Proposal Of The Global Stratigraphic Section And Point (Gssp) For The Base Of The Ladinian Stage (Middle Triassic) - GSSP at the base of the Avisianum Subzone (FAD of Aplococeras avisianum) in the Bagolino section (Southern Alps, NE Italy)

The debate over the best position for the GSSP of the base of the Ladinian remained intense throughout the last decade. The main topics of this debate have been summarized in a recent STS/IGCP meeting in Veszprém, Hungary (September 2002). A major problem on the definition of the GSSP was there emphasized by Vörös (2002), and is related to the physical recognition of the boundary in stratigraphic sections of historical meaning in various regions. Among the proposed possibilities: the highest position (FAD of Eoprotrachyceras curionii) is probably accetable for researchers of the Alpine region, but cannot be identified with confidence in Hungary; on the contrary, low positions (i.e., base of Reitzi Zone, or FAD of Reitziites reitzi) are clearly identified in hungarian sections but are rarely seen in the Southern Alps. Vörös (2002) points out that at least five possible criteria exist for the Anisian – Ladinian boundary, all more or less satisfactory from the scientific point of view. He thus suggests to move towards a compromise, i.e., a criterion which allows the physical recognition of the boundary in the largest number of stratigraphic sections. Following these suggestions, in November 2002 we endeavoured an integration of the ammonoid biostratigraphy in the classical section of Bagolino in Lombardy (Brack and Rieber, 1986, 1993), that resulted successful (Mietto et al., in press). We are thus able to raise a compromise proposal for the GSSP of the base of the Ladinian at the FAD of Aplococeras avisianum at Bagolino. This boundary can be positively recognized at least in Hungary, in the Southern Alps, and in Nevada. According to Brack et al. (this volume), we accept Bagolino as a reliable stratigraphic section for the base of the Ladinian. Readers are invited to refer to Brack et al. (this volume) for the description of the section and the geology of the area; only minor adjustments to this part are here proposed.