LATE QUATERNARY MARGINAL MARINE DEPOSITS AND PALEOENVIRONMENTS FROM NORTHEASTERN BUENOS-AIRES PROVINCE, ARGENTINA - A REVIEW

The late Quaternary marginal marine deposits along eastern Argentina ( Southwestern Atlantic) are reviewed according to our present knowledge . In the northeastern coastal area of Buenos Aires Province they have been assigned to a series of transgressions and regressions rang ing f rom the late Pliocene to the late Quaternary. The most widely accepted model is Frenguelli's (1957) classical chronostratigraphical scheme o f: 'Belgranense', late Pleistocene marine sediments at 3-6 m above m.s .l. and ca. 26,000-->35,000 C-14 years BP, the 'Querandinense', Pleist ocene-Holocene estuarine sediments below or at present m.s.l., and the most extensive 'Platense', mid-Holocene marine deposits at 4.5-2 m ab ove m.s.l. dated at ca. 8000-1340 C-14 years BP. The restricted 'Belgr anense' deposits, recorded in Samborombon Bay, in Magdalena at ca. 32, 000 BP, near Mar Chiquita at ca. 24,900 and 30,500 BP and southwards i n Bahia Blanca at ca. 26,000-35,500 BP, may belong to an interstadial (Gonzalez eh al., 1986). The molluscan composition suggests a marine i nvasion of the area but not a typical interglacial cycle characterized by euhaline and warm water elements. However, the oxygen isotope reco rd argues against an interstadial during the interval 34-27 ka and the chronological control for these deposits is very poor, suggesting tha t they most probably have been elevated neotectonically. The Pleistoce ne-Holocene 'Querandinense' deposits, extensively distributed along th e Bonaerensian coastal plain and continental shelf(ca. 11,000 C-14 yea rs BP), with very low faunal diversity, abundance of freshwater ostrac ods and absence of the warm water molluscs characteristic of the Holoc ene ridges, indicate low salinity and cool water conditions. Further d ating and isotope analysis of these deposits are required for a better understanding of the chronology of climatic events by the end of the Pleistocene in this area and to establish whether or not they could co rrespond to the Younger Dryas event of the northern hemisphere. 'Plate nse' littoral ridges formed between ca. 7600 and 2500 BP extend in sev eral subparallel rows from ca. 34 degrees S to ca. 39 degrees S. They formed either as beaches or possibly as sublittoral bars in Samborombo n Bay, where they reach 120 km long, 10-30 m wide and up to 5 m thick at 4.5-5 m above m.s.l., or in the supratidal and intertidal zones in Mar Chiquita at 2-4 m above m.s.l. Molluscs and ostracods suggest a br ackish marine to marine brackish environment of lower salinity than th e modern Atlantic littoral. During the mid-Holocene, the oceanic water s were mixing with large amounts of Antarctic ice melting in the South Atlantic (Isla, 1990) probably reducing salinity along the Bonaerensi an littoral. The oldest littoral ridges accumulated during the Hypsith ermal and in the Punta Indio area, their different geometry and alignm ent, greatest age, highest diversity and warmer affinity of the mollus can fauna suggest a short interval of reversal of the atmospheric circ ulation pattern. Further research is necessary for a better understand ing of the chronology of the marine late Quaternary deposits, coastal evolution and climatic changes in this area.