ABSTRACT

A transgressive systems tract (TST) deposit on the inner continental shelf of the south Tuscany region (central Tyrrhenian Sea, Italy) formed during the last postglacial sea-level rise. Its small-scale stratigraphy has been detailed using high-resolution seismic profiles, gravity cores, and grab samples. The TST deposit overlies a lowstand unconformity, shows a tabular geometry, and comprises three internal architectures 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) Architecture B, on the middle sector, comprises palustrine (bottom), washover, and the above-mentioned reworked sand sheet; and 13) Architecture C, on the inner sector, represents the absence or scarcity of deposits (reworked sand sheet) ov r the unconformity. Because the lateral distribution of these vertical successions is not random, but parallel to the coast, each architecture represents an individual sedimentary stage during sea-level rise. However, all architectures were formed via shoreface retreat in response to the landward migration of a beach complex over the unconformity. During this migration the beach system was characterized by a source diastem located in the surf zone and by two sediment dispersal systems. One moved the eroded sand over the flat back-barrier palustrine area by storm washover, while the other transported part of this sand to the lower shoreface, forming a reworked sand sheet above the older and inactive source diastem (ravinement surface). The TST architectures originated from a transgressive succession of beach facies, differentiated according to the intensity of shoreface retreat. Architecture A represents a low preservation potential of the original beach complex, Architecture B relatively high preservation, and Architecture C no preservation. The intensity of erosion and the consequent preservation potential were totally controlled by antecedent topography.