EFFECTIVE AND REALISTIC DEFINITION OF THE BASE OF THE BERRIASIAN STAGE (J/K BOUNDARY)

WIMBLEDON, W.A.P.; REHÁKOVÁ, D.; SVOBODOVÁ, A.; SCHNABL, P.; PRUNER, P.; KDYR, S.; FRAU, C.; BULOT, L.C.; RICCARDI, A.C.; IGLESIA-LLANOS, M.P.; KIETZMANN, D.A.; LÓPEZ MARTÍNEZ, R.

Simposio; 11th International Cretaceous Symposium; 2022

Any selected GSSP boundary should preferablyconform to traditional usage.  The J/Kboundary was first placed at the Portlandian/Purbeckian junction, near the subsequentlyused Jacobi ammonite zone.  Latterly, theTithonian/Berriasian boundary has been accepted as occurring withinmagnetosubzone M19n.2n, with the Calpionellaalpina Subzone as the best/most-widespread boundary marker­­­.  This last sits just below the “fingerprint” M19n.1r(the 30,000 yr “Brodno “).  In 2016, aformal vote of the 70-strong Berriasian WG (ISCS) fixed this usage. Tré Maroua(Hautes-Alpes) was subsequently proposed as the GSSP.All regions have been studied, in marineand non-marine facies, in the classical Berriasian regions (Berrias, Le Chouet,Puerto Escaño, Crimea etc.) and far beyond: key microfossil marker species allowcorrelation from Tethys to Mexico, the Andes and California.  Using magnetostratigraphy, isolated marine(Arctic Russia) and non-marine basins (Purbeck facies) have been correlated.  Suggested tested primary markers that were inconsistentor uncalibrated were rejected, and some were dismissed as thoroughlyunsuitable.  We examined these datums (ascendingorder):·        Subzone bases Praetintinopsella(not always identified) and Remanei (hardly and not always recorded).  Mid Tithonian, in M20, 1my+ earlier than the acceptedBerriasian base.·        M19n base: no coincidence with widespread/consistentbiomarkers·        Jacobi Zone base:  +- base M19n.2n, but Strambergella jacobi FO is considerably higher:  limited geographic extent.·        Alpina Subzone base, midM19n.2n: closest definable level to Jacobi Zone base – widespread and consistent,associated with nannofossil FOs, and an ammonite marker.  Just below M19n.1r: and close to d’Orbigny’soriginal Portlandian/Purbeckian boundary (sub-boreal)·        M19n.1r: not always detected, short,but a key secondary marker·        M18r base: already 1my+ laterthan traditional stage base. No fixed biomarkers.·        Ferasini Subzone: not alwaysclearly identified, variable positions M19n/M18r·        Elliptica base: lower M17r/M18n.  M17r is longest reversal.  ·        Occitanica base: varying levelsin mid M17r.  Index ammonites aretaxonomically problematic or sometimes absent (T. subalpina, T. occitanica).  No nannofossil FO coincides. In the boreal,this is an impossible correlative level: M17r (doubtfully identified?) is theshortest magnetozone present.