The occurrence and formation of ladderback ripples in subtidal, shallow-marine sands, Zululand, South Africa

Abstract

Until recently ladderback ripples have been considered diagnostic of late-stage emergence run-off in intertidal environments. Observations ∗ ∗ Reddering (1987) have demonstrated that ladderback ripples may also occur in the subtidal environment. New underwater observations off the Zululand coast support Reddering's ∗ assertion that ladderback ripples are not confined to tide-dominated environments. Two types of ladderback ripple are described. They form in shallow, submarine bioclastic and terrigenous sediments, on the landward margin of offshore coral reefs. The main ripple set in each instance is oriented sub-parallel to the shoreline and is produced by the dominant southeasterly swell, while superimposed ripple sets in the troughs of the larger sets are formed by two mechanisms: (1) the approach of low-amplitude, high-frequency northeasterly to easterly swells which are not sufficiently strong to change the orientation of the dominant larger set, and (2) longshore currents produced by oblique wind-driven swells in small coastal embayments. When conditions change, the superimposed ripples are rapidly reworked and incorporated into the main ripple set. They therefore have a low preservation potential. Both the occurrences documented here and those reported previously ∗ are in microtidal, wave-dominated settings.