COASTAL ENVIRONMENTAL-CHANGES ASSOCIATED WITH HOLOCENE SEA-LEVEL FLUCTUATION - SOUTHEASTERN BUENOS-AIRES, ARGENTINA

Sea level was 2 m higher than present 6000 years ago at southeastern B uenos Aires Province, Argentina. This fluctuation has caused several e nvironmental changes including: Drift reversal: Geomorphological featu res and radiocarbon dating suggest that the Mar Chiquita barrier grew from north to south during the regression from 4000 years BP. In contr ast, mineral dispersals, coastal defenses, inlet migrations and wave s tatistics shaw that present beach drift is to the north, in the same d irection as storm-induced shelf sediment transport. Coastal mollusk ma ss mortalities: Lagoonal deposits, that accumulated during the regress ion, contain abundant remains of Mactra isabelleana, Tagelus plebeius and Labiosa plicatella. These species are rare in present coastal lago ons. Many causes have been proposed for the absence of these species i n present lagoons and include: sudden sea-level decline, outlet obstru ction producing variation in salinity, and increased fluvial freshwate r inputs. The regional character of these mortalities and growth curve s indicates that taphonomic-feedback processes related to the scarce 2 m involved in the transgressive-regressive cycle are the reason for m ollusk mortality. Sand consumption. During the latter stages of the Ho locene transgression, sand ramps (up to 10 m height) at Mar del Plata cliffs, and sand barriers (up to 2 km width) north of Mar Chiquita vil lage, developed as a result of high wave energy and a plentiful sand s upply. The subsequent regressive phase produced lagoons, tidal flats, marshes and cheniers. The alongshore growth of barriers meaned their c annibalization (misfit foredunes) and narrowing driftwards. These proc esses occurred before the drift reversal mentioned above.