The determination of radiolarite ages of supraophiolitic rocks date the
expansion age of oceanic crust. Radiolarites from the Gets nappe, a
decollement cover nappe, provide the means of dating selected localities
of outcropping oceanic crust based on their radiolarian faunas. Some
studied samples from the ophiolitic melange (Perri re series) have a
very well preserved and highly diverse radiolarian fauna of
biochronological significance. The age of the radiolarites is
established by correlation with the biozonation of Baumgartner et al.
(1995b), which indicates a Bathonian age for the oldest radiolarian
assemblages. Accordingly, these radiolarites represent remains of the
oldest sediments recorded after the opening of the Piemont-Ligurian
Ocean. This age is in agreement with those recently established by
isotopic methods (166 +/- 1 Ma U-Pb and 165.9 +/- 2.2 Ma Ar-40/Ar-39) in
the associated gabbros from the ophiolitic melange. The isotopic age and
paleontological results are important because they represent the oldest
dating of the oceanic crust of the Piemont-Ligurian Ocean, proving a
Late Bajocian-Early Bathonian age for the oceanization in the western
Tethys.
The systematic part presents a complete Bathonian radiolarian assemblage
from two of the best preserved samples; the illustrated assemblage
contains 180 species attributed to 66 genera (44 nassellarians, 22
spumellarians and 1 entactinarian). Twenty new species and three new
genera (Helvetocapsa, Plicaforacapsa and Theocapsomella) are formally
described.