New radiometric dating of volcanic ash layers in Periadriatic foredeep basin system, Italy

New geochronological data are reported for two key ash beds interbedded in Upper Miocene (Messinian)-Early Pleistocene rocks at Maccarone, Bellante, and Mosciano S. Angelo on the Adriatic side of the Italian peninsula. Major element chemistry of glass shards was determined with electron microprobe analysis on the younger key bed. The older, Messinian-age ash bed, sampled at Maccarone, yields a corrected glass fission-track age of 5.38 +/- 0.42 Ma. The younger ash bed, sampled at Mosciano S. Angelo and at two localities at Bellante, yields corrected glass ages of similar to 2.1 Ma, similar to ages previously determined on Pliocene-Pleistocene ash collected along the Ionic coast and on the Tyrrhenian side of Italy. Apatite fission-track dating of one of the Bellante samples confirms the glass age, whereas apatite from the Mosciano S. Angelo sample yields an excessively old age due to the presence in the sample of a significant component of detrital grains. An unusual bimodal spontaneous track-diameter distribution is found in the Maccarone glass, similar to the distributions previously obtained on other Messinian-age tephra samples from the same region. This anomalous track-diameter distribution is interpreted as a thermal disturbance coeval with the volcanic activity that produced the Pliocene-Pleistocene key volcanic ash bed sampled at Bellante and Mosciano S. Angelo. The contemporaneity of this ash with the event detected by the Maccarone glass suggests that it was related to the tectonic phase which marked, with numerous erosive levels distributed along the whole Apennine region, the end of the Pliocene sedimentary cycle and the onset of the Pleistocene cycle. (C) 2000 Elsevier Science B.V. All rights reserved.