An experimental and field-based investigation into decimetre-scale bedforms formed by turbidity currents

Decimetre-scale cross-stratification is relatively rare in turbidites, yet it has been identified in both ancient and modern systems. However, the type of bedform associated with cross-stratification development and the processes involved in its formation remain poorly understood. Similarly, variations in the styles of decimetre-scale cross-stratification occurrence, the sedimentary character of individual turbidite beds and their evolution downstream remain poorly constrained. Consequently, the extent to which the occurrence of decimetre-scale cross stratification may be used as a diagnostic tool to interpret the likely downstream evolution of a system remains uncertain.
Three work elements are presented within this thesis. The first describes a series of laboratory experiments that investigate the controls on bedform development under both steady and waning saline density currents. Decimetre-scale bedforms developed within and scaled with a lower denser layer of the current below the height of the velocity maximum, whose dynamics dictated the bedform type. Dunes were distinguished from ripples, downstream migrating antidunes and upstream migrating antidunes on the basis of their out-of-phase relationship with the upper surface of the lower layer of the current. The associated flow regimes were calculated using the bulk Froude number based on the less stratified lower layer of the flow. Using this approach, the new experimental data refine the subcritical bedform phase spaces and antidune phase-spaces of the bedform phase diagram for density currents.
The second work element describes a series of experiments that test how beds of non-uniform, bimodally distributed sediment impact bedform development and kinematics, with application to both open channel flows and turbidity currents. Bedform phase diagrams that characterise sediment using the median grain-size, are found to inaccurately predict bedform development from non-uniform sediment beds made up of different grain-size distributions. The new data further refine bedform phase diagrams and improve understanding of the stabilising effect on the substrate of the fine grain-size fraction in sediment mixtures.
The final work element characterizes the occurrence of decimeter-scale cross-stratification within the turbidites of the Peïra Cava outlier, of the Tertiary Foreland Basin of SE France. Within the Peïra Cava system a greater level of variability is observed in the cross-stratified bed types than has hitherto been recognised. This facies is interpreted as being formed by dunes, which are confined to the proximal regions. Bed correlations indicate that dunes are not associated with ‘significant bypass’ of sediment to distal locations, as predicted by some existing facies tract models.