Evidenze geomorfologiche sommerse nelle aree costiere italiane di uno stazionamento del livello del mare ubicato circa -20m e attribuito allo stadio 3

Abstract

Underwater investigations along the Italian coasts have shown a recurrent paleo-sea level at -18÷-22 m below the present sea level. Geochronoiogical age determinations on submerged speleothems used as sea-level markers and sampled at around -20 m, suggest that the paleo-sea level corresponds to the highest stand reached by the sea during the isotope stage 3, between 48 and 27 ka. Geomorphological evidence of this paieo-sea level are remains of a subhorizontal abrasion surface, which is visible on cliffs bordering carbonate promontories. Analogous surfaces (terraces) are mainly observed in the Tyrrhenian Sea from Tuscany to Sicily and in Sardinia at similar depths (-16÷-22 m); examples are also visible along the Apulia's coast in the Adriatic Sea. Recurrent geomorphological features of these terraces are: a) discontinuous but frequent occurrence; b) width lower than 10 m; c) location at the base of ancient cliffs; d) notch and pot-holes at the inner margin; e) in many cases terraces are laterally connected with the wave-enlarged mouth and levelled bottom of submerged caves, and with the flat top of ancient buttes. The general tectonic stability since Eutyrrhenian times (stage 5e of the oxygen isotope curve) in the regions where the observed forms occur, confines the relative age to well definite times. The absolute age of the paleo-sea level is yielded by speleothems sampled at about -20 m b.s.l. in Tuscany. These display marine biogenic episodes interlayered with continental ones. The 14C radiometric ages of the different layers keep such alternating episodes within the time span ranging from 27 to 42 ka (Alessio et al. 1992; 1994), namely during the highstand related to the isotope stage 3 (Aharon & Chappel, 1986). It is concluded that abrasion terraces and related morphological features found within the same depth interval formed during the stage 3 oscillating highstand.