Notizie preliminari sul Pliocene del Bacino del medio Ombrone e della zona di Roccastrada

Preliminary results of interdisciplinary researches on the Pliocene sequences of the middle Ombrone Basin and the Roccastrada area are presented. The most interesting acquisition is that the sedimentary and palaeogeographic evolution of the two areas has developed through two sedimentary cycles. The first is represented by a sequence of marine clays of Early Pliocene age, transgressing on Miocene and pre-Neogene formations; it extends from the Sphaeroidinellopsis seminulina s.l. Zone to the lower part of the Globorotalia puncticulata Zone. The second cycle is represented by marine sediments, of small thickness, mainly consisting of fossiliferous sands and detritic-organogenic limestones, which have been referred to Middle Pliocene (Globorotalia aemiliana Zone). These sediments are transgressive on different Lower Pliocene biostratigraphic units or on Miocene and pre-Neogene formations. A lacustrine episode, represented by marly clays, marls, calcareous sands, peaty c1ays and coarse sands, comes after this marine event; it is not clear if the lacus trine sedimentation follows the marine one in continuity or is separated by a gap. On account of their contents in land vertebrates, the limnic sediments can be attributed to the Lower Villafranchian interval corresponding lo Middle Pliocene