Progress on the Definition and Global Correlation of the Juras-Sic/cretaceous Boundary – Berriasian Working Group Report

The Berriasian Working Group (BWG) recommends that the Jurassic/Cretaceous (J/K) boundary should be shifted downward into the present-day upper Tithonian. The base of the Calpionella Zone (i.e., the base of the Calpionella alpina Subzone) has proven problematic as the primary marker for the J/K boundary. The C. alpina Subzone is an abundance biozone, and its definition is not sufficiently well constrained. The internal structure of this event is poorly known; recently obtained quantitative data indicate that several “explosions” of C. alpina may be observed within a single section, or that the event record may be noisy, with several fluctuations in relative C. alpina abundance. Finally, astrochronologic calibration of pelagic succesions has shown that the position of the base of C.alpina Subzone may vary by ca. 0.4 Myr between sections. Therefore, the calpionellid events within the underlying Crassicollaria Zone, calcareous nannozone NJT17, or magnetic (sub)chrons M20n1r and M19r are regarded as more suitable candidate markers for the J/K boundary. The interval correlates with the transition between the M. microcanthum and P. andreaeai Tethyan ammonite zones. It coincides with major changes in planktonic communities, including the decline and eventual disappearance of Saccocoma, the diversification and proliferation of calpionellids, and the expansion of heavily calcified nannofossils, particularly nannoconids. Large-scale palaeoenvironmental perturbations have been related to progressive aridification, reduced terrigenous and nutrient input, as well as decreased surface-water fertility; those have also been documented in multiple localities outside the Tethyan domain. The proposed repositioning of the J/K boundary into the late Tithonian would coincide with the middle/upper Volgian boundary interval in the Panboreal Realm, where it could be traced with the precision of a single ammonite biohorizon. Correlation based on buchiid bivalves may be further extended from the Russian and Siberian platforms towards the Arctic regions, northern California, Far East, and northern China. Radiolarian stratigraphy can be roughly correlated between the UAZ zones in the Tethyan domain, the western Siberian and Sub-Boreal domains, as well as the Carribean region. Marine Sub-Boreal successions in the North Sea Basin are correlated using sequence stratigraphy, with an additional control of organic dinocyst, radiolarian and palynostratigraphy. The boundary between the second-order sequences J73 and J74, or between TMS2 and TMS3 according to various authors, lies stratigraphically very close to the middle/upper Volgian boundary. Organic dinocysts are particularly important due to their widespread occurrence between the Arctic and Tethyan domains, verifying the consistency of the Boreal–Tethyan correlations. Correlation with the Neuquén Basin (South America) is also possible, based on magnetic stratigraphy and microfossils, as well as recently performed astrochronologic calibration of the Argentinian and Tethyan sections. The M20n1r-M19r interval in the Neuquén Basin coincides with the boundary interval between W. internispinosum and C. alternansammonite zones.