New laboratory experiments reveal that cohesionless turbidity currents are able to enter cohesive soft muddy substrates without losing their shape. These intrabed currents are driven by bed shear stress exceeding bed cohesive strength, and by flow density exceeding bed density. The flows produce unique turbidites with internal mud layers, mixed cohesive-noncohesive sediment layers, and flame and load structures. A depositional model for intrabed (I) turbidites is proposed, comprising, from base to top: I1-sand-bearing mud, with a scoured base, dispersed mud, and mud clasts; I2-muddy sand from the intrabed portion of the turbidity current; I3-sandy mud with a speckled appearance; and I4-mud-poor sand from the suprabed portion of the flow. Complete I1-I4 turbidites are inferred to dominate locations in nature where the currents mix with the bed and deep erosional scours form, filled with deformed or chaotic sand-mud mixtures. Further downflow, base-missing I2-I4 and I4 sequences signify gradual deceleration, loss of erosivity, and termination of intrabed flow.

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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.

Density currents, whose movement takes place by the density difference between the flow and the ambient fluid around it, can interact with the substract generating bedforms similar to the fluvial environments. However, there are no specific bedform phase diagrams capable to predict this type of phenomenon. This study aims to compare the prediction of fluvial bedforms phase diagram with those generated by experimental saline currents. Bedforms were generated in two-dimensional tilting plexiglass flume submerged in a larger tank filled with water with three different mobile beds and varied values of discharge and salt concentration. It was observed three types of bedform (lower plane bed, ripples and dunes), which, with the concomitant calculation of hydrodynamic parameters (mean velocity, energy and mobility) allowed the use of the phase diagram. It was observed that the fluvial phase diagrams did not present good predictions for bedforms generated by density currents. This fact is associated to the hydrodynamics differences (velocity and concentration profiles) and the limitation of the dimensional parameters in the extrapolation of results. Therefore, it is indicated the need to draw up a proper phase diagram to density currents. Keywords: Density current; Bedforms; Physical modeling; Mobile bed; Bedform phase diagram. RESUMOAs correntes de densidade, cujo movimento ocorre pela diferença de massa específica entre o escoamento e o fluido ambiente ao seu redor, podem interagir com o substrato gerando formas de fundo, similares às encontradas em ambientes fluviais. Entretanto não existem diagramas de previsão específicos correspondentes para esse tipo de fenômeno. Assim, este trabalho visa comparar a ocorrência das formas de fundo fluviais previstas nos diagramas de previsão com aquelas geradas por correntes de densidade salinas obtidas experimentalmente. As formas de fundo foram geradas em um canal bidimensional de declividade variável, preenchido por água, com três composições de leito móvel e diferentes valores de vazão, massa especifica e inclinação. Três formas de fundo foram identificadas (leito plano inferior, ondulações e dunas), as quais, juntamente com o cálculo de parâmetros hidrodinâmicos permitiram a utilização dos diagramas fluviais. Verificou-se que os diagramas fluviais não apresentaram boas previsões das formas de fundo geradas por correntes de densidade. A esse fato são atribuídas as diferenças hidrodinâmicas dos escoamentos (perfis de velocidade e concentração) e, também, à limitação dos parâmetros dimensionais na extrapolação dos resultados. Dessa forma, indica-se a necessidade de se elaborar um diagrama de previsão próprio adaptado a estas correntes.Palavras-chave: Corrente de densidade; Formas de leito; Modelagem física; Leito móvel; Diagrama de previsão.

Controlled laboratory experiments reveal that the lower part of turbidity currents has the ability to enter fluid mud substrates, if the bed shear stress is higher than the yield stress of the fluid mud and the density of the turbidity current is higher than the density of the substrate. Upon entering the sub-strate, the turbidity current either induces mixing between flow-derived sediment and substrate sediment, or it forms a stable horizontal flow front inside the fluid mud. Such 'intrabed' flow is surrounded by plastically deformed mud; otherwise it resembles the front of a 'bottom-hugging' turbidity current. The 'suprabed' portion of the turbidity current, i.e. the upper part of the flow that does not enter the substrate, is typically separated from the intrabed flow by a long horizontal layer of mud which originates from the mud that is swept over the top of the intrabed flow and then incorporated into the flow. The intrabed flow and the mixing mechanism are specific types of interaction between turbidity currents and muddy substrates that are part of a larger group of interactions, which also include bypass, deposition, erosion and soft sediment deformation. A classification scheme for these types of interactions is proposed, based on an excess bed shear stress parameter, which includes the difference in the bed shear stress imposed by the flow and the yield stress of the substrate and an excess density parameter, which relies on the density difference between the flow and the substrate. Based on this classification scheme, as well as on the sedimentological properties of the laboratory deposits, an existing facies model for intrabed turbidites is extended to the other types of interaction involving soft muddy substrates. The physical threshold of flow-substrate mixing versus stable intrabed flow is defined using the gradient Richardson number, and this method is validated successfully with the laboratory data. The gradient Richardson number is also used to verify that stable intrabed flow is possible in natural turbidity currents, and to determine under which conditions intrabed flow is likely to be unstable. It appears that intrabed flow is likely only in natural turbidity currents with flow velocities well below ca 3Á5 m s À1 , although a wider range of flows is capable of entering fluid muds. Below this threshold velocity , intrabed flow is stable only at high-density gradients and low-velocity gradients across the upper boundary of the turbidity current. Finally, the gradient Richardson number is used as a scaling parameter to set the flow velocity limits of a natural turbidity current that formed an inferred intrabed 2002