HIGH RESOLUTION MODEL FOR BEACH RIDGE EVOLUTION, AN EXAMPLE DURING THE HOLOCENE CLIMATIC OPTIMUM FROM ARGENTINA

SEBASTIÁN M. RICHIANO; AUGUSTO N. VARELA; D'ELIA, LEANDRO; ANDRÉS BILMES; ALEJANDRO GÓMEZ DACAL; MARINA AGUIRRE

Congreso; International Sedimentological Congress; 2018

IAS

Beach ridges constitute geoforms that preserves a high quantity and quality of information respecting the littoral environmental where they were deposited. At Bahía Samborombón locality, Buenos Aires Province, Argentina, on of the most spectacular marine Argentinean Holocene deposits are preserved. Geomorphologic analyses based on satellite images allow determining a big beach ridge parallel to the present coast nearly 30 km inland. In these deposits 20 detail sedimentary logs were measured and sampled in order to generate an environmental model for deposition, which includes a sand ridge and a shell ridge. After the maximum Holocene transgression at the study area it is possible to recognize a unique beach ridge deposit, which presents internal surfaces, recognized from a 3D photogrammetric outcrop model, that allow us to divide it into eight bodies. The first two bodies correspond to the development of a sand ridge as a response to the erosion and reworking of previous aeolian Pleistocene sediments. Subsequently, a bioclastic ridge was amalgamated with the sand ridge (bodies 3 to 8). Simultaneously to these eight bodies, in a landward position, a lagoon and washover deposits were developed. Finally, after the deposition of the ridge the continuous sea level fall generated the actual coastal plain deposits. With the sedimentological model defined, the next step was to date the ridge and characterized the temperature and salinity of the paleoenvironment. In order to obtain well constrained dates we carried out a paleontological and taphonomical strict method that includes selection of three samples belonging to the main surfaces. In this sense, six 14C ages were made in different bodies, dated between 5240±110 yBP and 3910±90 yBP respectively. This represents that in ca.1330 years the beach prograded 290 meters. Temperature during the history of the chenier has two maximum values at bodies 1 and 6 (ca. 22.5 ºC), with a minimum at body 7 (18.5ºC). Considering the salinity values, it ranges between 32.53 and 33.16?, showing little variability.We document for the first time in Argentina a precise age-controlled model for the Holocene beach ridge evolution. The sudden change in the ridge deposits composition, from sandy sediments to almost exclusively bioclastic (carbonate) between bodies 1-2 and 3-8 can be interpreted as a combined result of an increase of carbonate productivity along with a decrease of siliciclastic supply along the coast. This stage would have been developed approximately 5ka B.P., in coincidence with the Holocene Climatic Optimum or Mid-Holocene Thermal Maximum when the very special climatic conditions may have led to the proliferation of large communities of benthic organisms, particularly bivalve mollusks, which are not so abundant today.In summary, it is possible to hypothesize that the strong activity of the Brazil current during the Mid-Holocene thermal maximum generates excellent condition to the development of a carbonate warm beach similar to that occurred at tropical areas nowadays.