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The threshold of motion of non‐fragmented mollusc shells was studied for the first time under oscillatory flow. In this regard, flume experiments were used to investigate the threshold of motion of three bivalve and three gastropod species, two typical mollusc classes of coastal coquina deposits. The sieve diameters ranged from 2·0 to 15·9 mm. These experiments were performed on a flat‐bottom setup under regular non‐breaking waves (swell) produced by a flap‐type wave generator. The critical Shields values for each species of mollusc were plotted against the sieve and nominal diameter. Moreover, the dimensionless Corey shape factor of the shells was evaluated in order to investigate the effect of mollusc shell shapes on the threshold of motion. According to their critical Shields parameter, the mollusc threshold data under oscillatory flow present smaller values than the siliciclastic sediments when considering their sieve diameter. In addition, the mollusc datasets are below the empirical curves built from siliciclastic grain data under current and waves. When considering the nominal diameter, the critical Shields parameter increases and the mollusc data are closer to siliciclastic sediments. Bivalves, which have a flat‐concave shape (form factor: 0·27 to 0·37), have a higher critical Shields parameter for smaller particles and more uniform datasets than the gastropod scattered data, which have a rounded shape (form factor: 0·58 to 0·62) and have varied morphologies (ellipsoidal, conical and cubic). The comparison between previous current‐driven threshold data of bioclastic sediment motion and the data of mollusc whole shells under oscillatory flow shows a fair correlation on the Shields diagram, in which all datasets are below the mean empirical curves for siliciclastic sediments. These findings indicate that the shape effect on the transport initiation is predominant for smaller shells. The use of the nominal diameter is satisfactory to improve the bioclastic and siliciclastic data correlation.

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.

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. KEYWORDS:Autogenic controls; Deep-water fans morphology; Turbidity currents; Physical modeling; Sediment concentration. (comprimento, largura, razão comprimento/largura, área, topografia 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

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