Sedimentary structures of combined flow deposits from the shoreface-connected ridges along the central Dutch coast

Sedimentary processes over the shoreface-connected ridges along the central Dutch coast were analyzed using a set of about 250 box-cores, supplemented by echo sounding and side-scan sonar surveys. The sedimentary structures reveal that reworking of the sea bed is dominated by the combined action of waves and currents. The structures allow a detailed description of these combined flow deposits. Low-angle bedding is the predominant structure, being associated with tidal current- and wave-related features to a variable degree. The most important structural characteristics are distal up-fanning of the laminae, abundant trough fillings, multi-directional sets and the presence of large, irregular erosion surfaces. The sedimentary features show no marked spatial variation across the ridge topography, except for a weak tendency towards a better grain-size sorting at the ridge crests and an increase in wave-generated structures in shoreward direction. The latter observation is in accordance with side-scan sonar pictures which show the fading of megaripples in the same direction. The uniformity of the sedimentary features of the present ridge system contrasts with that of other, comparable systems. This seems to indicate that it may be difficult to identify fossil shoreface-connected ridges on the basis of sedimentary structures and grain size alone. The relative scarcity and weakness of bioturbation suggests that physical reworking of the bed is intense and frequent. Algal penetration of the upper 10 cm of the sediment column may cause serious blurring of the structures.