The results of Ar-40/Ar-39 dating integrated with calcareous plankton
biostratigraphical data performed on two volcaniclastic layers (VLs)
interbedded in Burdigalian to Lower Langhian outer shelf carbonate
sediments cropping out in Monferrato (NW Italy) are presented. The
investigated VLs, named Villadeati and Varengo, are thick sedimentary
bodies with scarce lateral continuity. They are composed of prevalent
volcanogenic material (about 87 up to 90% by volume) consisting of
glass shards and volcanic phenocrysts (plagioclase, biotite, quartz,
amphibole, sanidine and magnetite) and minor extrabasinal and
intrabasinal components. On the basis of their composition and
sedimentological features, the VLs have been interpreted as distal shelf
turbidites deposited below storm wave base. However, compositional
characteristics evidence the rapid resedimentation of the volcanic
detritus after its primary deposition and hence the VL sediments can be
considered penecontemporaneous to the encasing deposits.
Biostratigraphical analyses were carried out on the basis of a
quantitative study of calcareous nannofossil and planktonic foraminifer
associations, whilst Ar-40/Ar-39 dating were performed on biotite at
Villadeati and on homeblende at Varengo.
The data resulting from the Villadeati section have permitted to
estimate an age of 18.7 +/- 0.1 Ma for the last common occurrence (LCO)
of Sphenolithus belemnos whereas those from Varengo allowed to
extrapolate an age of 16.4 Ma +/-0.1 Ma for the first occurrence (FO) of
Praeorbulina sicana. This latter biovent is commonly used to approximate
the base of the Langhian stage, that corresponds to the Early-Middle
Miocene boundary.