AR-40 AR-39 GEOCHRONOLOGY OF BIOSTRATIGRAPHICALLY CONTROLLED MIOCENE TUFFS FROM CENTRAL JAPAN - COMPARISON WITH ITALY AND AGE OF THE SERRAVALLIAN-TORTONIAN BOUNDARY/

Two volcanic tuffs of the Tomioka area (central Japan) were selected f rom a fossiliferous marine succession of Late Senavallian age (Miocene ) and dated using the incremental heating Ar-40/Ar-39 technique. The K itamura Tuff, located 10 m below the regional first occurrence (FO) of the planktonic foraminifera Globigerina nepenthes, yielded a biotite age of 11.79 +/- 0.08 Ma. The Baba Tuff, located 180 m above the previ ous one, yielded ages of 11.26 +/- 0.09 Ma (biotite), 11.29 +/- 0.12 M a (sanidine) and similar to 11.3 Ma (plagioclase). Isochron ages are s imilar to the plateau ages. The FO of G. nepenthes is estimated to hav e an age of 11.76 +/- 0.10 Ma in Japan, Similarly precise results obta ined from the Central Apennines (biotite: 11.48 +/- 0.13 Ma) lead to a n age estimate of 11.53 Ma for the same FO in an Italian section. The age difference between sections is of the same magnitude as the analyt ical errors. However, the potential diachronism of the biostratigraphi c signal has been independently estimated at 0.25 Ma. The FO of G. nep enthes and FO of Neogloboquadrina acostaensis are commonly taken as in dex for the beginning of N14 and end of N15 foraminiferal biozones, re spectively. In the Italian section, the time interval between the two signals - i,e. the duration of the two biozones - may be estimated at 0.3 Ma. The Serravallian-Tortonian boundary located near the dated lay ers can be estimated to have an age of similar to 11.25 +/- 0.20 Ma.