DEPOSITIONAL AND EROSIONAL COASTAL PROCESSES DURING THE LAST POSTGLACIAL SEA-LEVEL RISE - AN EXAMPLE FROM THE CENTRAL TYRRHENIAN CONTINENTAL-SHELF (ITALY)

A transgressive systems tract (TST) deposit on the inner continental s helf of the south Tuscany region (central Tyrrhenian Sea, Italy) forme d during the last postglacial sea-level rise. Its small-scale stratigr aphy has been detailed using high-resolution seismic profiles, gravity cores, and grab samples. The TST deposit overlies a lowstand unconfor mity, shows a tabular geometry, and comprises three internal architect ures of beach facies: (1) Architecture A, present on the outer sector of the study area, is made up of only a reworked sand sheet; (2) Archi tecture B, on the middle sector, comprises palustrine (bottom), washov er, and the above-mentioned reworked sand sheet; and (3) Architecture C, on the inner sector, represents the absence or scarcity of deposits (reworked sand sheet) over the unconformity. Because the lateral dist ribution of these vertical successions is not random, but parallel to the coast, each architecture represents an individual sedimentary stag e during sea-level rise. However, all architectures were formed via sh oreface retreat in response to the landward migration of a beach compl ex over the unconformity. During this migration the beach system was c haracterized by a source diastem located in the surf zone and by two s ediment dispersal systems, One moved the eroded sand over the flat bac k-barrier palustrine area by storm washover, while the other transport ed part of this sand to the lower shoreface, forming a reworked sand s heet above the older and inactive source diastem (ravinement surface). The TST architectures originated from a transgressive succession of b each facies, differentiated according to the intensity of shoreface re treat. Architecture A represents a low preservation potential of the o riginal beach complex, Architecture B relatively high preservation, an d Architecture C no preservation. The intensity of erosion and the con sequent preservation potential were totally controlled by antecedent t opography.