I fondali antistanti la costa di Montalto di Castro (alto Lazio): caratteristiche ed evoluzione tardo-quaternaria

Abstract

The inner continental shelf in front of the Montalto di Castro coastline (central Tyrrhenian sea) has been investigated for the purpose of reconstructing its evolution during the late Quaternary age. The marine investigation involved a seismic survey, using Uniboom and Side scan sonar apparatus, and a sampling of sea-floor sediment, using grab and gravity core. The seismic stratigraphy of the shelf is characterised by an erosive unconformity surface that bounded the bottom of the overlying Holocene sedimentary sequences. The unconformity surface shows a very rough morphology with two paleoriver valleys - located in front of the present river mouths of the Fiora and Tafone rivers - which cross the investigated area in a seaward direction, binding a large zone with a higher topography. This unconformity is also present in the sea-floor next to the investigated one, and was formed during the emersion of the continental shelf in the last glacial low-stand. Its morphology has strongly influenced the depositions which occurred in the last postglacial sea level rise and, in part, those of the present high silti-stand. During the last marine transgression (Versiliano), each of the two paleo-valleys was characterised by depositional environments with low energy and high sedimentary input, and each paleo-valley housed a coastal embayment characterised by a river mouth. The transgression in these two areas was strongly depositional and produced thick sedimentary bodies, which partially infilled the low stand paleoriver morphology. Whereas, in the area between the incisions - topographically higher and less protected from the marine processes - a transgressive sand sheet deposited. During the present high still stand the progradation of wedge-shaped coastal bodies began in both the incisions and in a zone close to that of the Tafone paleoriver. Probably at this stage the growth of Posidonia mats and that of bio-construction supported by a rocky sea-floor also occurred. The present-day sedimentation is mostly muddy and it is due to the dispersal of fine fluvial sediments, with scarce supply of bioclastic debris from the bio-constructions, and a very scarce supply of sand from the relict deposits.