Oscillatory-flow and combined-flow bed forms: Experimental investigation and implications for the shallow marine rock record

Long-period, storm-related oscillatory and combined-flows are thought to be responsible for much of the sand-sized sediment transport on the continental shelf. Yet, the relationship between these flow conditions and associated bed forms is still poorly understood. This study provides new experimental data aimed at characterizing equilibrium bed farms developed under long-period oscillatory and combined-flow conditions. Runs were conducted in a large combined-flow tunnel. Conditions explored were: oscillatory velocities (Uo) of 20--125 curls, unidirectional velocities (Uu) of 0--25 curls, oscillation period ( T) of 10.5 s and 8 s, and grain size of 0.14 mm and 0.22 mm. For conditions of increasing Uo the bed phases generated were: anorbital small-scale ripples (wavelength < 20 cm), orbital large-scale ripples (wavelength > 100 cm), and plane bed. For a given Uo and increasing Uu, the bed generally becomes progressively more asymmetric and is eventually planed flat.; Hummocky bed forms developed under moderate oscillatory velocities (50 cm/s < Uo < 90 cm/s) and low unidirectional velocities (0 cm/s < Uu < 15 cm/s). "Synthetic" stratification generated by these hummocky bed forms exhibits many of the features of hummocky cross-stratification (HCS). For purely oscillatory flows conditions, the addition of a weak Uu (≥5 cm/s) causes the stratification to become anisotropic. For greater Uu (>10--12 cm/s) the resultant cross-stratification becomes similar to that produced by unidirectional-flow dunes.; Potential genetic differences between HCS and swaley-cross stratification (SCS) were also investigated. Higher aggradation rates increased the preservation potential of hummocks which, as a consequence, increased the likelihood of forming HCS versus SCS. A cross-shelf depositional model is proposed in which HCS forms above storm wave base under moderate aggradation rates, whereas SCS forms at shallower depth under lower deposition/transport ratios resulting in lower aggradation rates.; Results from these experiments suggest that HCS ad SCS are genetically linked to the hummocky bed forms generated under long period, oscillatory flows and oscillatory-dominant combined flows. As long-period, high-energy waves require deep, wide basins to form, HCS and SCS may therefore serve as useful indicators of deposition in unrestricted, open water conditions.