New advances on the definiton on the Jurasic/ Cretaceous boundary : short report on the activity of the IUGS ICS Berriasian Working Group.

Lower boundaries of only two Cretaceous stages are not yet formally defined. One of them is the Global Stratotype Section and Point (GSSP) for the base of the Berriasian stage, being the system boundary between the Jurassic and Cretaceous (Gale et al., 2020). The Berriasian Working Group (BWG) works intensively since 2021 to verify the most reliable primary markers and indicate the reference section for the J/K boundary. Among possible markers, the acme event of Calpionella alpina as the base of its nominal subzone (= base of Calpionella Zone, see e.g., Remane et al., 1986; Reháková & Michalík, 1997; Lakova & Petrova, 2013; Benzaggagh, 2020) was critically evaluated, as it is still the primary marker for the J/K boundary proposed by the previous BWG (Wimbledon et al. 2020a, b).
Beside this option, the BWG considered a possibility to shifting down the boundary to the present-day upper Tithonian, or rise up it to the base of the upper Berriasian. Majority of the BWG in the recent ballot in June 2025 voted against the C. alpina option (12 against 6 Members, 2 Members did not vote), supporting shifting the J/K boundary down into the present-day upper Tithonian. The primary markers which might be further considered are e.g. base of the Crassicollaria Zone, base of the Cr. intermedia Subzone, base of the NJT17 calcareous nannozone, base of the Protacanthodiscus andreai ammonite zone (Szives et al., 2024) or a magnetozone boundary (e.g. base of M19r). As
one of the main objections against the base of C. alpina Subzone as primary marker, the BWG Members raised a lack of clear quantitative definition of the C. alpina event, which resulted in confusion when different teams studied the same section, and situated the C. alpina event at significantly different levels with an offset of up to 2-3 m (e.g., Houša et al. 1996 vs Michalík et al., 2009; Wimbledon et al., 2020b vs Granier et al., 2023). Another inconsistency resulted in problematic definition of the Cr. colomi Subzone, as the total range of the index taxon is not well constrained (e.g., Lakova & Petrova 2013; Petrova et al., 2025; Kowal-Kasprzyk in Lodowski et al., 2025).
Among other tasks, the BWG proposed provisional chronostratigraphic correlation schemes of the Tethyan domain with Russian-Siberian and Artic domains, South America (Neuquen basin), and NW Europe. GSSP section is not yet decided. Those which reveal the highest correlation potential are apparently the pelagic Italian sections: Torre de’Busi (Lombardian Basin), Bosso (Apennines). Likely candidates might be also the Las Alcantarillas and Las Loicas sections (Argentina), with multiple radiometric dates which are lacking in the Tethyan domain (e.g. Lena et al. 2019). The current results of the BWG activities were recently presented and discussed at the 12th Cretaceous Symposium in Hannover (31 Aug-5 Sept, 2025).