Abstract

Hummocky cross-stratification (HCS) and symmetrical gravel dunes (large-scale, coarse-grained, oscillatory wave ripples) commonly occur in close juxtaposition to one another in ancient sediments. If sediment is medium- to very coarse-grained, pebbly sandstone, then two-dimensional, symmetrical gravel dunes will be formed by bidirectional, oscillatory flow; if sediment is very fine- to fine-grained, then three-dimensional HCS will be formed by hydrodynami-cally similar currents. There is increasingly more documentation of symmetrical gravel dunes from modern shelves, whereas modern examples of hummocky cross-stratification are uncommon. Characteristics of two-dimensional gravel dunes can provide an insight to the nature of waves responsible for generating three-dimensional HCS. Ancient symmetrical gravel dunes are two-dimensional, with crest to trough heights of 10 to 15 cm and wavelengths of 70 to 130 cm. Crest shapes are rounded, although slightly peaked or flattened crests also occur. Crests are generally rectilinear, but can be slightly sinuous or have an anastomosed pattern. Coarser material is concentrated in troughs with more fine material near the crests. Crest orientation approximates paleoshoreline trends. Dip directions (a-b plane) of pebbles on opposing sides of dune crests dip in opposing directions, perpendicular to crest orientation. There is limited evidence from dune stratification that some symmetrical dunes may have been migratory. The conglomerate on which ancient symmetrical gravel dunes formed commonly has a sharp base, and is occasionally graded with solemarks, implying a sudden introduction of sediment, probably as a density current. This association is analogous to that of HCS. Modern symmetrical gravel dunes are formed on relict Pleistocene sediments in water depths of 5 to 100 m, formed by storm waves and onshore propagated swell. Symmetrical gravel dunes are orbital wave ripples, formed by bidirectional oscillatory flow. Wavelength of gravel dunes approximates orbital diameters of the waves (λ = 0.65 d_o).

Characteristics of flows that formed gravel dunes can be applied to the formation of HCS. Flow conditions that are capable of moving pebbles on symmetrical gravel dunes would generate the upper flat bed conditions of HCS.