Geochemical and diatom sedimentary evolution of the estuarine area

F.ISLA, B. RUBIO, A. ESCANDELL, M. ESPINOSA, M. GERPE, K. MIGLIORANZA

Mendoza

Congreso; 18th International Sedimentological Congress; 2010

International Association of Sedimentologists

The Chubut River flows from the Andes to the Atlantic Ocean with only one dam constructed at the Middle valley. The estuarine area is dominated by the aggradation of an alluvial plain induced by a complex of spits that enclosed the inlet in the last 5000 years. The river has reduced its flow since the blocking of the upper basin by terminal moraines during the Upper Quaternary. The last two sea-level highstands have flooded this ria, and aggradation have increased during regressions. The area is of particular interest in regard to irrigation channels practiced since the XIX century. Today, the mean monthly flow is less than 10 m3/s with peaks of 74 m3/s. The dynamics of the estuary is dominated by waves (wave-dominated estuary) as the tidal effects attenuate in less than 5 km. Three vibracorings were collected: a) at Gaiman, an area without any effect of the sea (37 km from the coast); b) at Trelew, at the former avulsion plain of the river (18 km from the coast), and c) at Playa Unión, a saltmarsh located 1 km from the beach. At the core from Gaiman (1.54 m long) fresh-water epiphytic diatoms dominate ( Epithemia sorex, Cocconeis placentula, Ulnaria ulna) suggesting the aggradation of an alluvial plain. The core from the alluvial plain (2.19 m long) was composed by fine sand with organic matter at the base that evolved into silty layers to the top. Several unconformities and laminae with heavy minerals were detected by their geochemical composition analyzed by micro X ray fluorescence (Itrax XRF core scanner). Fine-sand laminated layers were perfectly detected by their high content in S and Cl. On the other hand, mud layers had a lower content in Mg and Al with increments in Ca and V. The core from the marsh (1.67 m long) was analyzed in terms of the diatom evolution in order to detect Holocene sea-level and salinity effects. The sand flats from the bottom of the core were dominated by Nitzchia navicularis (mesohalobous and benthic taxa) and evolved into mixed flats, mudflats and marshes to the top. Sharp contacts have been detected between these facies, with wavy and lenticular bedding characterising the mixed flat deposits. The middle of the sequence is dominated by a coastal marine diatom (Paralia sulcata) while the top was dominated by Pinnularia borealis , an aerophilous and brackish/freshwater taxa.