Sedimentation of the late geosynclinal stage

Abstract

The sediments of the Upper Eocene to Middle Miocene Ranzano-Bismantova sequence of the Padan side of the Northern Apennines are considered late geosynclinal, because they were formed after the Middle Eocene folding stage in the Ligurian domain and were later involved in the main westward movement of the Allochthon. The sediments were essentially clastic. During the Oligocene they were deposited in adeep-water marine environment, in part by turbidity currents and slumping (e.g., Ranzano Sandstone); later, the environment tended to become shallower (e.g., Bismantova Formation) and deposition was by turbidity and normal currents. Other sediments in a clearly similar structural situation occur in eastern Tuscany and in the Romagna Apennines; they are of Middle to Late Miocene age and were formed in a shallow marine environment. This paper summarized published information on the lithology, stratigraphy and structural relations of the late geosynclinal rocks, and its includes a summary of paleogeographic evolution. It is also pointed out that in the Northern Apennines the late geosynclinal stage is defined essentially on the basis of structural relations. The term molasse, as understood in the Swiss Alps, applies only to parts of the sequences and, in general, it cannot be equalled with late geosynclinal sediments ( Aubouin, 1965).