Cross-stratification formed by migrating dunes is ubiquitous in fluvial channel deposits, yet cross-stratification, specifically formed by dunes, is comparatively uncommon in deep-marine channel deposits subjected to similarly unidirectional flows, the reasons for which is the subject of much debate. In part, this is related to the difficulty associated with acquiring high-resolution data sets; natural turbidity currents are highly destructive, whereas physical models are hampered by the shortcomings of conventional laboratory instrumentation. Consequently the internal structure of turbidity currents is poorly understood, thus hindering the ability to identify the linkages between flow properties and depositional characteristics. In this study we use a medical-grade computed tomography scanner coupled with a three-dimensional ultrasonic Doppler velocity profiler to characterize the flow field and depositional morphology across a range of grain sizes (d50: 70–330 µm) and sediment concentrations (5%–17.5% by mass). Results show that the development of angular bedforms is suppressed in all flows with particle concentrations greater than 9.5% by mass, but notably also in all flows with particles <230 µm and sediment concentrations down to 5%, the lowest slurry densities used in these experiments, by mass. This suggests a first-order control on the response of the bed by the makeup of the overriding current. More fundamentally, we propose that grain size controls the character of near-bed density stratification, which is a requisite condition to generate the hydrodynamic instabilities that create the initial bed defects, which then lead to the development of angular bedforms like ripples and dunes.