The results of a litho and biostratigraphic research, based on calcareous nannofossils, on the Mt. Rentella succession, tectonically sandwiched in between the Tuscan Nappe and the Umbrian autochthonous, is reported in this paper. The studied sequence consists of the polychromic marls of Mt. Rentella (MVR) at the bottom, the cherty marls of Mt. Sperello (MMS) in the middle and the sandy-marly turbidites of Castelvieto (ACv), at the top. The nannofossil content of the sequence shows that the MVR are Late Oligocene-Aquitanian in age, the MMS are upper Aquitanian and that the ACv are positioned at the Aquitanian-Burdigalian boundary. These results given, the Mt. Rentella sequence can be correlated neither to the topmost part of the Tuscan Nappe, which is older, nor to the turbiditic sequence of the Umbrian domain, which is younger. So it is hypothesized that it may be correlated to the oldest part of the type-sequence of the Mt. Cervarola, and that its palaeogeographic position, during the Late Oligocene-Early Miocene time, should correspond to a peripheral bulge of the Tuscan foredeep which was never reached by the deposition of the Macigno sandstones, Such an area, for almost the entire Aquitanian, was characterized by the sedimentation of polychromic marls (MVR) which are similar to the polychromic shales of the topmost part of the tuscan Scaglia, but differ for a larger content of calcium carbonate owing to a lesser depth of their basin floor.