The influence of volcanic supply on the composition of modern river sands: the case study of the Ofanto River, southern Italy

Abstract

The Ofanto river drains volcanic rocks from the Monte Vulture, lacustrine-fluviolacustrine deposits associated with the same volcano and sedimentary deposits of the Southern Apennines and the Bradanic foredeep sequences. Comparing the modal composition of river sands and the outcrop area of different lithologies in the different sub-basins, an over-concentration of the volcaniclastic fraction, mainly represented by loose crystals of clinopyroxene, garnet and amphibole, is shown. This has been related to the preferential erosion of pyroclastic deposits, characterized by high production of sand-sized loose minerals, together with the carbonate lability and the low sand-sized detritus production from claystones and marls. The occurrence of volcaniclastic components upstream of Monte Vulture can be explained with a contribution from the lacustrine-fluviolacustrine deposits outcropping in the upstream sector or from pyroclastic fall deposits of Monte Vulture and/or Campanian volcanoes. This research shows that the volcanic record in the fluvial sands of the Ofanto river comes from weathering and sorting processes of volcaniclastic deposits rather than of the lavas building the main edifice. Therefore, caution must be taken during paleoenvironmental and paleoclimatic reconstructions when relating the type and abundance of the volcanic component in sediments to the weathering stage and evolutionary history of the volcano.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5643959

Sr-Nd Isotopic Composition of Pyroxenes as a Provenance Indicator of a Double-Volcanic Source in Sands of the Ofanto River (Southern Italy)

“…For each locality, the sub-basin feeding the sampling point was outlined, and the area covered by each lithology, obtained by the lithological map (1:500,000 scale) of the National Geoportal of the Environment Ministry (http://www.pcn.minambiente.it/ mattm/servizio-di-scaricamento-wfs/ (accessed on 15 December 2021)), was calculated. The used procedure and software are described in [8]. A further sample was collected from the beach sand of Castellabate, on the Tyrrhenian Cilento coast (Figure 1c).…”

“…Abbreviations as in Figure 5. In the beach sample picked up at the mouth of the Ofanto River (OFA-22) a high percentage of carbonate rock fragments occur, due to the erosion of the Bradanic foredeep sediments [8]. Single, mainly rounded quartz crystals are also abundant (Figure 4e).…”

“…The sands of the Ofanto River always contain a component of volcanic origin, represented by volcanic rock fragments, clinopyroxene, amphibole and melanitic garnet crystals [8]. Pyroxene crystals occur in the sandstones of the Sicilide Complex.…”

“…In particular, sampling site OFA-13 (Figure 1) is fed almost exclusively from Monte Vulture volcanics (Figure 2b). Similarly, in the upstream area (sampling site OFA-3), the volcanic grains could be derived from the erosion of terraced volcaniclastic sediments formed during the building of the Monte Vulture volcano in the Pleistocene [8]. Alternatively, it could also be related to the direct decrease in highly fragmented pyroclastic material during the most violent explosive eruptions of Monte Vulture.…”

“…However, the geographic position of the Ofanto Basin makes it an ideal collector of volcaniclastic sediment deriving from explosive eruptions of other Southern Italy volcanoes located outside the basin itself. Recent studies showed that a volcaniclastic fraction, mainly made up of clinopyroxene, and subordinately of amphibole, garnet and volcanic rock fragments, also occurs in the sands of the upstream portion of the Ofanto River, where no volcanic rocks outcrop [8]. In order to distinguish the different potential volcanic sources of the present river's sediments, we analyzed three sand samples, from the upstream portion, central sector and mouth of the Ofanto River, respectively; for each of the sampling points, the area occupied by different lithologies was calculated.…”