ABSTRACT

We made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods T and maximum orbital velocities U_m in two closed flow ducts, 0.1 m and 0.4 m wide. The motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm-induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits. The ripples produced were classified geometrically as two-dimensional (2D) or three-dimensional (3D) in plan-view geometry and dynamically as oscillatory-current ripples (dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself) and reversing-current ripples (dependent on he existence of the current, in one direction or the other, during the oscillation).

Runs in the small duct with effectively 0.19 mm and 0.30 mm sand showed the familiar small 2D oscillatory-current ripples at short T and low U_m passing into much larger ripples, with spacing up to 2 m, at periods of 6-20 s and velocities up to 1 m/s, with no apparent limit on size with increasing T and U_m. Even in such a narrow duct there was some evidence of 3D geometry of the large ripples. Runs in the large duct with effectively 0.11 mm sand at a single period of about 9.5 s showed a progression from small 2D oscillatory-current ripples through small 3D reversing-current ripples to large round-crested 3D oscillatory-current ripples with prominent superimposed 3D to 2D reversing-current ripples. The large 3D ripples, which increased in spacing to 1-3 m at the h ghest attainable U_m of 1 m/s, shifted in position and changed in size with time, seemingly at random, causing substantial local and temporary erosion and deposition at a given point on the bed.

There was no net aggradation during the runs, but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3D ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium, and then plotting the profiles one above the previous with a constant upward increment. The resulting stratification is strikingly similar to much hummocky cross-stratification in ancient sandstones. This suggests that some hummocky cross-stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3D oscillatory-current bed forms develop from a planar bed during strong but waning flow.