A submerged beach of the last post-glacial sea level rise is present on a sector of the inner continental shelf of the Bassa Maremma Toscana (east of M.te Argentario). Its stratigraphy and evolution has been detailed using high resolution seismic profiles and controlled by the lithostratigraphies of 30 gravity cores. The littoral deposit overlies the unconformity formed during the last-glacial lowstand (würm) as a result of the subaerial exposure of the continental shelf. This deposit has tabular geometry and is delineated into three transgressive facies association (see included figure). The facies association A is 3-5 m thick and has been found in the middle portions of the study area. These sediments are characterized by retrograding and coalescent washover fans, truncated at the upper boundary by shoreface erosion (ravinement surface). Over the washover facies a sand sheet mainly made up of lower shoreface sands is present. The facies association B has been found on the external marine portions of the study area and is between 1 and 3 m thick. It is made up of beach and upper shoreface sediments, separated from the underlying lower shoreface sands by a ravinement surface. The facies association C represents the general absence of transgressive sediments and is present along the inner portions of the study area. These three facies association, parts of a larger transgressive sequence, are differentiated by the nature of the transgressive erosion. The different intensities of this erosion determined whether total (association C), moderate (association B) or low (association A) amounts of sediment were removed from the top of the trasgressive littoral sequence. The amount of erosion was controlled by the paleotopography (unconformity surface) and by the depositional-erosive mechanism acting during the transgression (the classical shoreface retreat model). During the landward beach migration associated with eustatic sea level rise, the depositional processes were also controlled by an accommodation-dominated regime. The beach profile showed an erosive diastem along the surf zone. Its progressive landward migration produced the recorded ravinement surface. The diastem was also an active sandy source, which distributed the sediment by two dispersal systems. The first one transported sand in the flat back barrier area by storm overwash phenomena and carried sand on the beach by landward asymmetric fair-weather surges. The second dispersal system transported the sand to the lower beach shoreface, on top of the older dormant diastem surface (ravinement). In the A and B facies association, the first dispersal system deposit underlies the second one. They are separated by the erosive diastem, whose stratigraphycal exspression is the recorded ravinement surface.
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|Titolo:||Stratigraphical features and depositional processes of a transgressive versilian beach present on the south Toscan inner continental shelf|
|Data di pubblicazione:||1993|
|Appartiene alla tipologia:||04a Atto di comunicazione a congresso|