Abstract

In Northern Apennines, the carbonate platform deposition of the Calcare Massiccio Fm (early Liassic) is followed by pelagic sedimentation as a result of drowning m the Early Jurassic. In order to define age and causes of this drowning, some stratigraphic sections were studied: Mt. Cetona (southern Tuscany), Burano gorge and Bosso gorge (Jurassic expanded successions of the Umbria-Marche Apennines), Mt. Nerone (Jurassic condensed successions of the Umbria-Marche Apennines). The ammonite assemblages and facies analysis support the following conclusions: 1) the first phase of drowning (late Hettangian-early Sinemurian) was synchronous across Tuscany and the Umbria-Marche Apennines; 2) in the Early Sinemurian the sedimentary environment had already been differentiated into shallow-water carbonate banks (Calcare Massiccio of M. Nerone) and in deeper troughs (Corniola of Burano and Ammonitico Rosso of Tuscany); 3) carbonate banks in the Umbria-Marche Apennines drowned in the Carixian; 4) drowning went on in different ways because of synsedimentary tectonics and different subsidence rates; 5) the drowning of the Calcare Massiccio platform was not caused by high subsidence rates, but by ecological changes related to the opening of seaways between the La Spezia basin and the Central Atlantic Ocean during the Early Jurassic.

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