O estuário da Bahía Blanca (Argentina) tem uma configuração morfológica resultante de processos hidrológicos e sedimentares relacionados a mudanças do nível do mar durante o Quaternário Tardio. Este sistema estuarino ocupa uma ampla planície costeira com uma densa rede de canais de marés, ilhas de baixa altitude e extensas áreas de intermarés com baixa declividade. Nesta área, pouco se conhece sobre as unidades sedimentares do fundo marinho. Portanto, análise estratigráfica da costa norte do estuário da Bahía Blanca foi realizada usando sísmica de alta resolução (3,5 kHz) com a finalidade de: i) definir sequências quaternárias, ii) descrever estruturas sedimentares, e iii) determinar as condições paleoambientais de sedimentação. Os dados estratigráficos sísmicos obtidos e suas correlações litológicas com dados de perfuração apresentaram cinco sequências sísmicas (S1, S2, S3, S4 e S5), das quais S1-S2 foram correlacionadas com um paleoambiente continental referente ao Mioceno-Pleistoceno. Sequências S3 e S4, de fácies litológicas e sísmicas (estruturas de paleocanais e configurações de reflexão progradantes) foram definidas nestas sequências, evidenciando o desenvolvimento de um ambiente ancestral deltaico que foi parte de um amplo sistema de drenagem do Pleistoceno. A sequência S5 foi formada durante os processos transgressivo-regressivos do Holoceno e preenche a coluna sismo-estratigráfica definida neste estudo.

The Bahía Blanca estuary (Argentina) has a morphological configuration resulting from hydrological and sedimentary processes related to Late Quaternary sea level changes. This estuarine system occupies a large coastal plain with a dense net of tidal channels, low-altitude islands and large intertidal flats. Little is known about the sedimentary units of the marine subbottom. Therefore, a stratigraphical analysis of the northern coast of Bahía Blanca estuary was carried out using high resolution seismic (3.5 kHz) in order to: i) define Quaternary sequences, ii) describe sedimentary structures, and iii) determine the paleoenvironmental conditions of sedimentation. The seismic stratigraphic data collected and their correlation with drilling lithological data show five seismic sequences (S1, S2, S3, S4 and S5), of which S1-S2 were found to be associated with a continental paleoenvironment of Miocene-Pleistocene age. Sequences S3 and S4, whose lithology and seismic facies (paleochannel structures and prograding reflection configurations), were defined on these materials, to evidence the development of an ancient deltaic environment which was part of a large Pleistocene drainage system. The S5 sequence was formed during the Holocene transgressive-regressive process and complete the seismostratigraphic column defined in the present study.

The morphological configuration of the Bahía Blanca estuary (southern Buenos Aires province) is the result of hydrologic and sedimentary events linked to Quaternary variations of the sea level. From high-resolution (3.5 kHz) seismic surveys, an analysis is performed of structures of paleochannels found in the sub-bottom of the Principal and Vieja channels. These channels have been identified in different seismostratigraphic sequences. The Principal channel is thus characterized by V-shaped erosive surfaces, with a maximum width of 40 m in their upper section. They would belong to an ancient hydrological river basin, possibly related to the Colorado river. These channels were buried by fluvio-deltaic sediments, which compose a prograding seismic structure. Another set of paleochannels (located in the Vieja channel), of a pattern similar to the previous ones but with a considerably greater widths (350 m), are associated with ancient tributaries of the Napostá creek. The filled sequence would have been deposited under fluvial conditions, finally silting in an estuarine-marine environment. These paleochannels, in which shallow gas accumulations are frequent, make evident that the base level of the ancient drainage system was located below the present sea level. The marine sedimentation, produced by the Holocene transgressive-regressive process, caused the burial of the fluvial channels near the sea, and induced the modification of the drainage network.

La configuración morfológica del estuario de Bahía Blanca, al sur de la provincia de Buenos Aires, es el resultado de los procesos hidrológicos y sedimentarios vinculados a las variaciones cuaternarias del nivel del mar. A partir de relevamientos sísmicos de alta resolución (3,5 kHz), se analizan estructuras de paleocanales halladas en el subsuelo marino de los canales principal y Vieja. Los antiguos cauces han sido identificados en secuencias sismoestratigráficas diferentes. Así, los paleocauces localizados en el canal principal, están caracterizados por una superficie erosiva basal en "V", con un ancho máximo de 40 m en su sección superior. Los mismos pertenecen a una antigua cuenca hídrica, posiblemente relacionada al río Colorado. Estos cauces fueron sepultados por sedimentos fluviales-deltaicos, los cuales componen una estructura sísmica progradante. Otro conjunto de paleocanales, ubicados en el subsuelo del canal Vieja, tienen un diseño similar a los anteriores pero su longitud transversal es considerablemente mayor (350 m), y son vinculados con antiguos tributarios del arroyo Napostá. Su secuencia de relleno se ha depositado bajo condiciones fluviales, culminando su colmatación en un ambiente estuarino-marino. La existencia de estos paleocauces, en los cuales son frecuentes las acumulaciones gasíferas someras (shallow gas), evidencia un nivel de base del antiguo sistema de drenaje ubicado por debajo del nivel del mar actual. La sedimentación marina, producida por el proceso transgresivo-regresivo del Holoceno, originó el soterramiento de los cauces fluviales cercanos al mar, e intervino en la modificación de la red de drenaje costera.

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.

Al sur de la provincia de Buenos Aires se ubica la región costera de Bahía Blanca. La misma es un extenso sistema mesomareal, con canales de diversas dimensiones y características. El objetivo de esta investigación es analizar los rasgos morfológicos del canal Tres Brazas y su entorno, definiendo dinámica sedimentaria y tendencias evolutivas asociadas. Ondas de arena de hasta 1,5 m de altura y megaóndulas evidencian la migración de los sedimentos. La arena transportada desde el interior del Tres Brazas resulta la fuente de aporte de una barra tipo espiga, que se desarrolla desde la margen derecha de su desembocadura. En tanto, en el interior de los canales Tres Brazas y Tierra Firme son intensos los procesos erosivos sobre sus veriles. Estos, constituidos por material limo arcillo arenoso, según dataciones 14C corresponden a la regresión marina holocena. Las formas acrecionales más significativas son point bars, genéticamente relacionadas a la sedimentación producida por la corriente de reflujo. En la confluencia del Tres Brazas y Tierra Firme se desarrolla una gran depresión de 26 m de profundidad. Sismoestratigráficamente, se define que ésta se originó durante el último período regresivo marino, cuando condiciones turbulentas producidas en el encuentro de los canales de marea habrían iniciado el socavamiento del fondo, generándose una geoforma enteramente erosiva. En una fase posterior, por una mayor disponibilidad de sedimento arenoso, se produce la acreción del flanco sur y sureste, mientras que la erosión del veril norte y noroeste resulta indicativa de la tendencia evolutiva actual.