Post-Variscan Verrucano-like deposits in Italy, and the onset of the alpine tectono-sedimentary cycle

Abstract

In the Italian regions the stratigraphic successions of the Verrucano s.l. consist essentially of continental alluvial plain to coastal-neritic siliciclastic redbeds, deposited at different times in the Alps (Mid? –Late Permian), the Northern Apennines (Mid?–Late Triassic p.p., locally in Southern Tuscany also followed by the Jurassic Pseudoverrucano Complex) and the ‘Calabro-Peloritani Arc’ (Mid?-Late Triassic, exceptionally reaching earliest Jurassic), whereas in Sardinia the recognition of the Verrucano succession is still a topic of discussion.The most typical metamorphic lithofacies, in the Pisan Mts. of Tuscany, is represented by the Verrucano s.s. conglomerates and breccias, including quartz clasts and minor metamorphic and volcanic lithics, grading laterally and vertically to quartzitic sandstones and pelites.In the Pisa Symposium the term ‘Verrucano’ was suggested only for those detrital deposits laid down, in the Alpine-Mediterranean areas, after the final act of the Variscan orogeny, and also to use local geographical names. Consequently, the typical terms ‘Verrucano Toscano’, the ‘Verrucano Lombardo’, the ‘Verrucano Briançonnais’ and other similar deposits, being linked to the Variscan belt, were generally accepted.The ages of these ‘Verrucanos’, since they are devoid of fossils, can only be interpreted indirectly. The Verrucano Lombardo of central Southern Alps was recently ascribed to a Mid? –Late Permian interval (Late Guadalupian?–Lopingian p.p.), taking into account that the connected Val Gardena Sandstones of the nearby Venetian region are interdigitated with the Late Permian Bellerophon Fm. The Verrucano Briançonnais of the Maritime Alps is again mostly related to Late Permian, being covered by the Lower Triassic alluvial-deltaic quartzites. In the easternmost Ligurian Apennines, near La Spezia, the Mesozoic section of Punta Bianca, which begins with a fluvial conglomerate, transgressive and unconformable on a metamorphosed Paleozoic Variscan Basement, is composed of two main cycles, of which the upper one correlates to the typical Pisan Verrucano s.s. and the overlying Mt. Serra Quartzites, the latter yielding Carnian pelecypods and vertebrate footprints.The Verrucano from the Mid-Tuscan Ridge to Mt. Leoni and in the Argentario promontory is delimited above by the Tocchi Formation and presumably developed from Middle (Late? Ladinian) up to Late Triassic (Carnian) times; in contrast, in Calabria (e.g. the Longobucco unit of Sila Grande) and in Sicily (e.g. the Taormina-Longi Unit of Peloritani Mts.) the so-called (Pseudo-) Verrucano deposits pertain to Mid? –Late Triassic and are capped by a basal Jurassic succession, determined by the discovery of an Early Hettangian palynoflora.All over their distribution areas, the above-mentioned Alpine Verrucanos rest, through an unconformity marked by a gap of varying and imprecise duration, on Late Paleozoic volcanic/sedimentary successions or directly on the underlying Variscan metamorphic basement.Schematically, the post-Variscan succession in Italy can be subdivided into three main tectono-sedimentary megacycles: the first generally ranges from the Late Carboniferous to, or slightly above the Early Permian, the second from Mid? –Late Permian to Middle Triassic, while the third cycle begins with the Mid?-Late Triassic p.p., attaining the Jurassic in some parts of Southern Tuscany and the ‘Calabro-Peloritani Arc’.After the Late Carboniferous and Early Permian transtensional tectonics represented by many strike-slip continental basins (cycle I), widespread Middle Permian geodynamic reorganization (‘Mid-Permian Episode’ Auctt.) led to the development of a dominant extensional regime, and the birth of tectonic plates and oceans (Neotethys, Meliata-Maliak, etc.) between Africa and Europe. The erosion of the Variscan relief was followed by ingression from S-E sectors of shallow-marine branches of Neotethys (cycle II). The major rifting events that led to the Jurassic birth of the Ligurian-Piedmont Ocean started during the Middle–Late Triassic, and were related to the eastern opening of the Atlantic Ocean. These rifting events gave rise to the development of the Adria and Europe-Corsica-Sardinia conjugate passive margins, characterized by an asymmetric structural and sedimentary evolution. In this geological context, the Verrucano-like deposits of Italy can be interpreted as a discontinuous and asynchronous ‘tectofacies’ that marks the final dismantling of the SE border of the Variscan chain.