Modelo sedimentario de colmatación de estuarios dominados por limo, Pcia. de Buenos Aires.

ISLA, FEDERICO I. Y ESPINOSA, MARCELA A.

Actas VII Reunión Argentina de Sedimentología.

AAS

Lugar: Salta; Año: 1998 vol. 1 p. 24 - 36

The Pampa Interserrana region of the Buenos Aires province is dominated by sandy silts loessic origin. The Holocene sea level fluctuation affected the coastline leaving regressive estuarine sequences infilling small fluvial valleys. Based on sedimentologic and diatomologic analysis of the estuarine sequences of the Las Brusquitas (ALB), La Ballenera (LB) y Claromecó (AC) creeks, an infilling sedimentary model was constructed. These sequences originated after the maximun sea level reached about 6,500 years BP. The ALB sequence deposited between 6,190 and 3,900 years BP. The LB sequences spanned between 6,120 and 4,120 years BP. The AC sequence was younger, deposited between 2,430 and 810 years BP. Only in the AC sequence, the lag gravels of the bottom of the estuary were recognized. In the other sequences, transgressive silts and sands covered the silty sediments with burrows, corresponding to oxydized root remains from the bottom of the creek. The basal estuarine sediments are black massive silts (with very fine sand subordinated), corresponding to the settling from the turbidity maximun. Overlying these massive silts, sediments (muds and sands) are laminated with gastropod remains (mostly Littoridina). To the top, the beds are sandier and with ligther colors, with diatom assemblages suggesting brackish waters. Finally, the three sequences were covered by fine sands corresponding to littoral dunes. Along the three sequences, salinity changes were distinguished. In ALB and LB, marine brackish diatom assemblages at the base evolved to less saline waters. The AC sequence has high ash content in several layers. Employing radiocarbon dating, it was possible to calculate the differencial infilling rate along these sequences. The initial stages, close to the maximun sea level and deposited from turbidity maximun, have a higher sedimentation rate. The final regressive stages have a lower and seasonal (laminated) sedimentation rate.