Sedimentary dynamics and morphological evolution of a deep channel in the Bahia Blanca estuary

Abstract

The Bahía Blanca coastal region is located to the south of Buenos Aires Province. It is an extended mesotidal system, with channels of diverse dimensions and characteristics. The objective of this work is to analyse the morphological features of the Tres Brazas channel and its environment, defining sedimentary dynamics and associated evolutionary trends. Sand waves of up to 1.5 m and megaripples are evidence of sediment migration. Sand transported from the interior of the Tres Brazas channel is the sediment source for a spit bar developed from the right bank of its outlet. At the same time, inside the Tres Brazas and Tierra Firme channels the erosive processes on their flanks are intense. The latter, composed of sandy clay silt, correspond to the Holocene marine regression, according to 14C dating. The more significant accretional forms are point bar deposits, genetically related to sediment transported by the ebb current. At the junction between the Tres Brazas and Tierra Firme channels a depression 26 m deep developed. Seismostratigraphy indicates that this scour originated during the last marine regressive period, when turbulent conditions developed at the meeting of the two channels, generating an erosional form. In a later phase, due to a greater availability of sandy sediment, accretion took place at the south and south-east flanks, while erosion at the north and north-west flanks foreshadowed the present evolutionary trend.