ABSTRACT

The marginal marine environment consists of an offshore where the wave form is approximately sinusoidal and a nearshore where the wave form is either solitary or that of a bore. Within the nearshore, shoaling waves become progressively higher and steeper until they break. After breaking, the waves progress as bores through a surf zone; these bores ultimately terminate within a swash zone on the beach itself.

In a high-energy coastal environment where long-period swell enters a nearshore uncomplicated by offshore bars, sedimentary structures develop on the seafloor in facies that trend parallel to the zones of different wave activity. In the offshore, small sand ripples are the most common depositional structure, but in the nearshore larger bed forms predominate. Seaward from the line of breakers, in the zone of wave build-up, are landward-oriented lunate megaripples. Near the outer portion of the surf zone the bed form is planar (outer planar facies), but, in the inner portion of the zone, a area of large-scale bed roughness (inner rough facies) commonly is present. Within the swash zone, the bed form is again planar (inner planar facies). The boundaries of the facies shift in response to changes in waves or tide, and certain of the zones are sometimes missing. The relative position of the zones, however, is invariable.

The major features of the bed forms can be interpreted in terms of flow regime. The stronger of the two opposing transient currents caused by passing waves produces structures analogous to those produced by continuous, unidirectional currents. Landward wave surge is dominant in the outer three structural facies, whereas seaward surge predominates in the innermost (inner planar) facies. Wave surge over the intermediate inner rough facies is more complex, and the direction of strongest surge may be variable.

In the outer three facies, the landward sequence from small asymmetric ripples to lunate megaripples to plane bed suggests an increase in flow regime from the lower part of the lower regime to the upper regime; this shoreward increase in flow regime is associated with a shoreward increase in orbital velocity at the bottom. The inner planar facies is produced by flow in the upper regime. The inner rough facies, situated between two zones of flow in the upper regime, is apparently a product of flow in the upper part of the lower regime.

Within each of the structural facies a distinctive set of internal structures is produced. Internal structure of the asymmetric ripple facies consists of shoreward-inclined ripple cross-lamination and gently inclined cross-stratification. The lunate megaripples produce medium-scale landward-dipping foresets. Within the outer planar facies bedding is nearly horizontal. Structures in the inner rough facies produce medium-scale foresets that mostly dip directly or obliquely seaward, although landward-dipping foresets also occur. Within the inner planar facies bedding is gently inclined seaward. Migration of the facies in response to changes in waves or tide produces distinctive assemblages of structures where the facies overlap. These assemblages provide criteria for paleoenvironmental i terpretation, particularly where interrelated assemblages occur in a meaningful spatial distribution.