40AR/39AR 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 from a fossiliferous marine succession of Late Senavallian age (Miocene) and dated using the incremental heating 40Ar/39Ar technique. The Kitamura 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 previous one, yielded ages of 11.26 +/- 0.09 Ma (biotite), 11.29 +/- 0.12 Ma (sanidine) and similar to 11.3 Ma (plagioclase). Isochron ages are similar to the plateau ages. The FO of G. nepenthes is estimated to have an age of 11.76 +/- 0.10 Ma in Japan, Similarly precise results obtained from the Central Apennines (biotite: 11.48 +/- 0.13 Ma) lead to an 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 analytical errors. However, the potential diachronism of the biostratigraphic signal has been independently estimated at 0.25 Ma. The FO of G. nepenthes and FO of Neogloboquadrina acostaensis are commonly taken as index for the beginning of N14 and end of N15 foraminiferal biozones, respectively. 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 layers can be estimated to have an age of similar to 11.25 +/- 0.20 Ma. Two volcanic tuffs of the Tomioka area (central Japan) were selected from a fossiliferous marine succession of Late Senavallian age (Miocene) and dated using the incremental heating 40Ar/39Ar technique. The Kitamura 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 previous one, yielded ages of 11.26 +/- 0.09 Ma (biotite), 11.29 +/- 0.12 Ma (sanidine) and similar to 11.3 Ma (plagioclase). Isochron ages are similar to the plateau ages. The FO of G. nepenthes is estimated to have an age of 11.76 +/- 0.10 Ma in Japan, Similarly precise results obtained from the Central Apennines (biotite: 11.48 +/- 0.13 Ma) lead to an 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 analytical errors. However, the potential diachronism of the biostratigraphic signal has been independently estimated at 0.25 Ma. The FO of G. nepenthes and FO of Neogloboquadrina acostaensis are commonly taken as index for the beginning of N14 and end of N15 foraminiferal biozones, respectively. 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 layers can be estimated to have an age of similar to 11.25 +/- 0.20 Ma.