Compositional and textural study of beach sands from active volcanic areas (southern tyrrhenian sea)

Abstract

The main goal of this research focuses on the provenance, compositional and textural investigation of modern sand supplied from volcanic terrains (southern Tyrrhenian sea). This is a contribution to quantify the controls on volcaniclastic sand composition among volcanic areas with different tectonic settings and compositions. Specifically, an important aim of this research is to quantitatively compare the relation between areal distribution, texture and composition of “source” lithotypes – which are the clastic debris producer – with respect to texture and composition of "volcano-derived" sediments. The study area covers the coastal perimeter/stretch of two Italian volcanic provinces in the southern Tyrrhenian sea: Aeolian islands and Campania province. Particular attention has been given to the factors that control the relationships between grain rounding, grain-size, sand composition, texture and source rocks. This research provide a good opportunity to define the provenance signatures of detritus eroded from lavas with different compositions, pyroclastic and minor sedimentary rocks. Different sandy petrofacies for the studied areas of Campania province have been formalized. In order to investigate on provenance, pre-burial processes, composition and texture of modern sand supplied from volcanic terrains, different studies have been carried out through:  Sieving analyses;  Polarizing microscope and Electron Microprobe;  Image analysis (roundness study);  Geographic Information System analysis (SGI calculation). There is a clear differences between Aeolian Islands costal beach sand and Campania coastal beach sand in terms of detritus maturity. Grain-size distribution within Aeolian beach sediment show a tendency towards coarser sand fraction to gravel, whereas Campania coastal samples show a tendency towards medium to fine sand fractions; this indicates a varied physical disintegration of the source rocks. The major components of Aeolian islands and Campania beach sands are monomineralic grains, sedimentary and volcanic lithic fragments with lesser amounts of calcareous bioclasts. Samples from Aeolian islands and Portici Sorrento coastal stretch have an high percentage of volcanic lithic fragments (Lvl > Lvmi> Lvv), whereas samples from Pozzuoli, display an average percentage among Lvl~Lvmi~Lvv. Three different petrofacies have been defined along Campania coastal stretch: sedimentary (Apennines), Vesuvian and Phlegrean fields petrofacies. Stromboli, Vulcano, Alicudi and Filicudi sands have a dual basaltic/shoshonitic and andesitic composition “signatures”. Panarea, Lipari and Salina sands have a wider range of composition “signatures” ranging from basalts to rhyolites. there is evidence that, on Aeolian islands, sand composition does accurately reflect bedrock composition except in the case of source areas dominated by pumice outcrops (e.g. Lipari islands), whereas in more protected and quite beach-environment such as Pozzuoli bay, this grain types (pumice, associated to more evolved [acid] volcanism and then explosive volcanism) have been found and resulted to be texturally more preserved. In the sandy detritus the persistence for the lithic grains is ranked as follows: Lvlblg, Lvmiblgl, Lvvblgl > Lvlbrgl, Lvmibrlgl, Lvvbrgl > Lvlclgl, Lvmiclgl, Lvvclgl > Lvf > Lvlgrgl, Lvmigrgl, Lvvgrgl, pumice. Thus, mafic source rocks will be overestimated and more acid source rocks will be underestimated in the stratigraphic record. New volcanic lithic compounds have been introduced (Lvlgrgl, Lvmigrgl, Lvvgrgl) then who will study the ancient stratigraphic records will know that the Lvlgrgl means dacitic provenance. New discriminating diagrams have been introduced which allow to obtain important information among the volcanic source rocks ranging from basic to acid composition. Lvlblgl, Lvmiblgl, Lvvblgl (1); Lvlbrgl, Lvmibrgl, Lvvbrgl (2); Lvlgrgl, Lvmigrgl, Lvvgrgl (3) can be produced not only by basaltic, andesitic and dacitic source rocks but also by source rocks with the same SiO2 content belonging at different alkaline series (e.g. trachybasalt, shoshonite, latite, trachyandesite, trachydacite, trachyte). Campania samples displaying an higher roundness degree which decrease towards Phlegrean Fields area from north (Volturno river mouth) to south and, show an higher percentage of (3), (4), (5) and (6) roundness category, whereas Aeolian islands samples have an higher percentage of (1) and (2) roundness category. There is a correlation between roundness and geographic location of the Aeolian islands beaches. Sand grains round more efficiently under gentler wave action of the eastern side whereas the more angular grains of the north-western beaches are immediately eroded from the nearby cliffs with null or quite minimal reworking. A new methodological and research approach for roundness degree calculation have been tested by conducting image analysis. By relating GIS, compositional and textural results, it is possible to affirm that lavas source rocks have an higher propensity to create sandy detritus than pyroclastic source rocks. This finding has implications for the stratigraphic record especially for the sandy pumice clasts which could be underrepresented in older volcaniclastic deposits and overrepresented in other detritus size fractions. This actualistic study helps in understanding factors controlling siliciclastic sediment composition and texture, in turn, will help in deciphering major controls on ancient volcaniclastic successions, especially those where volcanic terrains have been totally lost by erosion