Abstract
The transverse 'Sillaro line' is merely the outcrop line of the north-west dipping overthrust plane of the Liguride Complex. The anti-Apenninic strike of this overthrust plane is the result of the 'Sillaro flexure', a large synsedimentarily developed flexure of the basement of the Northern Apennines. The Liguride Complex overlies to
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the north-east progressively younger deposits dating from Early Miocene to eventually Pliocene: the Liguride overthrust is tectonically transgressive. The Liguride Complex is made up largely of clay, the Argille Scagliose; the overthrusting was gravitational. Just like glaciers and ice-sheets the Liguride Complex is a plastic mass capable of spreading under the influence ofgravitation. Olistostromization played a role ofvarying importance in the advance of the Liguride Complex. During the Pliocene, when a shallow molasse basin was invaded, it was the dominating mechanism. Locally the Liguride Complex interfingers with autochthonous deposits, as a result of a temporary standstill of the thrust sheet. The deposits just below the overthrust plane often show the influence of the approaching Liguride Complex. The Marnoso Arenacea is a turbidite formation of Miocene age. The configuration of its sedimentation basin was influenced strongly by an anticline that developed synsedimentarily, resulting in the Castel Vecchio 'high'. This 'high' divided the basin into an inner and outer basin in the south-west and north-east respectively, each with its own sedimentary history. The position of the inner basin axis was defined by the Rovigo syncline, which was also synsedimentary. A 'klippe sedimentaire', the Casaglia Member, invaded the basin from the south-west, and carne to a standstill near the basin axis. It proved impossible to describe the turbidites in terms of inner and outer fan and basin plain deposits by analogy with recent turbidite sedimentation in oceanic environments. A purely descriptive classification in facies associations is introduced here. The thickest and coarsest turbidites were usually deposited in the deepest parts of the basin. Towards the 'high' they became thinner and finer grained, until only marls were deposited. Upon the Castel Vecchio 'high' moluscs are found in living position, indicating that occasionally this 'high' approached sea-level. The Castel Guerrino Formation which constitutes a separate tectonic unit, was probably deposited in a more internal part ofthe Marnoso Arenacea basin. This formation thrust from the south-west upon the Marnoso Arenacea, after which it overthrust 5 km further to the north-east over a thin slice of Liguride rocks which had already overthrust the Marnoso Areacea.
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