Abstract
In the Peloritani Mountains an Alpine nappe stack, involving an Hercynian or older basement, is present. Some nappes involve a Meso-Cenozoic sedimentary cover, which starts with Upper Triassic-Hettangian continental redbeds (Verrucano), followed by Sinemurian neritic limestones and, up to the Oligocene, by marly-calcareous pelagic strata. Locally, Upper Triassic evaporites have been recognised. In the Sant’Angelo di Brolo valley, a peculiar sedimentary succession characterised by about 80 m of graded sandstones overlies the Verrucano redbeds. It has been described by Duée [1969] who ascribed it to the Alì Unit. Later on, Thery et al. [1985] interpreted the sandstones as fluvial deposits, Norian-Rhetian in age on the basis of pollens, and correlated them with the Sardinian « Keuper ». The finding of some ammonites and few nannofloras in the siliciclastic succession allow us to reach quite different conclusions. One ammonite specimen, collected in the uppermost part, shows morphological affinities with Spinammatoceras (M) tenax (Vacek), reported from the Middle Aalenian L. murchisonae Zone. Within the calcareous nannofossils, the presence of Lotharingius umbriensis in the lower part of the succession indicates an age not older than late Pliensbachian. However, the upper part of the same succession is characterised by the occurrence of Hexalithus magharensis, Triscutum tiziense, Watznaueria contracta, whose FO is early Aalenian. The petrographic study evidences that sandstones have two compositional groupings : a quartzose (quartzarenite to sub-litharenite) petrofacies of the continental redbeds (Verrucano), and a quartzo-feldspathic (feldspathic quartz-arenite to sub-arkose) petrofacies of the marine sandstones. The redbeds represent deposition by low gradient rivers and are similar to the composition of the Torrente Duno Fm in the Longobucco Group of the Sila Unit sedimentary cover. Their sources include abundant reworked quartz, felsitic volcanic, and low-grade metamorphic terrains. The overlying Middle Liassic-Aalenian marine sandstones testify an abrupt change in composition, reflecting changing source terrains. Its composition, including oversized feldspar grains, suggests gneissic/plutonic source terrains, added to the quartzose and metamorphic sources of the underlying fluvial sandstone. Identical changing detrital modes is testified in the Liassic formations of the Longobucco Group. These sandstone detrital modes mark the evolving early Jurassic rifted-continental margin of the Neotethys ocean. The studied succession shows characteristics unknown elsewhere in the Peloritanian Units, such as the presence of Mesozoic siliciclastic sediments younger than the « Verrucano » redbeds and the lack of terrains in carbonate platform facies above them. It has been deposited in a basin close to emerged areas, in which a clastic supply persists at least until Aalenian. Therefore, the Jurassic palaeogeography of the Peloritanian domain was more articulated than previously thought: pelagic areas were close to continental regions which supplied with siliciclastic detritus narrow basins, confined in grabens or half-grabens between emerged lands and sea-mounts. In the whole Jurassic of the Calabria-Peloritani Arc, siliciclastic marine terrains are known only in the Sila Unit. Here, Middle Carixian-Lower Domerian marls and sandstones in slope facies and an arenaceous turbiditic succession – late Domerian-early Toarcian in age (Longobucco Group) - have been described [Teale et Young, 1987]. There are close similarities in lithologies, tectono-sedimentary evolution, age and petrographic characters between these two sequences. The studied succession cannot be ascribed either to the Mandanici Unit, or to the Alì Unit. In fact, these units are affected by Alpine metamorphism and their Alpine cover is characterised by Upper Triassic evaporites followed by Jurassic and Cretaceous pelagic limestones and radiolarites. Its original bedrock is probably represented by the phyllites and marbles of the Piraino Unit, recently identified in the same region. In conclusion, the Sant’Angelo di Brolo succession was deposited in a marine environment between Pliensbachian (or Sinemurian) and Aalenian. Thus, both the late Triassic age and the fluviatile environment proposed for these terrains must be abandoned, as well as their correlation with the Sardinian « Keuper ».
The Upper Triassic deposits of the west Bangong-Nujiang suture zone and their paleogeographic implications
