The quest for a unified Oxfordian/Kimmeridgian boundary: implications of the ammonite succession at the turn of the Bimammatum and Planula Zones in the Wieluń Upland, Central Poland

Abstract

The detailed biostratigraphic succession at the turn of the Bimammatum Zone and the Planula Zone in the Wieluń Upland, Central Poland, displays the five ammonite horizons, viz. the litocerum horizon and the broilii horizon in the Hauffianum Subzone of the Bimammatum Zone, and the minutum horizon, the proteron horizon, and the planula horizon, in the Planula Zone. The stratigraphic position of the Amoeboceras Layer (MATYJA & WIERZBOWSKI 1988) has been partly revised: it lies in the litocerum horizon of the Hauffianum Subzone of the Bimammatum Zone. The layer marks a short-time invasion of the Boreal/Subboreal ammonites into the Submediterranean Province, including such forms described previously as Amoeboceras (Plasmatites) bauhini (OPPEL), A. (P.) praebauhini (SALFELD), and A. (P.) lineatum (QUENSTEDT), as well as the recently discovered Pictonia densicostata BUCKMAN and Prorasenia bowerbanki SPATH. The presence of these ammonites, and especially of P. densicostata, shows that the lower boundary of the Subboreal Kimmeridgian corresponding to the base of the Baylei Zone, lies not higher than the litocerum horizon of the Hauffianum Subzone in the Submediterranean Succession; in fact, it may lie still lower, somewhere between an upper Bimammatum Subzone and a lower Hauffianum Subzone of the Bimammatum Zone. Precise location of the lower boundary of the Submediterranean Kimmeridgian, corresponding to the base of the Platynota Zone, is not possible in the Subboreal Succession; that boundary in the Boreal Succession lies at the base of the Kitchini Zone.