40Ar/39Ar dating of a Langhian biotite-rich clay layer in the pelagic sequence of the Conera Riviera, Ancona, Italy

A nearly complete and undisturbed Miocene carbonate sequence is present in the easternmost part of the Umbria- Marche basin, Italy, which is ideal for detailed and integrated stratigraphic investigations of the Miocene Epoch. In this study, we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 Ma Ries impact structure in southern Germany, located about 600 km to the north-northwest of the Umbria-Marche basin. The first step is to find coeval strata in the Umbria-Marche sequence. At the La Vedova section, Co`nero Riviera, we dated a volcaniclastic biotite-rich clay layer, the Aldo Level, which is situated within planktonic foraminiferal Zone N8, at 14.9‘0.2 Ma, using the 40 Ar/39 Ar method. Together with detailed geologic and stratigraphic information about the Aldo Level, the resulting age can be used confidentially to calibrate the Langhian stage. Besides providing new constraints on Miocene geochronology, this age can now be used for impact stratigraphic studies. To directly correlate the biotite ages of the La Vedova section with rocks from the Ries impact event, Ries impact glass was also analyzed and found to be coeval. Although unrelated to this impact event, the biotite-rich clay layer should help in the search for evidence of distal ejecta related to the Ries crater. Œ 2001 Elsevier Science B.V. All rights reserved.