Abstract

Primary sedimentary structures exhibiting the three diagnostic criteria for single sets of hummocky cross-stratification (Harms et al., 1982) 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 × 0.30 m) reveal hummocky lamination in well sorted, fine sands in water depths less than 2 m, under conditions of wave breaking and strong longshore currents. The scales of the hummocks with wave lengths of 0.3 to 0.6 m are somewhat smaller than the classic ancient analogues, but the ratio of length to height ranges from 8 to 12 and overlaps the range for hummocky cross-stratification. Depth-of-activity rods have been used to identify those hummocks formed during sediment transport events when the near-bed currents were recorded directly using electromagnetic flowmeters. The results of such experiments clearly show that hummocky lamination was produced by an actively growing bed form with little or no lateral migration. The hummocks occur under conditions close to those expected for a flat bed. In one vertical sequence, the hummocky cross-lamination is underlain by subhorizontal, planar lamination and overlain by undulatory lamination, which grades upwards into small-scale, trough cross-lamination. This sequence was associated with a single storm and appears to represent a combined-flow regime sequence with the hummocky lamination representing a post-vortex (?) ripple bed form. At the time of hummocky formation, near-bed oscillatory flows were dominant and reached 1.4 m/s with a superimposed longshore current of 0.27 m/s. However, the latter was decreasing rapidly and may well have been the cause of vertical growth of the hummock. Hummocky cross-stratification is, therefore, a primary sedimentary structure that can be formed by combined oscillatory and superimposed unidirectional flows, in very shallow surf zones.