Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920100001&lang=es vol. 17 num. 1 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <link>http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000100001&lng=es&nrm=iso&tlng=es</link> <description/> </item> <item> <title><![CDATA[Geomorfología y dinámica del Canal San Blas, provincia de Buenos Aires (Argentina)]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000100002&lng=es&nrm=iso&tlng=es Se estudió la geomorfología submarina y se efectuaron mediciones de corrientes de marea en el canal San Blas para determinar la dinámica sedimentaria actual. Se relevó un sector del fondo del canal con el sistema batimétrico GeoSwath para describir exhaustivamente la morfología de un campo de dunas. Su asimetría permite inferir un transporte residual del material del fondo hacia el interior de la Bahía Anegada sobre el veril NE del canal. Se realizó un estudio de la dinámica de las corrientes de marea, determinándose sus velocidades y direcciones durante un ciclo completo de marea en toda la columna de agua y a lo largo de un perfil transversal a la costa mediante un ADCP. A pesar que el ambiente corresponde a un régimen micromareal (rango de marea alrededor de 2 m), en el canal San Blas existen fuertes corrientes de marea que desde el sector medio transportan todo el material inconsolidado del fondo, formando un delta de reflujo en la boca externa del canal y un delta de flujo en la Bahía Anegada. Se reconocieron diferentes geoformas costeras que permiten inferir un transporte litoral hacia el exterior del canal sobre la costa sur del mismo. Basándose en sus características morfodinámicas, es posible definir el canal San Blas como una entrada de marea que conecta el mar abierto con la Bahía Anegada.<hr/>The Bahía Anegada is the coastal zone of the southern part of Buenos Aires Province (Argentina), where several environments are recognized: islands, inlets, marshes, tidal plains and different types of beaches. The Bahía San Blas is located at the southern part of the Bahía Anegada, where the San Blas channel separates the mainland from an island (Fig. 1). The San Blas channel is 2.5 km wide and 12 km long. The study area has a tidal amplitude characterized by a mean of 1.62 m, being 2.20 and 0.58 m the high and low tide respectively. The present study shows the results of a bathymetric survey of the San Blas channel and an analysis of the behavior of the tidal currents that affect sediment transport and promote the generation of different bedforms. The aim of this study was to analyze the submarine geomorphology related to the dynamic conditions in order to characterize in detail the sedimentary conditions in this area. A detailed bathymetric study was conducted over a zone of 50 km², covering the San Blas channel, through a digital echosounder Bathy-500 positioned by DGPS operating in real time. Fifteen transversal tracks and three longitudinal tracks were made at the San Blas channel in order to obtain the necessary bathymetric profiles to make a bathymetric chart of the area. Over a zone of 1.5 km², characterized by a field of subaqueous dunes, a Phase Measuring Bathymetric System (FMBS), called swath bathymetry system "GeoSwath Plus" from GeoAcoustics Lt. (UK), was employed in order to determine the detailed dune morphology. This survey yielded details and disposition of the bedforms present on the channel bed with centimetric precision. Bottom sediment samples were collected and tidal currents were measured using an Acoustic Doppler Current Profiler (ADCP) mounted on a ship. The tidal currents were measured during a whole tidal cycle, obtaining the distribution of velocity and direction over the water column, on a track carried out transversally to the channel. These values were computed with the associated WinRiver software. The entire field work (bathymetry, sampling, FMBS and ADCP) was performed with the 6.5 m long boat IADO IV. The coastal features display geomorphological differences in response to a changing dynamic regimen along the channel (Fig. 2). Remarkable morphological and textural differences occur along the coast. Close to the mouth, a dissipative beach, with medium sand and a gentle slope is present. Mobile coastal dunes are common crowning the beach. Nearly the central part, the beach is steeper and composed of gravels developing then a reflective beach. Towards the north, in the inner part of the Bahía Anegada (north of Punta Ramirez), wave-cut platforms and marshes covered with Spartina alterniflora appear, sheltered by cliffs formed by deposits from the Río Negro Formation. The bathymetric map allows distinguishing the existence of significant changes in the morphology throughout the San Blas channel. Based on these differences, the area was divided into 4 zones (Fig. 3), each one with distinctive profiles (Fig. 4). Zones I and II are characterized by a 28 m-depth flat bottom, free of unconsolidated sediments and with steep flanks. These features allow establishing an analogy with a tidal throat. A subaqueous dune field, covering an area of more than 1.5 km², was found in Zone III. Zone IV is a shallow area where sand bars are exposed during low tide. The dune field was characterized by large dunes, with spacing between 40-80 m and heights of 2.5 m, identified at 21 m depth (Fig. 5). Higher dunes occur in deeper zones, reaching values of 4.5-5 m height at around 24 m depths. Most of the subaqueous dunes located on the southern portion of the dune field exhibit a symmetrical cross section and are covered by smaller bedforms of 0.3-1 m high and 0.7 to 0.9 m of spacing. On the other hand, dunes located on the northern side of the field are asymmetrical, with the steeper side towards the inner part of the channel. The mean grain size in the field dune is between medium and coarse sand (1.8 a 0.28 phi) that is in equilibrium with the strong velocities of tidal currents. The results obtained from the current measurements over a tidal cycle show that maximum velocities measured in the San Blas channel reached 2 m s-1 during flood and 1.8 m s-1 during ebb (Fig. 6). The maximum velocities of the flood currents were attained mainly on the northern flank and central part of the channel almost reaching the bottom. On the other hand, the maximum velocities of the ebb currents were found on the deepest site of the profile, on the southern flank, from mid-water up to the surface. At both ends of the channel, sand accumulates due to the reduction in the carrying capacity which leads to the formation of an ebb and a flood tidal deltas. The sand bars exposed in low tide in the inward shallow part in the Anegada Bay is the tidal flood delta formed by the loss in the sediment transport capacity caused by the widening of the channel section. This unconsolidated granular sediment is available to be transported by the tidal currents. Towards the outer part of the channel, the bathymetric map shows shallower depths while nautical charts confirm the presence of submarine bars at the entrance of the San Blas channel (Fig. 7). These bars might be evidence of the occurrence of an ebb tidal delta. The present study allowed concluding that the San Blas channel is a narrow strait between the mainland and an island, connecting the Anegada Bay with the outer sea and showing different submarine topography. Towards the mouth, the channel presents a flat bottom over 80% of its width, showing a U shape. The bottom is free of unconsolidated sediments due to the strong currents that wash away any loose material. The submarine topography of the San Blas channel corresponds very well with the tidal inlet significance. <![CDATA[Estructuras de deformación (¿sismitas?) en la Formación Río Negro, provincia de Río Negro, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000100003&lng=es&nrm=iso&tlng=es A Las estructuras de deformación (ED) en sedimentos blandos comprenden las alteraciones que se producen casi simultáneamente con la sedimentación. Estos procesos se hallan en relación directa con las características internas de los materiales sedimentarios y de los factores externos que actúan sobre ellos. Sus resultados incluyen deformaciones como inyecciones, fracturas, volcanes y laminaciones convolutas que afectan total o parcialmente la estratificación. Los sedimentos blandos deformados por sismos se incluyen bajo la denominación general de sismitas (seismites). Como objetivo de esta investigación se plantea reconocer, por primera vez, estructuras de deformación ubicadas en la Formación Río Negro presentes en el sector norte del Golfo San Matías, en inmediaciones del Faro Río Negro. La metodología empleada consistió en el reconocimiento y descripción de las estructuras, para lo cual se extrajeron muestras para determinar granulometría, mineralogía y contenido de materia orgánica. Se fotografiaron los distintos sectores con deformaciones con el fin de establecer modelos comparativos. El sector estudiado, de 4 km de extensión, se ubica entre el faro de Río Negro y el inicio del Banco Verde y morfológicamente corresponde a un frente acantilado con orientación ENE-OSO. En él se determinaron las siguientes ED: a- de carga simple, pseudonódulos contiguos y aislados, y estructuras complejas; b- de escape de fluidos y c- estructuras de deslizamiento basal y por presión dirigida. El origen de las deformaciones se debe a las características de las sedimentitas y a los procesos que las afectan tales como los efectos por carga, escapes de fluidos y las presiones dirigidas. Como origen de estos procesos se señalan: la presión de la columna litológica, las olas de tormentas y los terremotos. Por los rasgos hallados las deformaciones del litoral rionegrino tendrían un origen sísmico, proceso ocurrido en un único evento durante el ciclo Andino cuyos inicios se fijan hace aproximadamente 45 Ma. Por otra parte sus techos y bases no se hallan asociados a otros procesos de deformación y sus espesores no exceden el metro de potencia. Además se hallan acotadas a la zona de transición entre los miembros medio y superior de la Formación Río Negro descansando en algunos casos sobre arcilitas y en otros sobre limolitas, originadas en un paleorelieve de interdunas.<hr/>Soft-sediment deformation structures (SSD) are alterations produced almost simultaneously with sedimentation. They are directly related to internal characteristics of sedimentary materials as well as to external factors acting on them. Results derived from such alterations are evidenced as injections, fractures, volcanoes and convolute laminations, among other forms, affecting stratification either totally or partially. Soft-sediment deformation structures resulting from seisms are known as seismites. The present study aims at determining for the first time the presence of SSD structures in the Río Negro Formation, located in the northern area of San Matías Gulf, near Río Negro Lighthouse, Argentina (Fig. 1). To this end, structures were firstly identified and further described. Samples were subsequently collected for the determination of grain-size, mineralogy and organic matter content. Photographs of the different sectors evidencing deformations were taken in order to determine further comparative models. Morphology in the study area is associated to cliffs with vertical, fractured fronts and with an average height of 70 m in whose base torn-down blocks are accumulated. The geological structure of the study area is related to the Cuenca del Colorado and the Comarca Nordpatagónica, whose basement is mainly composed of Paleozoic and Mesozoic crystalline rocks. The sedimentary tertiary cover from the Miocene-Pliocene is represented by light-blue sandstones of the Río Negro Formation (Andreis, 1965). This unit was formed in an aeolian environment with intercalations of clay-silt shallow lagoons and a marine episode located in the mid area of the Río Negro Formation. At the top of the Río Negro Formation there are Pleistocene-Holocene sedimentites having a thickness of up to 5 m. Within the local structural framework of our study area there are fractures with a NE-SW and a NW-SE direction, which are related with fractures N55º, N90º and N350º azimuth located in the abrasion platform. According to Dzulinsky and Walton (1965), Lowe (1975), Brencley and Newall (1977), Clauss (1993), van Loon (2002), Owen (2003), Neuwerth et al. (2006), Alfaro et al. (2006), Montenat et al. (2007), among others (Table 1), and, taking into account the geometry of deformations, laboratory reconstructions and field observations from our study area, it can be concluded that the classifications of SSD structures tend to establish morphologic and genetic systematizations. The following characteristics were identified in our study area: limited deformations among stratigraphic horizons; a lateral continuity of SSD structures at considerable distances; and a confinement between non-deformed strata and its lithological association with psamitic-pelitic sediments. The study area, which is 4 km long and is located between Río Negro Lighthouse and the beginning of Banco Verde, is from the morphological point of view, a cliffed front with an ENE-WSW orientation. Different types of SSD structures were identified in this area. For example, from the morphological point of view and according to the loading mechanisms observed, simple-load structures (Fig. 2), attached and detached pseudonodules (Figs. 3, 4 and 5) and complex structures (Fig. 6) were identified. Furthermore, from the genetic point of view and according to the intrusion processes observed in soft sediments, water-scape structures (Fig. 7) and plate- or fountain-like deformations (Fig. 8) were found. From the genetic point of view, and based on the collapse and pressure mechanisms observed, basal slumping (Fig. 9) and directed-pressure structures (Fig. 11) were also found. The above-mentioned SSD structures were analyzed and interpreted following Strachan´s model (2002) (Fig. 10) and Laird´s model (1968) (Fig. 12). The origin of SSD structures depends on the characteristics of sedimentites and on the mechanisms that produce them. In the study area, the materials susceptible to deformation come from an interdune environment that is characterized by granulometric variations derived from the fluctuating and restrictive climatic conditions (Cojan and Thiry, 1992) that typify the Río Negro Formation. Fine-grained materials having low cohesion and poor sorting such as the sediments of deformed strata (Fig. 13) produced SSD structures as a result of high pore pressure and liquefaction effects (Tsuchida and Hayashi, 1971; Obermeier, 1996). Grain packing with a porous value as that allows intercommunication among grains and saturated material, is also crucial to the formation of SSD structures. The mineralogic content of deformed levels is composed of i) quartz, chalcedony, orthose, plagioclase, pyroxenes and biotite, opaques (magnetite and ilmenite, autigenic pyrite) in crystalline aggregates; ii) undetermined Fe oxides; and iii) colorless and light-brown unaltered volcanic glass shards, clays identified as smectite-illite interstratified and scarce kaolinite. Grains are mainly subangular and, to a lesser extent, sub-round and round. The surface of the majority of grains in the study area was found clean and with some marks. The percentage of CaCO3 was found to vary from 0.5 to 3% and that of total organic carbon (TOC) was found to reach 1.5%. Deformations may be produced as a result of load deformation mechanisms, fluid escape, basal slumping or pressure-directed displacements. Due to load deformation mechanisms, structures are linked to gravity-related movements occurring during the initial stages of deposition. For these deformations to occur, grain-size at the overlaying levels should be thicker than at the underlying levels, for example, sandstones rather than silstones or claystones. These deformations are related to water saturation at the deformed level (fluidization-liquefaction). Therefore, deformation mechanisms, which involve both expulsion and rotation of fragments as well as fluid escape, are characterized by the action of lithostatic pressure which produces movement (deformation) and by the action of the underlying sedimentary levels. Deformations may also result from a fluid escape mechanism, i.e., from a mechanism associated to i) the spatial arrangement of grains (packing), ii) their shape, iii) their tendency to inequigranularity, and iv) the communication among macro- and micro- pores as well as the high or low sinuosity connection among themselves (Net and Limarino, 2000). Further requirements for deformations to occur include particular thixotrophic conditions, especially the presence of colloids among grains. The rupture of unions of particles either by hitting or by shearing is, among others, a cause which produces an unbalance between hydrostatic pressure and lithostatic pressure. If the latter is altered, the energetic unbalance makes fine sediments flow among the weakly lithified sandstones whose extrusion will occur via both vertical and horizontal pore ducts (Lopez Gamundi, 1986; Clauss, 1993). Basal slumping produces deformations that are associated not only to soft sediments deposited in natural slopes but also to interbedded sand- and mud-levels. Layers tend to have a prismatic-shaped geometry whose materials are under ductile-to-fragile conditions, in which antique layers support younger ones. Once horizontality is affected, movement, which is marked by a rupture of the original slope, begins. The lower levels are expected to transport the upper ones without affecting the original succession of layers. At the delay of movement derived from the compressive effect of the displacement front, fluids extrude forming cones or cut dikes (Fig. 10). Several deformations of this type initiate movement as result of differences in the hydrostatic gradient (Strachan, 2002). Deformations may be also produced as a result of pressure-directed displacements which are conditioned by the compaction level, thickness and ability of materials to deform. Thus, deformations occur because the original level is saturated in water as a result of the ductile behavior of materials (Bracco et al., 2005). Laird (1968) claims that SSD structures should meet some of the following requirements to be considered of seismic origin: slightly curved strata walls and floors to follow the original stratification and interruption of continuity of the stratum that is marked by a scar in which the sedimentary fillings keep their characteristics both above and below stratification. There could be rotated sediment clasts below the discontinuity as a result of a thrust-induced drag of the upper sedimentary packing. These processes could be, in turn, triggered either by the charge or pressure of the lithologic column, storm waves and seismicity. Storm-wave impact may also produce deformation in soft sediments. Nonetheless, no high energy structures such as cross-beddings or tsunami-type chaotic sedimentation were observed in our study area. Noteworthingly, for stormwave-derived liquefaction to occur, waves should reach magnitudes higher than 6 m (Alfaro et al., 2002), this being a phenomenon that was not recorded in our study area. Taken together, findings from the present study indicate that SSD structures in our study area are seismic alterations that occurred in an event during the Andean cycle whose beginnings are traced approximately 45 My ago. The fact that i) both the roofs and bottoms of these structures are not associated to other processes of deformation, ii) their thickness does not exceed one meter, and iii) they are confined to a transitional area between the middle and top members of the Río Negro Formation, lying in some cases on claystones and in some other cases, on siltstones, originated in an interdune paleorelief, confirms their seismic origin. <![CDATA[Sismoestratigrafia y evolución geomorfológica del talud continental adyacente al litoral del este bonaerense, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000100004&lng=es&nrm=iso&tlng=es El Margen Continental Argentino es uno de los márgenes más extensos del mundo (2 x 10(6) km²) cuyo mayor desarrollo, entre 35 y 48ºS, corresponde a un típico margen pasivo volcánico. Allí se desarrollan los siguientes rasgos: plataforma, talud, emersión y el Cañón Submarino Mar del Plata. Este trabajo describe los aspectos morfosedimentarios, estratigráficos y evolutivos del talud adyacente al litoral del este bonaerense. El estudio se basó en información sísmica mono y multicanal de alta resolución complementada con el análisis de muestras del fondo marino, fundamentalmente testigos verticales. El talud se desarrolla entre 120 y 3500 m de profundidad. Está formado por tres sectores principales. El más cercano a la plataforma es el talud superior situado por encima de los 700/800 m, de fuerte pendiente. A partir de allí se desarrolla el talud medio, constituido por la Terraza Ewing, cuya superficie de baja pendiente llega hasta los 1300 m de profundidad. Finalmente, el talud inferior vuelve a ser de fuerte pendiente y llega hasta los 3500 m, desde donde grada hacia la emersión continental. El talud es atravesado por el Cañón Submarino Mar del Plata, que comienza alrededor de los 500 m de profundidad aunque adquiere una típica configuración de valle en V entre 1200 y 3700 metros. La cobertura sedimentaria del talud es silicoclástica, formada por fangos algo arenosos que muestran mayores porcentajes de arena e inclusive rodados en los alrededores del cañón, particularmente en sus cabeceras. La asociación mineralógica es volcánico-piroclástica de origen pampeano-patagónico. A través de la aplicación de métodos sismoestratigráficos se identificaron siete "Secuencias Depositacionales" que abarcan desde el Cretácico superior hasta el presente, las cuales están separadas por horizontes sísmicos mayores que representan discontinuidades resultantes de la ocurrencia de significativos eventos climáticos-oceanográficos de amplia extensión regional. La sucesión de secuencias señala que el talud comenzó a evolucionar a partir de la transición Eoceno-Oligoceno en respuesta a complejos procesos de agradación y progradación con depósitos turbidíticos y contorníticos, y formación de cañones submarinos. Se definen cuatro etapas evolutivas principales: 1) Agradacional, del Cretácico-Eoceno, con fuerte acreción vertical del talud asociada a subsidencia térmica de la Cuenca del Salado, con alta tasa de sedimentación. 2) Desarrollo del talud durante el Eoceno superior-Mioceno medio, cuando se estructura el margen pasivo y comienza a evidenciarse la influencia de las masas de agua de origen antártico, lo que se manifiesta en la formación de secuencias sedimentarias complejas con períodos alternantes de progradación-retrogradación aunque con predominio de los primeros (con alta dinámica turbidítica y formación de cañones submarinos) que hacen avanzar progresivamente el talud en dirección al mar. 3) Desarrollo de la Terraza Ewing en el Mioceno medio-superior, cuando la dinámica sedimentaria asociada a la circulación de las corrientes oceánicas de origen antártico favorece la migración hacia el norte de grandes depósitos contorníticos. 4) Configuración definitiva del talud en el Plioceno-Cuaternario, al desarrollarse la Terraza Ewing y el Cañón Submarino Mar del Plata con sus características presentes.<hr/>Introduction The Argentina Continental Margin (MCA) is one of the largest margins around the world (2x10(6) km²), which in most of its area of development (between 35 and 48ºS) belongs to a typical extensional volcanic passive margin (Mohriak et al., 2002). The region is located in a key area of the Southwestern Atlantic Ocean due to its significance in the global oceanographic-climatic interaction (Wefer et al., 1996, 2004; Carter and Cortese, 2009). As a result, the study of the Cenozoic sedimentary sequences preserved in the geological record is very important for paleoceanographic-paleoclimatic-paleoenvironmental reconstructions. The study area is included in the northern part of the MCA adjacent to eastern Buenos Aires province (Fig. 1a). The major physiographic units are the shelf, slope, rise and the Mar del Plata Submarine Canyon. This work describes the Cenozoic morphosedimentary, stratigraphic and evolutive aspects of the continental slope in the region. The study is based on high resolution seismic (multi and monochannel) complemented with sediment analysis on piston and gravity cores as well as grab samples (Fig. 1b), obtained during different cruises on board the Research Vessels Puerto Deseado (Argentina) and Meteor (Germany) (Table 1); additional seismic, geological and sedimentological information was gathered from LDEO (USA) and BGR (Germany). Geotectonic, Morphosedimentary and Oceanographic Setting The region (Fig. 1a) is located in the southern part of the Salado basin where post-Cretaceous sediment thickness varies between 2 and 4 kilometers. Stratigraphic information from offshore oil drillings (Fig. 2) indicates that in the outer shelf, immediately west of the study area, sedimentary sequences are represented by Maastrichtian-Paleocene marine deposits, Eocene-Oligocene continental deposits, Miocene marine deposits, Pliocene continental deposits and Quaternary marine deposits (Yrigoyen, 1975, 1999; Tavella and Wright, 1996). These sequences change towards the slope into fully deep-marine facies. Morphosedimentary configuration of the continental slope in the entire passive margin is dominated by a contourite depositional system (Hernández-Molina et al., 2009, Fig. 3a), composed of both depositional and erosive features (drifts, terraces, scarps, submarine canyons) that resulted from complex interactions among sedimentary, oceanographic and climatic components. One of the main conditioning factors is the oceanographic setting (Fig. 3b), characterized by predominance of parallel-to-the-slope (contouritic) south-to-north circulating currents from Antarctic origin, which in the northern part of the margin interact with the North Atlantic water masses, so determining the Confluence Zone. Across-margin sediment transport processes such as turbidity currents are also very significant in the evolution and configuration of the margin. These processes became more important towards the north, particularly in the study area. Morphology and Sedimentology of the Eastern Buenos Aires Province Continental Slope The continental slope in the study area extends between 120 and 3500 m depth (Fig. 4). It is constituted by an upper sector characterized by a steep slope above 700/800 m (upper slope). From there, the middle slope extends seaward, constituting the Ewing Terrace, a terraced, low-gradient feature that extends down to 1300 meters. The lower continental slope has again a steep slope that reaches 3500 m, from where it grades to the continental rise. The continental slope is cut by the Mar del Plata Submarine Canyon that begins at about 500 m depth, showing a typical V-shape configuration between 1200 and 3700 meters. The sedimentary cover in the slope is siliciclastic and consists of sandy muds, which close to the canyon incorporate a higher sand content and pebbles. The mineralogical content belongs to the volcanic-pyroclastic association of pampean-patagonian origin. Figure 5 illustrates a type morphological cross section showing the bottom and near-bottom sediment distribution. Analysis of the forams collected from several cores indicate that in the upper 5 m of the sedimentary cover upper Pliocene to Recent faunas are present, with ages lower than 450 ka at 1.5 m, 120 ka at 0.75 m, and late Holocene in the uppermost 0.5 m. Stratigraphy The seismic-stratigraphic analysis and correlation with geological information from offshore oil drillings allowed to define seven depositional sequences (SD) (named with letters A to G from top to bottom), which encompasses the time-span between Late Cretaceous and the present. They are separated from each other by major seismic reflectors that represent unconformities produced by significant climatic-oceanographic events of regional extension (Tables 2 and 3). Interpretation and correlation among different seismic reflectors defined by several authors (Ewing & Lonardi, 1971, Hinz et al., 1999, Parker et al., 1999, 2005) was needed before defining those that separate the SDs. Figure 6 represents the synthesis of the correlation between seismic and geological information, whereas figure 7 is a type section showing the architecture and regional disposition of the SDs. The depositional sequences are described from bottom (SD G) to top (SD A). SD G: the top of the unit is the seismic reflector AR3 that represents the K-T boundary. The age of the sequence is considered Aptian-Maastrichtian. Internal seismic characteristics are mainly represented by subparallel, semi-transparent reflections. It represents shallow marine environments deposited in a longitudinal basin which evolved as a result of the South Atlantic opening. SD F: Paleocene-upper Eocene. It has a maximum thickness of 900 meters. The internal seismic structure is transparent, with aggrading sequences (Fig. 8a-d) and locally chaotic, sometimes divergent reflections towards the base. Paleovalleys associated to ancient submarine canyons are also evident (Fig. 7). The upper boundary (reflector AR4) shows deep depressions that affect the base of the sequence; this reflector represents a regional expansion of the eastern Antarctic ice masses during Eocene-Oligocene times. The unit represents the final evolution of the "sag" stage in the Salado basin. SD E: upper Eocene-beginning of the mid Miocene. Thickness reaches up to 500 meters. The internal seismic structure shows changing characteristics, with prograding sequences in the base, retrograding sequences in the middle part with wavy megastructures, and aggrading sequences in the upper part with large sediment lobes and paleovalleys (Fig. 8a-d) as well as cut-and-fill structures (Fig. 8d) and debris flows (Fig. 8c). The top reflector (AR5) represents another regional expansion of Antarctic ice masses. The unit shows evidences of deepening of the marine environment from base to top, and seismic reflector R* that divides two lithologically different sections could represent the change from prograding to retrograding facies. SD D: mid to upper Miocene. The top seismic reflector (H2) has a morphology similar to the present surface (Fig. 7). Thickness is about 400 m. Seismic structure is semitransparent, with subparallel reflections and local chaotic configuration. The lower section shows prograding structures indicating the growing of the slope towards the east, whereas the upper section shows retrograding sequences with megawaves (Fig. 8 a-d). A contouritic drift develops in this upper part. Sediment infilling of paleovalleys is also evidenced. The Ewing Terrace is well developed and shows evidences of erosive processes with formation of submarine canyons. SD C: lower Pliocene. Seismic configuration shows morphological features very similar to the present-day topography (Fig. 7). Thickness is less than 200 m. Internal seismic structure is homogeneous, with subparallel seismic reflectors of large lateral extension, prograding structures and channels with internal migrating and filling structures (Fig. 8a-d). In the outer boundary of the Ewing Terrace, retrograding, sometimes chaotic structures are evident. SD B: mid to upper Pliocene. Thickness is less than 200 meters. Internal seismic configuration is of reflectors parallel to top and bottom, with aggrading levels in the upper slope and Ewing Terrace (Fig. 8a-d). SD A: upper Pliocene-Quaternary. Thickness doesn't exceed few tens of meters. Parallel reflections, turbiditic and contouritic facies as well as slides, debris flows and active erosive-depositional processes are evident. Cores obtained in the upper levels of this unit are composed of muddy and silty sediments with thin sand layers probably representing turbiditic processes. Discussions and Conclusions The sequence stratigraphy, architecture and structures reveals that the continental slope begun to evolve during the Eocene-Oligocene transition as a result of complex processes like aggradation and progradation, with turbiditic-contouritic processes and formation of submarine canyons, mainly associated to oceanographic and climatic conditioning factors. Two main features characterize the slope configuration: the Ewing Terrace and the Mar del Plata Submarine Canyon. The Ewing Terrace mainly resulted from sedimentation conditioned by along-slope, south-to-north flowing contouritic currents with additional strong action of across (down)-slope turbiditic processes. Post-Miocene sequences in the Terrace represent deep marine sedimentary facies genetically associated to sea-level fluctuations. Contouritic deposits seem to be mainly associated to highstands, whereas turbiditic and down-slope slides processes probably dominated during lowstands when high-energy, coastal processes occurred near to the shelf-slope transition. The Mar del Plata Submarine Canyon is an expression of high-energy, turbiditic processes that were probably enhanced during sea-level lowstands (Pickering et al., 1989, Viana et al, 2002, Canals et al., 2006). Intercalations of sandy deposits in between the dominant muddy sedimentation on the terrace around the canyon reveals the importance of turbiditic activity. The configuration of seismic reflector N shows that the canyon reached a morphology similar to the present one in the upper Pliocene. Four stages define the evolution of the study sector of the MCA: 1) Initial aggradational stage, from the Cretaceous to the Eocene, with marked vertical accretion of the slope associated to the "sag" stage with high sedimentation rate in the Salado basin. 2) Growing-up of the slope during upper Eocene-mid Miocene times, when the "passive margin" stage developed and the strong influence of the Antarctic water masses begun to affect the region, what is manifested by the formation of complex sedimentary sequences with alternating prograding-retrograding cycles. Prograding cycles dominate in the region with high turbiditic dynamics and formation of submarine canyons that allowed the seaward advance of the slope. 3) Development of the Ewing Terrace in the mid-upper Miocene, when the sediment dynamics associated to the near-bottom circulation of oceanic currents of Antarctic origin favoured the northward migration of large contouritic deposits. 4) Definitive configuration of the slope in Pliocene-Quaternary times, when the Ewing Terrace and the Mar del Plata Submarine Canyon reached their present characteristics.