The region now occupied by the Northern Apennine Mountains of Italy was deep sea from late Triassic or earliest Jurassic through much of the Miocene. In the early Oligocene a trough developed parallel with the axis of the later mountain system and was filled with sediments ascribed to turbidity currents, originating at the northwest. In early and middle Miocene the locus of deposition shifted northeastward; the source remained in the northwest.
About the same time (late Oligocene or early Miocene) a series of submarine orogenic ridges developed, first west of the present Ligurian-Tyrrhenian coast, then progressively northeast in Tuscany. Argillaceous and marly sediments of late Jurassic, Cretaceous, and Eocene age slid from the crests of the rising ridges in a series of great imbricate slices which became more chaotic with distance of transport northeast. The tectonic ridges of Tuscany rose beneath a thick cover of the gravity-transported chaotic mass and this allochthonous material was involved in the uplifting and faulting of each ridge. Most of the ridges are overturned and thrust northeast. The youngest ridges, those of the mountain front facing the Po Plain and the Adriatic Sea, came into being at the end of the Miocene or in earliest Pliocene. Thus the turbidity-current transport of vast quantities of sediment from the northwest, the compressional movements and the tectonic transport northeast, and the resulting gravity transport of a great chaotic mass in the same direction occurred during brief and overlapping intervals of Cenozoic time.