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7465257 
Journal Article 
Calcareous plankton biostratigraphy and Ar-40/Ar-39 dating of miocene volcaniclastic layers from Monferrato (NW Italy) 
D'Atri, A; Dela PIerre, F; Ruffini, R; Novaretti, A; Cosca, MA; Hunziker, JC 
2001 
Yes 
Eclogae Geologicae Helvetiae
ISSN: 0012-9402
EISSN: 1420-9128 
94 
2 
137-144 
English 
WOS:000171316800001 
The results of 40Ar/39Ar 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 87up 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 40Ar/39Ar dating were performed on biotite at Villadeati and on horneblende 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. 
biostratigraphy; geochronology; Early-Middle Miocene; volcaniclastic layers