Abstract

Wave ripples were generated in a wave tunnel under large-scale oscillatory flow (orbital diameter 1–4.5 m) using two different grain sizes, very fine sand and coarse sand. The geometry of bed configurations that were produced varied strongly as a function of grain size: small anorbital ripples (wavelengths ~ 10 cm, heights < 1 cm) formed exclusively in very fine sand at low oscillatory velocities, whereas large orbital ripples (wavelengths 50–350 cm, heights 7–26 cm) formed in both very fine and coarse sand, but were subdued, sharp- to round-crested, and 2-D to 3-D in very fine sand, and steep, sharp-crested, and 2-D in coarse sand. The large ripples in fine sand, if aggraded, would deposit low-angle (5–15°) cross stratification resembling hummocky cross stratification, whereas the large ripples in coarse sand would deposit high-angle (15–25°) cross stratification that might be mistaken for the deposit of a dune because of its high dip angle and large set thickness (> 5 cm). These results support the hypothesis advanced by Leckie (1988) that large waves generate markedly different stratigraphic signatures in fine-grained and coarse-grained sediment.

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