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The Middle Oxfordian Platysphinctes event-horizon – an integrated tool in the correlations around the boundary of the Plicatilis and Transversarium zones in Europe

Glowniak E., Wierzbowski H.

Volumina Jurassica

2006

Vol. 4, no. 1

162-163

Palaeobiogeographic differentiation of the Late Jurassic faunas of the northern hemisphere was connected with temporal excursions and mixing of the Boreal-Subboreal and Mediterranean ammonites in the area of the Submediterranean epicratonic seas of the northern Tethyan shelf. These excursions are well recognized from the Late Oxfordian onwards (e.g. Atrops et al. 1993) but are less well-known in the Middle Oxfordian. However, the lower Mid-Oxfordian Platysphinctes event-horizon first recognized in central Poland (GĐowniak 2000), has been recently evidenced also in southern Poland and NW Germany. According to the new study (GĐowniak 2006b, in press) it appears in central Europe within the Arkelli Horizon of the Arkelli Subzone (Mid-Oxfordian Plicatilis Zone in the usage of the Submediterranean zonal scheme based on perisphinctid lineage, GĐowniak 2002, 2006a) and was a proxy for a biogeographical phenomenon referred to as the “Mediterranean Spread”. At that time, geographical barriers did not hinder communication between the Submediterranean and Tethys basins, which may point to the occurrence of the “Mediterranean Spread” in conditions of a sea-level rise. In accordance with the previous opinion remain geochemical isotope data based on stratigraphically well-dated belemnite rostra derived from the Mid-Oxfordian sections of central Poland (Wierzbowski 2002). A gradual positive shift in ä13C values, which starts at the Lower/Middle Oxfordian boundary, reaches its maximum in the Platysphinctes event-horizon. According to Wierzbowski (2002) the early Middle Oxfordian carbon isotope excursion may have been linked to transgression, which occurred during a longer period of the crisis in oceanic carbonate sedimentation. The early Middle Oxfordian positive carbon isotope shift was subsequently recognized in the sections of Scotland (Wierzbowski 2004, Pearce et al. 2005). The geochemical events from Poland and Scotland are simultaneous and should be correlated with the Platysphinctes event-horizon (cf. Wierzbowski 2002, 2004). As a consequence, the latter becomes a precise marker for the stratigraphic correlations of the Mid-Oxfordian zonal schemes in the upper Plicatilis Zone between the Submediterranean and Boreal-Subboreal Europe.