Il Quaternario di P.ta dell' Avoltore (Monte Argentario - Toscana meridionale)

Abstract

Deposits and forms that can be found in the area of P.ta delf'Avoltore-Sbarcatelio (South-Eastern extremity of Monte Argentario, Tuscany) and related to three marine sedimentary cycles are described. These are intercalated with and/or superposed on continental breccias. Heavy minerals of volcanic origin were tested; the analysis executed on the II cycle sandstones has shown the abundance of augite and diopside. These minerals are rare in the Argentario pre-Quaternary basement rocks, therefore their origin is linked to the peritirrhenic voi can ism and the sediments are approximately Middle Pleistocene in age. Moreover, the probable morphostructural evolution is outlined, in relation to the overall area of Argentario, on the basis of indications come out from other zones of the promontory. On the basis of the stratigraphical relations and the altimetrical position of the deposits, that consist in sandstones with a marine molluscan fauna, five uplift phases have been recognized, alternating with glacioeustatic transgressions. In the examined area, the absolute values of the uplifts successive to the first phase, which should not have produced notable altitude differences, because the sediments of II cycle are onlapped on those of the I, have been estimated, respectively, 80 m, 25 m, 40 m and 20 m. Along the coasts of the promontory, marine terraces Middle Pleistocene in age are only in the Southern (170 m a.s.l.) and Western (60 m a.s.l.) sectors; this indicates the uplifts did not occurred in a uniform way, but they produced a complex tilting towards NE, that determined the present morphological asimmetry as well as the different morphodynamic behaviour of the slopes. Gravitational slidings occurred in the past (between III cycle and Tirrenian), are here mentioned, whose forms are still easily recognizable, some of them present characteristics of "deep seated gravitational slope deformations". Another mention is given about the present sliding movements, genetically differents from the previously cited slides, because they are connected to the anthropic activity, but strictly linked to them as they are located on the old slides masses.