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ABSTRACT

Primary sedimentary structures exhibiting the diagnostic criteria for single sets of hummocky cross-stratification (Harms et al.) have been found in the surf zone of a storm-wave dominated coastline in the Canadian Great Lakes. Epoxy peels of box cores (0.45 m × 0.30 m) reveal hummocky stratification in well-sorted, fine-grained sands in water depths less than 2 m under conditions of wave breaking and strong longshore currents. The wavelengths of the hummocks (0.3–0.6 m) are somewhat smaller than the norm for their ancient analogues, but the ratios of length to height (8–12) are comparable. Depth of activity rods have been used to identify those hummocks that formed during sediment transport events when the near-bed currents were recorded directly using electromagnetic flowmeters. Results from such experiments clearly identify the hummocky stratification as being produced by an actively growing bedform with little or no lateral migration. Hummocks occur under conditions close to that expected for the upper flat bed. In one vertical sequence, the hummocky cross-stratification is underlain by subhorizontal, planar lamination and overlain by undulatory lamination which grades upward into small-scale, trough cross-lamination of wave ripple origin. This sequence was associated with a single storm and would appear to represent a combined-flow regime sequence with the hummocky structure representing a post-vortex (?) ripple bedform. At the inferred time of hummock formation, near-bed oscillatory flows were dominant and reached maxima of 1.1 m s −1 with a superimposed longshore current of 0.27 m s−1. Rapid sedimentation associated with vertical growth of the hummocky bedform was triggered by a significant reduction in the orbital currents (by 19%) and'steady'currents (by 67%) while the total bed shear remained high.