Carbonate sandwaves in Bass Strait

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Abstract
Side‐scan sonar, seabed photography and surficial sediment sampling document a field of sandwaves at 40–46 m water depth in eastern Bass Strait, Australia. The sandwaves are formed of coarse sand and gravel consisting of 50–92% biogenic carbonate, derived largely from the remains of molluscs, bryozoans and echinoderms. Four different scales of bedform are identified: ripples (0.1–0.3 m wavelength, 0.02–0.04 m high), small crest megaripples (4–5 m, 0.2 m), large trough megaripples (3–12 m, 0.1 ‐0.5 m) and sandwaves (55–1730 m, 2–12 m). The ripples are superimposed on both the megaripples and sandwaves. Large megaripples are present in the troughs between the sandwaves, while small megaripples are superimposed on the sandwave crests. Local tidal currents are non‐rectilinear. Sandwaves and their superimposed smaller bedforms are formed transverse to the ebb flow (from 255°) but subparallel to the flood flow, and trough megaripples are aligned transverse to the flood flow (from 135°). Minor deviations of the sandwave crests from normal to the ebb flow appear to deflect flow locally, resulting in megaripples superimposed upon sandwaves aligning oblique to the sandwave crests.