Abstract In this study investigates the predatory activity on a Holocene gastropod from the Argentinean Malacological Province. Buccinanops deformis (King, 1832), endemic in the southern hemisphere, is a typical taxon of cold-waters and high latitudes. We observed direct evidence of durophagy as lethal and sublethal damage. Sublethal damage was studied using traces of repaired shell as an indicator of activity by durophagous predators. Three parameters were used to evaluate durophagous predation: frequency of predation, repair scar frequency and prey effectiveness. A predatory-prey interaction was identified between Danielethus crenulatus (A. Milne-Edwards, 1879) and B. deformis during the Holocene on the Argentinean littoral coast. The low value observed for shell repair frequency (0.06), the low frequency of predation (0.16), and the intermediate value for prey effectiveness (0.37) may be interpreted because of a low but effective predatory activity.

Resumen En este estudio se investiga la actividad depredadora sobre una especie de gasterópodo del Holoceno de la Provincia Malacológica Argentina. Buccinanops deformis (King, 1832) es una especie que habita en aguas frías en latitudes altas, endémica del Hemisferio Sur. Este estudio está basado en la evidencia directa de la durofagia tanto en daño letal como subletal. El daño subletal fue inferido utilizando la evidencia de los exoesqueletos reparadas como un proxy de este tipo de actividad. Se emplearon tres parámetros para la evaluación de la depredación durófaga: la frecuencia de depredación, la frecuencia de cicatrices reparadas y la efectividad de la presa. Se identificó la interacción depredador-presa entre el cangrejo Danielethus crenulatus (A. Milne-Edwards, 1879) y B. deformis durante el Holoceno en las costas litorales argentinas. El bajo grado de la frecuencia de depredación (0.16) y reparación del exoesqueleto (0.06) y el valor medio de la efectividad de la presa (0.37) se interpretaron como resultado de una baja pero efectiva actividad depredadora.

In the large Bahia Blanca estuarine system (province of Buenos Aires) there are sectors without a comprehensive assessment of submarine morpho-sedimentological features. Within this context, in this paper the seabed along the north coast of the entrance to the Principal channel of the estuary is studied. Acoustic surveys (echo sounder and side scan sonar) and sediments sampling were performed. The presence of depositional and erosional geoforms was determined. Among those of accretional nature have been established different types of dunes and sand ribbons. The erosive forms are scarps, relatively continuous and irregular, and rocky isolated outcrops. Also, seabed cohesive sediments, both massive and stratified, were differentiated. As anthropogenic forms furrows dredging and the presence of pipes for oil and metal debris were distinguished. The analysis of geoforms and sediments determined that in the east sector of the coastal strip studied prevails sediment transport as bedload towards the Continental Shelf, while in shallow waters of the western sector, there is a marked litoral drift towards the inside of estuary. The conjunction of morphological and sedimentological data allowed to extend the characterization of two ancient coastal levels, indicative of stabilization periods of sea level during the last transgressive process.

En el amplio sistema estuarial de Bahia Blanca (provincia de Buenos Aires) existen sectores carentes de un conocimiento integral de las características morfosedimentológicas submarinas. Dentro de este contexto, en el presente trabajo se estudia el fondo marino a lo largo de la costa norte ubicada en el sector de entrada del Canal Principal del estuario de Bahía Blanca. Se realizaron relevamientos acústicos (ecosonda y sonar lateral) y muestreos de sedimentos de la superficie del fondo. Se determinó la presencia de geoformas depositacionales y erosivas. Entre las de carácter acrecional se han establecido diferentes tipos de dunas y lineaciones arenosas (sand ribbons). Entre los rasgos generados por erosión se encuentran escarpas, relativamente continuas e irregulares y afloramientos rocosos aislados. Asimismo, también se diferencian fondos de sedimentos cohesivos, tanto masivos como estratificados. Como formas antrópicas se distinguen surcos de dragado y la presencia de cañerías de oleoductos y restos metálicos. El análisis de las geoformas y los sedimentos permitió establecer que en el sector este de la franja costera estudiada, prevalece un transporte sedimentario como carga de fondo hacia la Plataforma Continental, mientras que en aguas someras del sector oeste, existe una marcada deriva de sedimentos hacia el interior del estuario. La conjunción de datos morfológicos y sedimentológicos posibilitó ampliar la caracterización de dos paleoniveles costeros, indicativos de períodos de estabilización del nivel del mar durante el último proceso transgresivo.

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.

On the north shore of the Bahia Blanca estuary, Buenos Aires, Argentina, sand shell ridges composed by a high proportion of boulders of sedimentary origin are present. Many of these clasts show significant signals of bieorosion. The procedence of this material is linked to the extensive abrasion platforms present to the east of the estuary (coasts of Pehuén Co). Two ichnofacies were established: Trypanites ichnofacies comprising Gastrochaenolites, Maeandropolydora, Trypanites, Gnathichnus and Radulichnus, typical of hard substrates and Skolithos ichnofacies represented by Ophiomorpha galleries developed in soft substrates. Both characterize a shallow coastal environment. During the Holocene transgression, coastal substrates were affected by the advancing sea and eroded fragments were transported by littoral drift into the estuary. Finally, storm events deposited material with signs of bioerosion in the littoral ridges. Today, many of these boulders, which exhibit gradations in size into the estuary, are in transit across the beaches by east-west littoral drift.

En la costa norte del estuario de Bahía Blanca, provincia de Buenos Aires, Argentina, se presentan cordones litorales compuestos por arena conchilífera y elevada proporción de rodados de origen sedimentario. Muchos de estos clastos muestran importantes signos de bieorosión. La procedencia de este material se vincula con las extensas plataformas de abrasión presentes al este del estuario (costas de Pehuén Co). Se determinaron dos icnofacies: la icnofacies de Trypanites integrada por Gastrochaenolites, Maeandropolydora, Trypanites, Gnathichnus y Radulichnus, propia de sustratos duros y la icnofacies de Skolithos representada por galerías de Ophiomorpha desarrollada en sustratos blandos. Ambas, caracterizan un ambiente litoral somero. Durante la transgresión holocena, los sustratos costeros se vieron afectados por el avance marino y los fragmentos erosionados fueron transportados por deriva litoral hacia el interior del estuario. Finalmente, eventos de tormenta depositaron en los cordones litorales el material con signos de bioerosión. En la actualidad, muchos de estos rodados que presentan gradación en tamaño hacia el interior del estuario, se hallan en tránsito en las playas por deriva litoral este-oeste.

A evolução geológica da plataforma continental e zona costeira do sul do Brasil no período Quaternário é aqui enfocada. Registros sísmicos de alta resolução da Lagoa dos Patos revelaram canais que dissecaram profundamente a região antes da formação da planície costeira atual. Dois períodos de incisão de canal são inferidos, baseados principalmente em registros sísmicos. Os cursos fluviais mapeados por sísmica podem ser conectados àqueles reconhecidos na plataforma continental adjacente por estudos anteriores. à montante, a incisão da drenagem ocorreu principalmente no prisma sedimentar costeiro depositado durante eventos pretéritos de nível do mar alto. A rede de paleodrenagem, reconhecida na planície costeira e na plataforma continental, representa um sistema de ligação da bacia de drenagem continental aos depocentros da bacia marginal, sobrepassando a plataforma continental exposta durante uma regressão forçada. A incisão da drenagem na planície costeira e plataforma continental do Rio Grande do Sul desempenhou um papel importante na arquitetura marginal, distribuição de fácies e no espaço de acomodação durante as flutuações do nível do mar no Quaternário.

The geological evolution of the continental shelf and in the coastal zone of southern Brazil during the Quaternary period is addressed in this study. High-resolution seismic records obtained at the Patos Lagoon revealed the presence of channels that deeply dissected the coastal zone before the formation of the modern, low-relief, coastal plain. Two periods of channel incision are inferred, based mainly on seismic records. The paleodrainage paths mapped from the seismic records can be connected with those recognized by previous studies on the adjacent continental shelf and slope. Upstream, the drainage incised mainly into the coastal prism deposited during previous sea-level highstand events. The paleodrainage network, recognized on the coastal plain and continental shelf, represents a river-shelf system, linking the drainage basin to the depositional settings on the marginal basin, bypassing the continental shelf exposed during a forced regression event. The drainage incised into the coastal plain and continental shelf of Rio Grande do Sul played an important role in the basin-margin architecture, facies distribution and accommodation during the Quaternary sea-level fluctuations.

Bahia Blanca estuary located in the south of Buenos Aires province, Argentina, is characterized by a low coastal area with large tidal plains, islands, and a complex network of tidal channels of different dimensions. The latter form an interconnected tidal channel system. The aim of this study was to determine the net bed load sediment transport in an interconnected channel system (Cabeza de Buey, El Alambre, La Lista, and Principal channels) through an analysis of the morphological features of the bottom using acoustic data (echo sounder and side scan sonar). Cabeza de Buey channel is connected to La Lista channel through El Alambre channel. The first two flow into the Principal channel. Cabeza de Buey channel has a fiat bed of mud with very fine sand and medium-sized dunes of fine sand in its inner sector that display an asymmetrical section according to the ebb flow. These dunes continue to develop uninterrupted towards El Alambre channel, where large dunes (1.5 m high) of very fine sand originate. Their crest lines are either sinuous or barchanoid and they have their lee towards the ebb current In La Lista channel, medium-sized asymmetrical dunes and large sand ribbons form on a rocky bottom according to the ebb flow. The sediment mobilized by the currents is characterized by the fine sand - silt fraction. The distribution and configuration of bed forms reveal that the bed load sediment transport is mainly concentrated in La Lista channel. Thus, only one tidal channel regulates the dispersal of sandy material, transporting a significant volume of sediment towards the shipping channel.

El estuario de Bahía Blanca, ubicado al sur de la provincia de Buenos Aires (Argentina), constituye una costa baja con extensas llanuras de marea, islas y una compleja red de canales de diferentes magnitudes. Estos últimos integran un sistema de canales interconectados. El objetivo de este trabajo es establecer la circulación sedimentaria como carga de fondo en un sistema de canales interconectados (Cabeza de Buey, El Alambre, La Lista y Principal) mediante el análisis de los rasgos morfológicos de fondo, utilizando métodos acústicos (ecosonda y sonar lateral). El canal Cabeza de Buey se comunica con el canal La Lista a través del canal El Alambre. Los dos primeros vierten sus aguas en el canal Principal. El canal Cabeza de Buey posee un lecho plano constituido por material pelítico con arena muy fina. En su tramo más interno se presentan dunas medianas compuestas por arena fina, con un perfil asimétrico según el reflujo. Estas formas, continúan su desarrollo ininterrumpidamente hacia el canal El Alambre, donde se originan dunas grandes (altura 1,5 m) formadas por arena muy fina. Sus crestas son sinuosas o barjanoides y poseen su talud hacia la corriente de bajante. En el canal La Lista se determinan dunas asimétricas medianas orientadas según el reflujo y grandes linea-ciones sedimentarias (sand ribbons) sobre un fondo consolidado. El sedimento movilizado por la corriente corresponde a la fracción limo-arena fina. La distribución y configuración de las geoformas evidencian que el transporte sedimentario como carga de fondo se concentra principalmente en el canal La Lista. Es así como en el sistema de canales estudiado, sólo un canal de marea regula la dispersión de arena, transportando un importante volumen hacia el canal de navegación.

Tidal sandbanks are sedimentary features com monly found at the inlets of the tidal channels of Bahía Blanca Estuary (south Buenos Aires Province). To improve our understanding of the processes and mechanisms responsible for the formation and maintenance of the tidal sandbanks in estuaries, we carry out this study. We focus in a tidal sandbank located at La Lista channel mouth, denominated La Lista bank. It has an elongate shape with 4 km long. Bathymetric surveys, side-scan sonar records, grain-size characteristics of bottom sediment and bottom current measurements were used to define the hydrodynamics and sediment transport. Ebb-dominant current is the source of sand for this its south face is flood dominated, whereas the opposite face is ebb dominated, resulting in a clockwise residual transport. The rate of net bedload sediment transport estimated on the southern side of the sandbank is 0.00451 g cm-1 sec-1 in the flood direction and on the northern side is 0.02188 g cm-1 sec-1 in the ebb direction. Over the last years the shape of this bank have undergo several modifications and the accumulation of sand resulted in the lengthwise and elevation growth of it.

Los bancos de arena son rasgos comunes en la boca de los canales de marea del estuario de Bahía Blanca (sur de la provincia de Buenos Aires). Este estudio tiene por objetivo incrementar el conocimiento sobre los procesos y mecanismos de formación y mantenimiento de los bancos estuariales. La investigación se centró en un banco elongado, de 4 km de largo, ubicado en la boca del canal La Lista, denominado banco La Lista. A través de datos acústicos (ecosonda y sonar lateral) se determinaron rasgos morfológicos de fondo. Estos, conjuntamente con muestras de sedimento y mediciones de corrientes, posibilitaron definir la hidrodinámica y el transporte sedimentario. La arena movilizada desde el interior del canal La Lista por la corriente de bajante constituye la principal fuente de sedimento del banco. Sobre el flanco norte del banco, el transporte neto de arena como carga de fondo se produce hacia el sureste debido a la corriente de reflujo. Aquí, el transporte de sedimento estimado fue de 0,02188 gr cm-1 seg-1. En oposición, en el flanco sur, la dominancia del flujo produce un transporte de 0,00451 gr cm-1 seg-1. Por lo tanto el mantenimiento del banco es el resultado de una asimetría de flujo/reflujo sobre sus flancos, generándose un transporte sedimentario residual en sentido horario. En consecuencia en los últimos años el banco ha experimentado cambios en la configuración de la cresta y crecido vertical y longitudinalmente.

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.

Um levantamento sísmico de alta resolução foi realizado na Lagoa dos Patos, sul do Brasil, a bordo da Lancha Oceanográfica Larus da Fundação Universidade Federal do Rio Grande (FURG). Foram levantados aproximadamente 400 km de perfis de 3,5 kHz, os quais apresentaram uma resposta acústica de boa penetração e resolução. Anomalias sísmicas, incluindo turbidez acústica e bolsões de gás, revelam que sedimentos carregados de gás são comuns em diversas áreas do corpo lagunar. As acumulações de gás na Lagoa dos Patos são controladas pela distribuição espacial das fácies sedimentares. Tanto as acumulações de gás na forma de 'cortinas', como as 'zonas acusticamente turvas', ocorrem em área de baixios paleotopográficos, vinculados a vales e canais fluviais que se desenvolveram na planície costeira do Rio Grande do Sul durante os eventos regressivos/transgressivos do Quaternário.

A high resolution seismic survey was conducted in the Patos Lagoon, southern Brazil, aboard of the research vessel LARUS of the Fundação Universidade Federal do Rio Grande (FURG). Around 400 km of 3.5 kHz seismic profiles were collected, which provided acoustic signals of good penetration depth and resolution. Seismic anomalies, including turbidity and pocket gas, revealed that gas-charged sediments are common in several areas of the lagoon. The gas accumulations in the Patos Lagoon are controlled by the spatial distribution of the sedimentary facies. Either in 'curtains' or in 'acoustic turbid zones', the main gas accumulations occur in areas with paleotopographic lows related to fluvial channels and valleys developed in the Rio Grande do Sul coastal plain during regressive/transgressive events of the Quaternary.

In spite of their size, micromolluscs can be affected by bioerosive activity of organisms on their valves. Of 107 molluscs species found in Holocene sand shell ridges, 35% (38 species) belong to micromolluscs. Only 22 species show bioerosion on their valves. Among bioerosive structures are borings and etchings. Borings are due to predatory activity by naticid and muricid gastropods done in life of micromolluscs. The holes are circular and truncated cone-shaped. A single complete hole is invariably present on gastropods, while common multiple and uncomplete borings occur on bivalve shells. These borings are assigned to Oichnus simplex Bromley and Oichnus paraboloides Bromley. Another group of trace fossils is represented by etchings made by cheilostome bryozoans. The pits have elliptical morphology and are uniserially arranged on micromollusc valves; they are assigned to Leptichnus dromeus Taylor, Wilson and Bromley and are done either in life as postmortem. From an ethologic point of view the trace fossils have been grouped in two categories: predation structures ( Praedichnia ): Oichnus simplex and Oichnus paraboloides and dwelling traces ( Domichnia ): Leptichnus dromeu.

A pesar de su pequeño tamaño, los micromoluscos pueden verse afectados por la actividad bioerosiva sobre sus valvas. En cordones areno-conchilíferos costeros holocenos se determinaron 107 especies de moluscos, de los cuales 38 especies corresponden a micromoluscos. Entre estos últimos, 22 especies muestran signos de bioerosión sobre sus valvas bajo la forma de orificios y grabados. Los orificios debidos a la actividad predadora de gastrópodos natícidos y murícidos tienen forma circular, cilíndrica o troncocónica y son producidos en vida del animal. Los gastrópodos presentan un único orificio mientras que en los bivalvos hay orificios múltiples e incompletos. Son atribuidos a Oichnus simplex Bromley y Oichnus paraboloides Bromley. Los grabados realizados por briozoos cheilostomados sobre las valvas de los micromoluscos se asignan a Leptichnus dromeus Taylor, Wilson y Bromley y pueden producirse tanto en vida como postmortem. Aparecen como hoyos de forma elíptica y dispuestos uniserialmente. Desde el punto de vista etológico, las trazas fósiles se agrupan en: estructuras de predación ( Praedichnia ): Oichnus simplex y Oichnus paraboloides y estructuras de habitación ( Domichnia ): Leptichnus dromeus.

The Beagle Channel connects the Pacific and Atlantic oceans in the southernmost part of South America, and has a subantarctic environment. It is a deep basin (300 m depth) separated from the Atlantic Ocean by a shallow sill (30 m depth). The Beagle Channel is a tectonic valley that was completely covered by ice during the Last Glaciation. Later, it was occupied by a glacial lake from 12,000 to 8,000 years B.P. and then flooded by the sea, reaching a maximum sea level between 6,000 and 5,000 years B.P. A geophysical survey (side scan sonar and 3.5 kHz profiler) was carried out in the channel to analyze the sea-bed and sub-bottom sedimentary facies. Metamorphic Mesozoic basement rocks are overlain by glacial deposits (till) and fining upwards sequences. These sequences represent different stages of the glacial retreat, showing in their upper parts proglacial lacustrine facies. Holocene marine deposits overlie the glacigenic deposits. Palaeovalleys and submerged fluvial sequences transgressed by marine deposits were also observed.

El canal Beagle conecta los océanos Pacífico y Atlántico en el extremo meridional de Sudamérica y se ubica en el ambiente subantártico. Conforma una cuenca de unos 300 m de profundidad máxima y está separada del océano Atlántico por un umbral de 30 m de profundidad. El canal es un valle tectónico que fue completamente cubierto por el hielo glacial durante la última glaciación. Posteriormente, el canal fue ocupado por un lago glacial desde los 12.000 a los 8.000 años A.P., cuando fue invadido por el mar que alcanzó un nivel máximo entre los 6.000 y 5.000 años A.P. Con el objetivo de analizar las facies sedimentarias superficiales y del subfondo del canal se realizó un relevamiento geofísico con sonar de barrido lateral y un perfilador de 3,5 kHz. Sobre un basamento constituido por rocas metamórficas del Mesozoico, se identificaron depósitos de till y secuencias granodecrecientes que representan distintos estadios del retroceso glaciar, evidenciando hacia la sección superior facies lacustres y por encima depósitos de la transgresión marina del Holoceno. Además, se han identificado paleocauces y secuencias fluviales cubiertas por sedimentos marinos transgresivos.

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.

In the north patagonian coast (NW corner of San Matías Gulf), at the inlet of San Antonio Bay, an extended tidal delta develops to depths of 14 m. A semidiurnal tidal regime, with spring ranges of about 9 m, is the main circulation factor. The more important morphological features are the presence of intertidal banks attached to the main channel and the asymmetry of the ebb tidal delta resulting from the interactions of waves coming from the east and wind-driven currents from the west. By means of seismo-acoustic methods (side-scan sonar and precision depth surveys) and sampling of bottom sediments, an analysis of the bedform sequence (sand waves, megaripples, sand ribbons, comet marks) was carried out in order to recognize the environmental conditions responsible for their generation and to improve the knowledge of the regional sedimentary dynamics. Sand waves develop at different sectors (Fig. 2). the larger ones, up to 4 m high, are found isolated at the deepest parts and have a "barchan type" configuration (Fig. 3). Asymmetrical sand waves up to 2 m high are also observed on depressions confined to rocky bottoms; their slopes are oriented to the south. Over the stoss sides, oblique megaripples run 30º-40º with respect to the sand wave crests (Fig. 4). On the outer platform, sand waves do not exceed 1.5 m in height and are grouped in well defined fields. They generally slope to the S-SE, although their crests pattern is variable (Fig. 2). Megaripples are found on the shallower areas. All these bedforms are composed of coarse to fine sands (mean size from 0.28 phi to 2.31 phi). The sedimentary lineations are mostly sand ribbons, (according to Kenyon, 1970) and develop on the adjacent shallow shelf, at minimum depths of 8 m (Fig. 2). Their heights are less them 1 m, and can be grouped into two types, according to their spacing: 1) ribbons with spacing in the order of 10 m; and 2) ribbons with 30-40 m spacings (Fig. 5). Their lengths reach more than 200 m and their orientations vary between N140º and N170º, depending on location (Fig. 2). Sonographs also permitted to establish the presence of comet marks (Werner & Newton, 1975), mainly associated with sand-ribbon fields. Both bedforms are composed of sand moving over a reflective substrate mostly composed of gravels. The asymmetrical sand waves developed on the bottom of the main channel, have straight or "barchan-like" crests, and indicate a net offshore sediment transport. The detailed morphologic analysis of the shallow shelf adjacent to San Antonio Bay establishes the coexistence of different bedforms occupying contiguous sectors. The scheme of figure 10 shows the lateral sequence of the bedforms: 1) sand waves; 2) short-spaced sand ribbons; 3) long-spaced sand ribbons. The mostly sharp, although slightly transitional in some places, boundaries between different bedforms suggest that these differences cannot be assigned to variations in flow intensity. On the other hand, the bedforms are not dependent upon depth, as within a few tens of meters they change without significant depth variations. Sediment availability is therefore the prime factor in the development of bedform types. The sectors with higher sediment availability are characterized by sand waves. The source material comes from the coastal system, from the inner environments of San Antonio Bay (Fig. 10). At intermediate zones, with less sediment input, sand ribbons occur. In sum, sand waves develop in sectors having a thick sediment layer which allows their development; sand ribbons and comet marks are formed in areas with a thin bedload transport layer (Fig. 11).

En el extremo NO del golfo San Matías (sector exterior de la bahía de San Antonio) grandes bancos arenosos forman, hasta una profundidad de 14 m, un gran delta de marea, el cual constituye la característica morfológica general. La circulación del área es gobernada por un régimen semidiurno macromareal con amplitudes de sicigias de hasta 9 m. Por medio de métodos sísmico-acústicos (sonar de barrido lateral y ecosonda) y muestreo de sedimentos del fondo se analizaron rasgos morfológicos subácueos (ondas de arena y megaóndulas, lineaciones sedimentarias, marcas de cometa), conducentes a inferir las condiciones físicas ambientales referidas a su generación y a completar el conocimiento de la dinámica sedimentaria regional. Ondas de arena de hasta 4 m de altura se presentan agrupadas en campos bien definidos o en cubetas sedimentarias delimitadas por material rocoso con una configuración tipo "barján". En general, el talud de las mismas se orienta hacia el sur-sureste. Sobre las rampas de las ondas de arena y en sectores de menor profundidad es común el desarrollo de megaóndulas. La composición de todas estas geoformas varía desde arena gruesa a fina (0,28 phi-2,31 phi). Las lineaciones sedimentarias se encuentran a partir de los 8 m de profundidad sobre la plataforma adyacente al delta de marea. Se agrupan en dos tipos: 1) franjas con espaciamiento del orden de los 10 m; 2) franjas con espaciamiento de 30-40 m. Estas acumulaciones arenosas, cuyo relieve no supera el metro, alcanzan los 200 m de longitud. Asociadas a las mismas se producen marcas de cometa ("comet marks"). La generación de ambas geoformas se relaciona a un sedimento arenoso movilizado sobre un sustrato gravoso. La estrecha relación entre la dinámica sedimentaria y formas de fondo es evidente y refleja un transporte neto como carga de fondo hacia el exterior de la bahía de San Antonio. El análisis morfológico determina la coexistencia de las diferentes formas mencionadas, las que se ordenan en sucesión lateral según el siguiente patrón: 1) ondas de arena y megaóndulas, 2) lineaciones sedimentarias de pequeño espaciamiento y marcas de cometa, 3) lineaciones de gran espaciamiento. La disponibilidad de sedimento es el factor preponderante que define la generación secuencial de las diversas formas de fondo. Así, los sectores que reciben mayor aporte del interior de la bahía de San Antonio se caracterizan por el desarrollo de ondas de arena. En cambio en aquellas zonas intermedias con menor disponibilidad de sedimento, las lineaciones arenosas y marcas de cometa resultan el rasgo característico.

This investigation is a contribution to the geological kowledge of the Argentine marine shelf. The study area is a sector of the internal shelf up to 25 m of depth, along a coastal zone extending approximately from Bahía Blanca estuary to Anegada bay (Fig. 1). The purpose of this report is to define, through the analysis of surface sediments and the marine subbottom, the present sedimentological conditions and establish the coastal sedimentary paleoenvironments that characterized the last transgressive holocene period. The specific oceanographic tasks, carried out with the research vessel El Austral, comprised the extraction of sedimentological samples (surface and core samples) and 3.5 kHz seismic survey (Fig. 2). The sediments were analyzed both in their texture (sieve and/or pipette) and their mineralogical contents (microscopy and X-ray difractometry). Generally, in bottom surface sediments, the psammitic fraction (medium-fine) is predominant, albeit important concentrations of silt-clay are found on the coastal region, while the gravel-type sizes (biogenetic remains, sandstone and “patagonic pebbles”) appear in the more external sector. The 3.5 kHz seismic survey, together with the cores, indicate that the shelf subbottom adjacent to the west and northwest coast is formed by a sequence of fine sediments (silt clay-fine sand), determined acoustically up to a depth of 7 m. Both texture and mineralogy, show lateral continuity towards the inner Bahía Blanca estuary. The seismic records determine a facies with a reflective configuration of the parallel-subparallel type and hummocky, associated with conditions of uniform sedimentation and to a shallow low energy palaeoenvironment, in marked contrast with the present hydraulic regime and sedimentation. During a period when the sea level was lower than the present one, the coastal palaleogeographic configuration would have determined restricted marine conditions, with a relevant continental contribution silt-clay type sediments. This ancient river-estuary sedimentary environment, defines seismically up to Anegada bay, would be formed by channels of various dimensions, as is evident from the seismograms. The silt-clay materials found on the shelf subbottom are thus associated with a wide deltaic palaleoenvironment, which may be put into correlation with the ancient drainage basin of the Colorado river, where marine conditions were influential only due to the effect of tides. It is to be noted that south of the present Colorado river mouth an ancient river bed of it is defined in the subbottom. The transgressive holocene process originated an important increase in the energy of the transport agent. Consequently, sandy material, with pebbles and bioclasts of different sizes and fragmentation, cover in erosive unconformity the silt-clay materials of the subbottom. In the cores, the thickness of this surficial sandy stratum is on the order of 40-50 cm, although towards greater depths seismic information establishes that it increases. Terraces and topographic slopes, together with the presence of pebbles and concentrations of heavy minerals and biogenic remains, represent morphosedimentological relict elements, belonging to periods of stabilization in the marine ascent and to ancient coastal environments. The regional distribution of the bottom sedimentary facies, as well as certain morphological marine features, are thus valuable indicators for inferring the present sedimentary dynamics. On the basis of the textural and statistical results, sedimentological evaluations were carried out to infer transport and sedimentation modes. Likewise, as part of the mineralogical analysis of the sediments, the regional distribution or tendency of the diverse mineral species was also taken into account, since through their different specific weights and differential hydrodynamic behavior, they point to sedimentary sources and/or energy conditions that operate on the shelf investigated. It then emerges from these observations the predominance of the bedload sediment transport in the more external sector of the study area. Contrarily, on the more coastal zone, the influence of silt-clay coming from the inner Bahía Blanca estuary and the Colorado river contribution is evident. In the latter case, considering the textural areal analysis of the sediments, the “sedimentary plume” developed starting on the west coast, adjacent to the Colorado river mouth, and extending northeastward, is remarkable. The distributive tendency of the heavy mineral concentration clearly points to an important source of sedimentary contribution, related to the ferrotitaniferous deposits present on the shores of the study area, from San Blas bay up to the Negro river mouth. These sediments would be affected by an intense coastal drift towards the zone of the present work. On the other hand, the bed forms, particularly sand waves, tend to reach an equilibrium state relative to the dominant flux conditions. The asymmetry of these geoforms is then an indicative feature of the sedimentary dynamics and circulation as bedload transport. The morphological evaluation of the sand waves found in the northern and southern sectors of the area investigated, in conjunction with analysis of the distribution of the sedimentary surface facies (textural-mineralogical), support the conclusion that the sedimentary bottom dynamics in the shelf environment studied is represented by a northward net bedload sediment transport.

Se analizan sedimentos superficiales y subsuperficiales, y rasgos geomorfológicos de fondo, de un sector de la plataforma continental interior entre el estuario de Bahía Blanca y bahía Anegada. El objetivo es definir condiciones de sedimentación actual y paleoambientes costeros. Las tareas oceanográficas, realizadas con el B.O. El Austral, comprendieron extracción de muestras sedimentológicas, superficiales y de testigos, y relevamiento sísmico con 3,5 kHz. Se establece que el subsuelo se halla conformado por una secuencia de sedimentos finos (limo arcilla-arena fina). Los sismogramas determinan una facies reflectiva con estratificación paralela-subparalela y “hummocky”, relacionada a condiciones de sedimentación uniforme y a un paleoambiente somero de baja energía. Durante un período con nivel del mar por debajo del actual, la configuración paleogeográfica costera habría determinado condiciones marinas muy restringidas, con un relevante aporte continental de sedimentos limo arcillosos. Estos se asocian a un amplio paleoambiente de sedimentación deltaico, correlacionable a la antigua cuenca hídrica del río Colorado, donde el medio marino tuvo una influencia parcial, sólo por efecto de las mareas. El proceso transgresivo holoceno produjo un sensible aumento de la energía en el ambiente hidrodinámico de sedimentación, lo que originó la depositación de material arenoso sobre los sedimentos deltaicos. Terrazas y desniveles topográficos, junto a la presencia de rodados y concentraciones de minerales pesados y bioclastos, resultan elementos morfosedimentológicos relícticos de períodos de estabilización en el ascenso marino y antiguos ambientes litorales. Evaluaciones de las facies superficiales permiten establecer un dominio del transporte de sedimento como carga de fondo en el sector más extremo del área. En cambio, sobre la zona más costera se evidencia un aumento del transporte en suspensión, resultando notable la influencia de material limo-arcilloso proveniente del interior del estuario Bahía Blanca y del río Colorado. Los minerales pesados marcan un importante aporte por deriva costera, proveniente de las playas del sur del área investigada. Por último, las características morfológicas de grandes ondas de arena presentes en el área, conjuntamente con el análisis de la distribución de las facies superficiales (texturales-mineralógicas), conducen a definir la dinámica sedimentaria dominante sobre el ambiente de plataforma estudiado, representada por un transporte de sedimento como carga de fondo en dirección norte.