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Stratigraphy and architecture of the extensional basins in the eastern Southern Alps (N Italy)

Masetti D., Fantoni R., Romano R., Sartorio D., Trevisani E., Venturini S.

Volumina Jurassica

2006

Vol. 4, no. 1

97-98

The Southern Alps represent a well-preserved section of the southern continental margin of the Mesozoic Tethys, characterized by horst and graben structures inherited from the rifting associated with the opening of the central North Atlantic. In their eastern sector Southern Alps are composed of three palaeogeographic-structural units. These are, from east to west: * a carbonate platform persistent in its southern area from Jurassic until the Cretaceous (Friuli Platform), * a basin formed in the Early Liassic (Belluno Basin), * a wide ridge (Trento Platform/Plateaux), subdivided in a puzzle of blocks drowned in different times from Sinemurian until Aalenian when the Trento Platform area evolved in a pelagic plateau covered by condensed sedimentation during the Late Jurassic. In the subsurface areas (Veneto Plain and Adriatic Sea) a new paleogeographic-structural unit has been recognized informally called North Adriatic Basin. This new unit is characterized by the superposition of Pliensbachian deep-water units, similar to those outcropping in the Belluno Basin, on the top of the Hettangian-Sinemurian peritidal platform. The integrated valuation of surface and subsurface data performed through the stratigraphic revision of the well stratigraphy and the interpretation of seismic lines acquired by means of the CROP projects (Crop Mare and Transalp) and the ENI hydrocarbon exploration in the Venetian Plain and of the Adriatic Sea, led to the distinction of a new three dimensional model pertaining to the Mesozoic extensional architecture of the eastern Southern Alps and the their Veneto foreland.According to this model, the recognized extensional pattern is the result of the following two main tectonic phases: At the Triassic/Jurassic boundary this region experienced a strong extensional tectonics with the debut of a deep asymmetrical trough, the Belluno Basin, filled by cherty deposits (Soverzene Fm.), closed to the south and bounded to the west by the Trento Platform and to the east by the Friuli Platform. In the other sectors of the Southern Alps and of the Veneto foreland, and elsewhere, peritidal sedimentation persisted (M. Zugna Formation) (Fig. 1A). At the Sinemurian/Pliensbachian boundary, a new extensional phase, seemingly interactive with a decrease in carbonate productivity, produced a widespread drowning of the platforms and the deep reorganization of the Lower Jurassic palaeogeography (Fig. 1B). The northern and the eastern sector of the Trento Platform and the northern area of the Friuli Platform were drowned and became plateaus on which crinoidal sand waves accumulated (Fanes encrinites). To the south the Northern Adriatic Basin opened with a structural style far from the Belluno Basin; compared with this last mentioned palaeogeographic unit, the Northern Adriatic Basin is wider, nearly asymmetric and less subsiding. The Northern Adriatic Basin debut pushed the margin of the Trento and Friuli platforms respectively to the west and to the east, thus leading to a zigzag geometry of the structural units formerly interpreted as the interfering between the main, NNW-SSE extensional faults and a ENE-WSW, trascurrent system.