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Jurassic-Cretaceous boundary bioevents and magnetochrons: A stratigraphic experiment
Robert W. Scott

Building: Muséum d'Histoire Naturelle de Genève
Room: Lobby /Hall/
Date: 2018-12-05 12:03 PM – 12:06 PM
Last modified: 2018-12-01


The search for a Global section and Point (GSSP) for the upper Jurassic-lower Cretaceous/Tithonian-Berriasian stage boundary has eluded stratigraphers for many years (Michalík & Reháková, 2011). Characterizing this boundary is difficult because few evolutionary events are dramatic and paleoenvironmental and tectonic changes are subtle and none are global. Paleomagnetic reversals are potential markers because they are recorded in marine and non-marine strata. Ammonites, calpionelids and calcareous nannofossils are the principle biota used to distinguish Tithonian-Berriasian-Valanginian stages (Michalík & Reháková, 2011).

This paper is a stratigraphic experiment to evaluate multiple criteria by applying a quantitative method to integrate the first and last appearances of these fossil groups with magnetochrons. Biostratigraphic checklists by well-known biostratigraphers have been published for thirty-eight well documented measured sections and deep-sea cores. The first and last occurrences (FOs/LOs) of taxa were cross plotted in X/Y diagrams with a geologic time scale (Ogg et al., 2016) to create hypotheses of synchroneity between section pairs. The correlation line of synchroneity extended the FOs and LOs relative to each other. The range extensions approximate first and last appearance datums (FADs, LADs) and were composited into a single range chart (LOK2CSDB) and calibrated to numerical ages (Ma). This stratigraphic experiment places the ammonite FADs in the same order as Reboulet et al. (2014) and Ogg et al. (2016) (Fig. 1). However the ammonite events project into the magnetostratigraphic scale slightly differently.

The FAD of Berriasiella jacobi at 145.7 Ma is close to the base of M19n at 145.4 Ma (Fig. 1). The FAD of Calpionella alpina (145.8 Ma) rather than its “explosion” is at magnetic polarity Chron M20-M19. Nannofossil bioevents are the FADs of Nannoconus globulus globulus at 145.1 Ma, Crucielllipsis cuvillieri at 144.8 Ma, and Zeugrhabdotus erectus at 145.0 Ma (Fig. 1). The LAD of Crassicollaria intermedia is at 145.8 Ma.

Base of the middle Berriasian Substage is the FAD of S. subalpina (144.1 Ma) at the base of the Subthurmannia occitanica Zone (Fig. 3). In LOK2CSDB the FAD of S. subalpina projects close to the base of Chron M18. Base of the upper Berriasian Substage is the FAD of Malbosiceras [Mazenoticeras] paramimounum (142.4 Ma) (Reboulet et al., 2014) at the base of the Subthurmannia boiseri Zone. New data projects this bioevent at about the boundary between chrons M17 and M16 (Fig. 1).

The base of the Valanginian Stage is at the FAD of Thurmanniceras pertransiens (139.7 Ma) and Calpionellites darderi (139.7 Ma) and above the FAD of Calcicalathina oblongata (141.2 Ma) (Reboulet et al., 2014) (Fig. 1). In this experiment base Valanginian correlates with the upper part of Chron M15 instead of polarity Chron M14r. The base of the upper Valanginian is at the base of the Saynoceras verrucosum Zone and its FAD is 132.3 Ma (Fig. 1).