Widespread syn-eruptive volcaniclastic deposits in the Pleistocenic basins of South-Western Calabria

Abstract

Since late Tortonian, depositional sequences developed along the Southern Calabrian forearc, inside the half-graben depressions of the Mesima-Gioia Tauro and Reggio Calabria basins. In these basins volcaniclastic sedimentation took place during lower to middle Pleistocene. Volcaniclastic deposits consist of isolated pumice swarms, ash layers and thick successions of lapilli and ash. In this paper we describe and discuss the depositional characteristics of these volcanogenic sediments and the morphology, mineralogy and chemistry of pumices, shards, lithics and crystal fragments. These deposits all have the characteristics of a proximal tephra fall deposit from a Plinian plume, which has been reworked in a shore environment. In the Mesima basin the deposits reach 6 m in thickness and in the Gioia Tauro plain 21 m of pumiceous sediments have been cored in a drill hole. The compositional homogeneity of fragments and the absence of non-volcanic detritus as well as of soil or organic matter suggest that the deposits were quickly reworked after their primary emplacements. All the tephra deposits exhibit calc-alkaline affinity and dacitic to rhyolitic composition. The mineralogical assemblage, including orthopyroxene as phenocryst, indicates provenance from magmatic-arc volcanism. ICP-MS and LA-ICP-MS analyses of crystals and glass point to a common origin, probably from a single eruption. This makes these volcaniclastic layers a very useful tool for stratighaphic reconstructions at a regional scale. The geological setting of the deposits, their petrological and geochemical signature and the occurrence of large-size clasts suggest a provenance from explosive eruptions preceding the emplacement of the 50 ka dacitic lava dome of the island of Panarea (Aeolian Archipelago). However a provenance from a different calc-alkaline volcanic system cannot be excluded. These deposits can therefore represent the only evidence of a large explosive eruption in the Southern Tyrrhenian Sea whose very proximal equivalent is not preserved.