Sedimentology and hydrodynamics of strait-margin, sand-and-gravel beaches and shorefaces

Sedimentological and ichnological analyses of three beaches and their associated shorefaces in the Juan de Fuca Strait was undertaken to develop a model for strait-margin beach-shoreface complexes. In the Juan de Fuca Strait,wave processes dominate deposition in the backshore, foreshore and upper shoreface. In the lower shoreface to offshore tidal energy is dominant. Sedimentological characteristics indicative of tidal currents dominated-depositionin the offshore and lower shoreface include the dominance of sand and the prevalence of current-generated structures. The offshore and lower shoreface are recognized ichnologically by biogenic structures attributable to the SkolithosIchnofacies. The term “tide-influenced shoreface” best describes thesen depositional environments.Biogenic-enhanced gravel transport was also observed in the offshore, lower and middle shoreface. Large kelp attached to clasts was moved parallel to the shoreline via tidal currents. It is estimated that with attached seaweed, thethreshold velocity for gravel transport is reduced by one order of magnitude.