Gartnerago stenostaurion, first appearance datum (FAD) – Lithraphidites eccentricus, and LAD – Staurolithites mutterlosei were determined to be reliable early Cenomanian markers at mid-latitudes, while the FAD and LAD of Calculites anfractus are unreliable bioevents and should not be used as a secondary marker for the Albian - Cenomanian boundary. In addition, we have revised the stratigraphic position of the FAD of L. eccentricus, which was previously assigned to the middle-Cenomanian. The proposed FAD was shortly after the Albian - Cenomanian boundary, before the FAD of Corollithion kennedyi. We conclude that UAs are more robust than RASC bioevents for this time sequence; however, both methods do not account for the asynchronicity of depositional and biotic chances. Despite these caveats, combining the results for both methods has proven to be a powerful quantitative biostratigraphic tool.