Abstract Disarticulated shells of three bivalve mollusk species (Anomalocardia brasiliana, Codakia orbicularis, and Divaricella quadrisulcata) were experimentally tested in laboratory flumes to determine the threshold of motion and final orientation of the valves. A total of 150 current flow experiments were conducted on single shells resting on a fixed sand bed. This study demonstrated that shells in the convex-up position are more resistant to flow when the umbo is pointing downstream rather than upstream. Moreover, species with higher frontal areas were more likely to be entrained at lower flow velocities. Results of dimensionless shear stress exhibited values far below the threshold of grains movement for beds of uniform roughness (Shields curve). It was observed that circular shells in convex-up positions were mostly orientated with the umbo pointing downstream. Conversely, elliptical shells in convex-up position tended to align their longer axis parallel to the flow with the posterior side of the valve pointing downstream. These results are not only directly applicable in interpretations of incipient shell motions and in paleocurrent analyses from field and sample data, but also support construction of accurate geological models. Anomalocardia brasiliana orbicularis quadrisulcata valves 15 bed convexup convex up upstream Moreover velocities Shields curve. curve . curve) Conversely data models 1

ABSTRACT Sea level oscillations in time scales between hours and days have a great vertical amplitude regarding the low lying coastal gradient of the beaches in the Rio Grande do Sul coast. However, the mechanism of oscillations is poorly understood since the scarcity of observational data makes it impossible to determine the forces that control sea level oscillations. Therefore, hourly sea level and wind time series with a time period of 650 days were examined. It has been found that the mean tidal and subtidal amplitudes were very similar to each other and that a considerable portion of the energy from sea level oscillations was due to astronomical forces. A new perspective was introduced when high and low frequencies were compared, leading to the comprehension that astronomical tides should be considered in coastal studies in southern Brazil. The sea level time series analyzed in this study showed that the maximum amplitude of the high spring tide was 0.53m, and that the subtidal rise caused by the wind reached up to 0.66m. In general, when large tidal and subtidal amplitudes are added, it can generate extreme events of sea level rise on the coast, which constitute a direct threat to coastal communities and habitats.

RESUMO: Controles autogênicos influenciam significativamente a morfologia dos leques de águas profundas e seus lobos deposicionais. Neste trabalho, objetivamos investigar a ação dos controles autogênicos sobre a topografia e a geometria de leques de águas profundas, bem como a influência da concentração de sedimentos gerados por correntes de turbidez sobre a morfologia destes. Por meio da repetibilidade de experimentos de modelagem física 3D de correntes de turbidez, foram realizadas duas séries de experimentos: correntes de turbidez de alta densidade (HDTC) e correntes de turbidez de baixa densidade (LDTC). Todos os demais parâmetros de entrada (vazão, concentração volumétrica de sedimentos e tamanho de grão) foram mantidos constantes. Cada depósito foi analisado mediante abordagens qualitativas e quantitativas e análise estatística. Em cada série experimental, foi observada a variabilidade dos parâmetros morfológicos dos depósitos gerados (comprimento, largura, razão comprimento/largura, área, topografia). Os leques de HDTC apresentaram evolução deposicional complexa, com quatro etapas evolutivas bem marcadas ao longo do tempo que resultaram no processo de autocanalização das correntes de turbidez, enquanto que nos leques de LDTC não foi identificado autocanalização ou quaisquer etapas evolutivas distintas. Altas disparidades na geometria dos leques, caracterizadas pelos elevados valores de desvio padrão relativo, sugerem que os controles autogênicos induziram a um comportamento morfológico estocástico nos leques.

ABSTRACT: Autogenic controls have significant influence on deep-water fans and depositional lobes morphology. In this work, we aim to investigate autogenic controls on the topography and geometry of deep-water fans. The influence of the sediment concentration of turbidity currents on deep-water fans morphology was also investigated. From the repeatability of 3D physical modeling of turbidity currents, two series of ten experiments were made, one of high-density turbidity currents (HDTC) and another of low-density turbidity currents (LDTC). All other input parameters (discharge, sediment volumetric concentration and grain size median) were kept constant. Each deposit was analyzed from qualitative and quantitative approaches and statistical analysis. In each experimental series, the variability of the morphological parameters (length, width, L/W ratio, centroid, area, topography) of the simulated deep-water fans was observed. Depositional evolution of the HDTC fans was more complex, showing four evolutionary steps and characterized by the self-channelizing of the turbidity current, while LDTC fans neither present self-channelizing, nor evolutionary steps. High disparities on the geometrical parameters of the fans, as characterized by the elevated relative standard deviation, suggest that autogenic controls induced a stochastic morphological behaviour on the simulated fans of the two experimental series.

This paper deals with the characterization of the geomorphology and paleo-evolution of a La Plata River paleochannel, located on the south Brazilian continental shelf since the Upper Pleistocene. Through a bathymetric survey and coring of sediments from the paleochannel, a sedimentological and palynological analysis was carried out. This kind of analysis allowed us to characterize a transgressive depositional sequence on the La Plata River paleochannel. The palynological sequences indicated marine environments grading to mixohaline and continental freshwater. From a sedimentological point of view, the area is characterized as a fluvial-estuarine environment. These data offer new information to reconstruct the paleogeographic evolution of the La Plata River paleochannel on the Rio Grande do Sul continental shelf (Brazil).

Esta investigación tiene como objetivo caracterizar la evolución geomorfológica y sedimentológica de un paleocanal, perteneciente al Río de la Plata, que se ubica en la plataforma continental del sur de Brasil desde el Pleistoceno superior. Se realizó un levantamiento batimétrico y se obtuvieron testigos para un estudio palinológico y sedimentológico del área. A partir de estos datos fue posible caracterizar depósitos transgresivos junto al paleocanal del Río de la Plata. Las secuencias palinológicas mostraron ambientes con influencia marina que evolucionan a mixohalinos, lagunares y continentales de agua dulce. Desde el punto de vista sedimentológico, el área fue caracterizada como un ambiente fluvio-estuarino. Los datos obtenidos aportan información nueva para la interpretación de la evolución paleogeográfica del antiguo cauce del Río de la Plata en la plataforma continental de Rio Grande do Sul (Brasil).